Health Issues

Some fruits are good sources of many nutrients

2008-06-23T06:54:00.000-07:00

FRUITS

As with vegetables, some fruits are good sources of many nutrients, including carbohydrates and fiber. In addition, fruits, unlike most desserts, can satisfy your sweet tooth without tipping the scales. Also unlike many desserts, fruits provide our bodies with a variety of important vitamins and minerals.

Choices include:

apple, fresh juice sauce
apricots, * fresh canned in juice dried
banana
berries, ** except strawberries strawberries
cherries
dates
figs, fresh and dried
fruit cocktail, fresh or canned grapefruit, ** fresh canned in juice juice
grapes
kiwi**
kumquats
mango
melon, *** except watermelon watermelon
nectarine***
orange, ** fresh Mandarin sections juice
papaya
peach, * fresh canned in juice
pear, fresh canned in juice
persimmon
pineapple, fresh canned in juice juice
plums
prunes
prune juice
raisins
tangerine**

* Good source of vitamin A
** Good source of vitamin C
*** Good source of vitamins A and C

Vegetables are vital to heart health

2008-06-23T06:38:00.000-07:00

VEGETABLES

As excellent sources of vitamins, minerals, complex carbohydrates and fiber, vegetables are vital to heart health. To get the most nutrition from vegetables, eat them raw or minimally cooked (steamed). Keep in mind that each vegetable provides different vitamins and minerals, making variety important. As a role, if the outer portion of the vegetable is edible, that's where you'll find the majority of vitamins and minerals.

One unit in the vegetable group equals 1/2 cup cooked or 1 medium raw vegetable. Choices include:

artichoke
asparagus**
bamboo shoots
beets
broccoli***
Brussels sprouts
cabbage**
carrot*
cauliflower
celery
cucumber
eggplant
greens***(collard, endive, escarole, lettuce, spinach)
kohlrabi
leeks
mushrooms
okra
onion
parsnip
peas
peppers***
pumpkin*
radish
rutabaga
scallions
shallots
squash*
snow peas
string beans
sweet potatoes*
tomato**
tomato juice or tomato-juice cocktail, low-sodium turnips**
water chestnuts
yams*
zucchini

*Good source of vitamin A
** Good source of vitamin C
*** Good source of vitamins A and C

Note: Starchy vegetables higher in complex carbohydrates are found in the grains group. Tomato juice and tomato-juice cocktail are high in sodium. Select the low-salt variety or make your own!

Learning to Relax

2008-06-23T00:29:00.000-07:00

All nervous tension has a muscular element, the control of which helps your nervous and emotional as well as your physical state. You cannot command your whole body to relax at one time, but you can easily learn to relax one or two muscle groups at once. When you have learned this simple procedure, you can quickly relieve accumulated muscle tension at almost any time. In the process you will keep both nervous and muscular tension from building up, and maintain a much more emotionally responsive state.

Lie down in a quiet, darkened room. Hold your neck muscles slightly stiff, moderately stiff, quite stiff, then as stiff as you can make them. Now move backwards along the same scale, from totally stiff to quite stiff, to moderately stiff, to slightly stiff, to normal - and then one more step in the same direction, towards looseness and relaxation beyond the original base level. Go through this routine three or four times, until you definitely 'get the feel of it' and can relax your neck muscles at will.

Now you are ready to relax other parts of your body.

Relax your right arm, your right leg, your left leg, your left arm, your scalp, your face, your neck, your back and your tummy muscles. This order - around the clock, then top to bottom - is easy to remember. Do not strive for maximum relaxation of each part: you will relax more thoroughly in a given length of time by focusing your attention on each part only long enough to loosen its musculature through a single 'relaxation command', then shifting your attention to the next body part. After three or four 'go-rounds' you will find yourself drifting into a state of highly restful calm which you can easily maintain for some time.

When you first try part-by-part relaxation, twenty-minute rest breaks in a quiet bedroom work best. After a few weeks, however, you will become good enough at relaxing muscle groups that isolation and quiet are no longer necessary. You can sit in a straight chair with both feet flat on the floor, place your hands in your lap, let your head loll forward, and relax muscle groups in rotation just as if you were in bed. Even when children are playing in the vicinity or dinner is cooking on a nearby stove, you will find that you can readily relax. After two or three 'go-rounds', lift one hand up to shoulder height and let it fall back into your lap like a limp dishrag. Do the same with the other hand. Then resume part-by-part relaxation, perhaps for two to three minutes. Such brief 'refresher slouches' will definitely help to keep tension from building up, of ten with startling effects on your disposition, level of contentment, and sexual responsiveness.

A few more weeks of practice will improve your ability to relax to the point where a set position and chair- or bed supported posture are no longer entirely necessary. Whenever you become conscious of muscular tension you will be able to relax it without interrupting your activities.

Reducing your risk for developing coronary heart disease

2008-06-23T00:10:00.000-07:00

In order to increase your chances of success in reducing your risk for developing coronary heart disease, follow these tips:

1. Consult your physician.

2. Monitor your eating and activity behaviors.

3. Familiarize yourself with foods that contain small or large amounts of cholesterol, fats and sodium.

4. Follow the suggested food-group units for a diet of 1600 or 2000 calories, or calculate your own individual energy needs. The less active you are, the fewer calories you can afford to eat without gaining weight.

5. Study the sample menus that follow to assist you in designing your own diet. What is important is that you generally eat as suggested, paying more attention to the types of food you eat and the way the foods are prepared than to exact amounts. Of course, this does not mean you should eat more than suggested if you are trying to lose weight or if you find you are gaining undesired weight. Use your common sense in these matters.

Rebelliousness, Morality, and Psychological Health

2008-04-27T11:28:00.000-07:00

The first and most obvious consideration in the relationship of rebelliousness to morality and psychological health is one which by now has passed from iconoclastic protest to virtual stereotype. Nonetheless, it should not be disregarded. It is simply this: rebellionresistance to acculturation, refusal to "adjust," adamant insistence on the importance of the self and of individuality--is very often the mark of a healthy character. If the rules deprive you of some part of yourself, then it is better to be unruly. The socially disapproved expression of this is delinquency, and most delinquency certainly is just plain confusion or blind and harmful striking out at the wrong enemy; but some delinquency has affirmation behind it, and we should not be too hasty in giving a bad name to what gives us a bad time. The great givers to humanity often have proud refusal in their souls, and they are aroused to wrath at the shoddy, the meretricious, and the unjust, which society seems to produce in appalling volume. Society is tough in its way, and it's no wonder that those who fight it tooth and nail are "tough guys," I think that much of the research and of the social action in relation to delinquency would be wiser if it recognized the potential value of the wayward characters who make its business for it. A person who is neither shy nor rebellious in his youth is not likely to be worth a farthing to himself nor to anyone else in the years of his physical maturity.

A second consideration which is certainly no news to most people, but which tends to get lost to psychologists who use phrases like guilt feelings, hostility, and anxiety, is that the healthy person psychologically is usually virtuous in the simple moral sense of the term. Psychologically healthy people do what they think is right, and what they think is right is that people should not lie to one another or to themselves, that they should not steal, slander, persecute, intrude, do damage willfully, go back on their word, fail a friend, or do any of the things that put them on the side of death as against life. This probably sounds like old-time religion, and in fact I am willing to be straightforwardly theological about this. I think there is an objective character to guilt, and when a person is false to his nature or offends against the nature of others then he is in sin and the place in which he has his existence is well described by the word "hell." I take "sin" here to be descriptive of the state of separation from the most basic sense of selfhood, or what some existentialist philosophers have called "the grounds of being." In whatever terms it is put, the fact is that a person is most alive and is functioning in such a way that he knows who he is and you know who he is and he knows who you are when his thoughts and actions are in accord with his moral judgment. The corollary is that when you do what you think is wrong you get a feeling of being dead, and if you are steeped in such wrongful ways you feel very dead all the time, and other people know that you are dead. There is such a thing as the death of the spirit. Many of the people whom we know as patients in our mental hospitals or as prisoners in our jails are in a condition of spiritual death, and their only hope is that someone can reach out to them, break through the walls of their isolation, recognize them. I think that too much has been made of the word love in this connection, for usually it connotes a feeling on the part of the person who is to give the love. The essence of the act of love as I understand it is the act of attention, and the affect that accompanies it in the person who is paying attention may be love, hate, sadness, or what have you. A real fight is an act of attention, a genuine condemnation is an act of attention, an understanding of final defeat is an act of attention. These as well as their positive counterparts are on the side of life, and the person who experiences them is in communication with other living beings and offers to them the possibility of community. The sort of philosophy of psychotherapy that prescribes blandness, nonjudgmentalness, and essential indifference on the part of the psychotherapist is simply a form of human debasement. Paying attention, caring, and being there yourself is all that counts.

One of the therapists there was clearly an incompetent by all standarts. Everything he did was wrong. After about six months of his residency, however, it became apparent that many of his patients were unaccountably getting better. Among his aberrant behaviors were such gross actions as telephoning a patient's foreman at work and telling him to stop bullying the patient, suggesting an unusual sexual technique to another patient whose wife was apparently frigid, and bluntly suggesting to a third patient that he should give up his job as an automobile repairman and get into the dispensing of food. The climax of the latter case was especially gruesome to the clinic, for the patient opened a doughnut shop of his own and on his final appointmerit appeared with a dozen doughnuts of his own making which he presented as a gift to the therapist, who without any insight at all offered them around to various other therapists and his supervisor of whom had difficulty, swallowing them. Goodness knows I am not suggesting, in recalling the case of this incompetent fellow, that all psychotherapists go forth and do likewise, for he was he and we are we. But I will say that he was alive, even though so obviously misguided; to his patients, the only thing that was of consequence was that he cared about them and that he thought there was something different they could do which would be right.

Parent Education and Mental Hygiene

2008-04-27T11:15:00.000-07:00

One of the most important points about parent education today is the extent to which mental hygiene principles have worked their way into its materials and procedures. Teachers and school administrators who think of parent education in terms of the earlier activities of parent-teacher groups will find that and with the use of authentic data made available from scientific research at the child development institutes, a significant change has come about. An incredible amount of exceedingly valuable information about children's behavior has been brought within the reach of present-day parents, especially mothers, and a glance into the more recent study courses and publications used by parent and child study groups is distinctly reassuring.

There is a tendency in some quarters to view with suspicion the efforts of groups of parents to learn something about mental hygiene as applied to themselves and to their children. Probably not a large number of parents are as yet actually helped; possibly, too, a certain number are harmed--the psychiatrists report a few parents getting just enough of the jargon and the general point of view to find "problem" children where these do not exist. On the whole, however, it is doubtful whether in any other educational field (except the nursery school) mental health principles have penetrated as far and as well as in the modern plans and practices in parent education and education for family life.

Formerly programs for study [by parents' groups] focused attention exclusively upon the child and included for discussion such topics as obedience, punishment, rewards, curiosity, imagination, habit formation, play, etc. More recently, especially with leaders trained in mental hygiene, interest focuses upon the life of the family groups and upon such items as personality development in family relationships, emotional honesty in dealing with children, etc. In attending such study groups parents are able to learn not only important facts about child growth and the family in a changing world, but also more satisfactory self-direction in their daily relationships with children.

So definitely has mental hygiene entered into modern education for family relations that in nearly all the current definitions mental health as an objective is either explicitly stated or unmistakably implied.

Parent education is a voluntary cooperative effort by parents, studying with qualified leaders (1) to understand more about childgrowth and development, family relationships, the conduct of family life, public education, home-school relationships, and the family in community life; and (2) to grow in ability to take a constructive part in family and community relationships with confidence and satisfaction.

Classification of objectives of parent education reflects clearly, especially in the first three of her list, the recent mental health emphasis in education for family relations:

1. To interpret to parents the findings of specialists in regard to various aspects of child and family life.

2. To modify or change the attitudes of parents toward their children and their behavior.

3. To act as a therapeutic device for relieving personal maladjustment.

With the increased knowledge available about human nature and the ways in which it develops and is modified, education in family relations ceases to be the simple matter it was in the past and is sometimes assumed to be today. Many parents who pride themselves on following to the letter the prescribed rules for the physical hygiene of their children. Even among those who realize the importance of early control of behavior problems, there are many who are deeply concerned when their children lie, steal, or have temper tantrums but attach little or no significance to such unhealthy signs as undue self-consciousness, day-dreaming or jealousy. Still less do they realize that these delinquencies and manifestations of abnormal behavior may be due to their own attitude toward their children. Yet many cases of delinquency in children have been traced to the attempt of parents to make the child's life compensate for their own failure to reach certain goals of achievement; to the fact that the child has for years been buffeted between the rigid discipline of one parent and the extreme laxity of the other; to the dominating attitude of one member of the family group, which leaves the child no opportunity for asserting himself as an individual, and to similar forms of conflict between the needs of the child and the conditions prevailing in his family. Parental attitudes are of fundamental importance.

Personality begins very early and persists very late

2008-04-27T11:06:00.000-07:00

Persons concerned with education need to remind themselves constantly that the family has had the child under its influence long before the school and continues to have him throughout school experience. It is in the home, very largely, that the stage is first set for learning patterns of human behavior. Practically unanimous agreement exists among psychiatrists and mental hygienists as to the significance of early family life for mental health.

Possibilities for the school, therefore, are definitely limited at the start. The "education" of the child has been under way for a number of years before he comes to school. What education can do for mental health will depend to a considerable extent on what the family and home have already done; also on what the family continues to do while the youngster is in school. Where the public educational provision includes the nursery school, as it should, the school is able to exert its influence at an earlier stage, of course, but the part of the home remains highly significant even then.

Most authorities believe that the influence of the home and family in making or breaking wholesome personality begins very early and persists very late. Babies in the first year, who showed all the marked differences that characterize later personalities--some slow in their reactions, phlegmatic, dull; others quick, amiable, responding with distinct pleasure to the different stimuli, or with clear evidence of discomfort. At the other end of the scale, there have been some noteworthy cases of adjustment of very difficult boys and girls in foster home surroundings long after older adolescence and beyond.

Clinical case records indicate that the home is still the major force in forming the personality of children. On the ground that home life, quite apart from its physical aspects, may react upon the health of the child by its confusion and discord, or by its harmony and peace, and "this in turn is largely the result of the parents' own emotional adjustment.

It is in the home that the child's needs for affection, security, and opportunities for growth or development, which play so important a part in shaping his personality, are met or thwarted. Even the most affectionate and intelligent parents may not always fully understand the child's needs for security and growth. Security is founded upon the emotional maturity of parents, upon justice, truthfulness, regularity, order and serenity in the home. Opportunities for development can be given the child only by parents who want to see him grow and give him every chance to utilize and enlarge his own powers.

Fully Functioning Persons - Educational Achievement

2008-04-16T10:58:00.000-07:00

Schools are anxious that the children they educate grow into fully functioning persons. This has long been an avowed and widely approved purpose of education. We say that education in a democracy should help individuals fully develop their talents. Recently there have been pressures to limit this to intellectual talents. There has been much talk about limiting the school's concern to the full development of the intellect only.

Even with this limited definition of the goals of education, the abilities involved in creative thinking cannot be ignored. There has been increasing recognition of the fact that traditional measures of intelligence attempt to assess only a few of man's thinking abilities. Certainly we cannot say that one is fully functioning mentally, if the abilities involved in creative thinking remain undeveloped or are paralyzed. These are the abilities involved in becoming aware of problems, thinking up possible solutions, and testing them. If their functioning is impaired, one's capacity for coping with life's problems is indeed marginal.

Educational Achievement

Almost no one disputes the legitimacy of the school's concern about educational achievement. Teachers and guidance workers are asked to help under-achievers to make better use of their intellectual resources and to help over-achievers become better "rounded" personalities. But, how do you tell who is an under- or over-achiever? In my opinion, recent findings concerning the role of the creative thinking abilities in educational achievement call for a revision of these long-used concepts.

We are finding that the creative thinking abilities contribute importantly to the acquisition of information and various educational skills. Of course, we have long known that it is natural for man to learn creatively, but we have always thought that it was more economical to teach by authority. Recent experiments have shown that apparently many things can be learned creatively more economically than they can by authority, and that some people strongly prefer to learn creatively.

Traditional tests of intelligence are heavily loaded with tasks requiring cognition, memory, and convergent thinking. Such tests have worked rather well in predicting school achievement. When children are taught by authority these are the abilities required. Recent and ongoing studies, however, show that even traditional subject matter and educational skills can be taught in such a way that the creative thinking abilities are important for their acquisition.

Most of these findings are illustrated dramatically in a study conducted during three years in the University of Minnesota Laboratory Elementary School. We differentiated the highly creative children (as identified by our tests of creative thinking) from the highly intelligent. The highly creative group ranked in the upper 20 per cent on creative thinking but not on intelligence. The highly intelligent group ranked in the upper 20 per cent on intelligence but not on creativity. Those who were in the upper 20 per cent on both measures were eliminated, but the overlap was small. In fact, if we were to identify children as gifted on the basis of intelligence tests, we would eliminate from consideration approximately 70 per cent of the most creative. This percentage seems to hold fairly well, no matter what measure of intelligence we use and no matter what educational level we study, from kindergarten through graduate school.
Although there is an average difference of over 25 IQ points between these two groups, there are no statistically significant differences in any of the achievement measures used either year.

It is of special interest that the children with high IQ's were rated by their teachers as more desirable, better known or understood, more ambitious, and more hardworking or studious. In other words, the highly creative child appears to learn as much as the highly intelligent one, at least in some schools, without appearing to work as hard. My guess is that these highly creative children are learning and thinking when they appear to be "playing around." Their tendency is to learn creatively more effectively than by authority. They may engage in manipulative and/or exploratory activities, many of which are discouraged or even forbidden. They enjoy learning and thinking, and this looks like play rather than work.

Mental Health - what does all this have to do with creativity

2008-04-16T10:54:00.000-07:00

There are very legitimate reasons why educators should be concerned about assessing and guiding the growth of the creative thinking abilities. I would like to discuss a few of these.

Schools are legitimately concerned about the mental health of children, adolescents, college students, and adults. They would like to be able to help their students avoid mental breakdowns and achieve healthy personality growth. These are legitimate concerns of education. But what does all this have to do with creativity?

Actually, it has a great deal to do with creativity. There is little question but that the stifling of creativity cuts at the very roots of satisfaction in living and ultimately creates overwhelming tension and breakdown. There is also little doubt that one's creativity is his most valuable resource in coping with life's daily stresses.

In one study, a battery of tests of creative thinking was administered to a group of schizophrenics who appeared to be on the road to recovery. Many of them were being considered for vocational rehabilitation by the State Department of Welfare. These individuals manifested an astonishingly impoverished imagination, inflexibility, lack of originality, and inability to summon any kind of response to new problems. Their answers gave no evidence of the rich fantasy and wild imagination popularly attributed to schizophrenics. There was only an impoverished, stifled, frozen creativity. They appeared to be paralyzed in their thinking, and most of their responses were the most banal imaginable.

Although it will be difficult to prove, I suspect that schizophrenics and others who "breakdown" under stress constitute one of the most unimaginative, noncreative groups to be found. I also suspect that it was their lack of creativity rather than its presence which brought about their breakdowns. Certainly the schizophrenics tested lacked this important resource for coping with life's stresses. Creativity is a necessary resource for their struggle back to mental health.

History of Biochemistry and Its Medical Applications

2008-04-10T05:06:00.000-07:00

In contrast with the cosmic earthquake that the study of physics underwent in the years before 1930, change moved tranquilly in the fields of biology and chemistry. Indeed in biology most of the work of the 1920's continued along lines already established in the nineteenth century.
In genetics, the key event was the rediscovery in 1900 of the work of the Bohemian monk Gregor Mendel, who, contemporaneously with the later researches of Darwin in the 1860's, conducted the epoch-making experiments with the crossbreeding of peas that were to form the basis of the whole modern study of inheritance. The most significant outcome of Mendel's discoveries was the identification of indivisible and unalterable units called genes, through whose infinitely varied combination the process of heredity proceeded. This genetic theory seriously undermined Darwin's principle of natural selection. Some geneticists were ready to discard natural selection entirely, some preferred to retain it in modified form, but there was general agreement that acquired characteristics were not inherited. Mendel's original conclusions were reinforced when twentieth-century geneticists began to extend his work to systematic experiments with the fast-reproducing fruit fly, and to apply the calculus of probabilities to their findings. As a result, by the 1920's the new science of genetics had reached a high level of technical exactitude. Moreover, in treating the gene as a basic and indivisible unit, it seemed to confirm Planck's contention that nature proceeded by jumps and in definite quantities rather than through the continuous and imperceptible processes of change that had been postulated by nineteenth-century philosophers of nature.

Another link between biology and the theory of physics was provided by the new science of biophysics, which, along with the related field of biochemistry, accounted for a large part of the progress made in the study of the human body. Perhaps the most dramatic experiments were those of Sir Frederick Gowland Hopkins in 1912, which became the starting point for the systematic investigation of nutrition and the identification of the basic vitamins. For a long time the chemistry of all the vitamins except D remained a mystery. But in 1929, with the chemical breakdown of Vitamin A, there began a period of rapid progress in the analysis and synthetic production of these substances that continued down to the outbreak of the Second World War.

Closely related to this study was the development of the new science of glands and internal secretions known as endocrinology. In the 1920's, the function of hormones began to be understood, and work on the pituitary and thyroid glands proceeded steadily. Discoveries such as these obviously had relevance for medicine. Indeed, a salient characteristic of the decade was that now, for the first time in history, new research in physiology and biochemistry was quickly applied in clinical practice. An astounding advance resulted. In the mid-1920's there began a period of breathtaking innovation that brought more progress in medicine in a single generation than the profession had known in all previous human history.

The discovery of antitoxins, begun in the 1890's, moved on steadily, as did the analysis of the corresponding viruses. By this method, medical research succeeded in eliminating certain diseases almost completely: as smallpox had been routed in the nineteenth century, so the conquest of diphtheria, yellow fever, and tetanus followed in the interwar period. But most diseases resisted this sort of immunization. Although the ravages of tuberculosis, for instance, were enormously reduced, no satisfactory antitoxin was discovered to combat it. In dealing with these stubborn diseases, the development of antibiotics marked the crucial turning point. Beginning with Sir Alexander Fleming's almost accidental discovery of penicillin in 1928, one new drug followed another in a rapid sequence of successful experiments leading to commercial production.

In all these cases, however, there had been a time lag between laboratory research and its clinical application. Not until the Great Depression had focused attention on problems of hunger and want were the new discoveries in the field of nutrition and vitamins fully exploited. Through the necessities of treating vast masses of sick and wounded soldiers in the Second World War, penicillin, the sulfa drugs, and DDT came into their own. These examples suggest the close relationship between social needs and the development of scientific and medical knowledge in our time.

Similarly, in the organization of research, economic and social factors began to exert an increasingly important influence. In the past, the isolated scientist or physician could produce useful and even epoch-making results with the simple equipment of his own home laboratory. By the 1920's, only a wellfurnished laboratory or research institute could contribute to the growth of scientific knowledge. With this change, the problem of the organization and financing of research took on a new urgency. In such respects, a large and wealthy society like that of the United States enjoyed obvious advantages; a socialized country, such as the Soviet Union, held potential assets for the future. Thus even as early as the 1920's, men of science in the three countries that together had accounted for most of the scientific progress of the nineteenth century--Britain, Germany, and France--were beginning to wonder whether the economy and the way of life that had yielded such marvelous results a generation or two earlier, would prove capable of dealing with the unfamiliar and pressing demands of twentieth-century mass society.

School Companions and Friendships

2008-03-25T04:19:00.000-07:00

The child's social development during the second half dozen years of his life is marked by important differences in the stability, intimacy, and strength of his friendships, as well as in the character of his play and other interests. Companionship is not so largely determined by propinquity as in the earlier years when the child's range of getting about was narrowly limited to his immediate small neighborhood. Children of similar chronological and mental ages, of similar developmental ages, having a community of interests and common activities, tend to be companions. With the increased experience, mental ability, and, general maturity resulting from age, lines of interest tend to become better defined. All of these conditions mean that his personality is developing and that his patterns of behavior are becoming more definite. Accordingly, the selection of companions becomes less childish and more mature as first puberty, then adolescence, maturity, and middle age approach. In later childhood, however, proximity may still be a factor, because it does provide opportunities for acquaintanceship and more or less intimate understanding. Thus, studies of gangs show that living in the same neighborhood (common environment) and being in the same grade at school are basic conditions underlying the forming of gangs. Other studies seem to show that children prefer as friends those who are quiet, self-controlled, "smart," good in their lessons, strong, and "not silly." At least, these are some of the qualities which they say they prefer.

Factors in the Formation of Groups

During the first four or five years of the child's life the factors that induce the child's contacts with others and the formation of groups seem to be helplessness and need, companionship in play, and common interests in toys or other objects. These earliest groups usually have two members, while later on larger groups are formed. During the years from six to twelve, common interests and activities, similarity in chronological, mental, and developmental ages, are factors underlying the formation of groups. Such groups are more stable and lasting than the rapidly shifting groupings of the pre-school years. Various environmental factors also may be influential, such as living in a certain neighborhood close to some other groups whose activities are well known. At about the eighth to tenth years boys and girls frequently form clubs or gangs of one sort or another. Many of these are short-lived. A name, membership dues, a time and place of meeting, a list of members, and no very definite specified purposes, are common characteristics of many clubs at this age. We have studied many of them among public-school children, ages eight to fourteen. Girls' clubs are common at these ages and are very transitory and short-lived. Boys' clubs seem to be more definite in their objectives and more lasting. It may be that boys feel a keener need for such social groupings. The intimacy, loyalty, and solidarity in some of these groups are truly remarkable, as various detailed accounts of them have shown.

Community feeling, however, seems to be slow in developing. Tattling is found in the earlier school years. Identification of one's self with the group in such a vital way as to regard group success as highly satisfying is also slow in developing. Doing things for the good of the group, as in teamwork, develops gradually. Even several years after the child begins school it may have little motivating power in child behavior. Anyone organizing a baseball team of ten-year-old boys is not likely to have any scarcity of candidates for catching, pitching, first base, and short stop, but he will lack fielders.

Social Concepts of Children Six to Twelve Years of Age

2008-03-25T04:15:00.000-07:00

The child's ability to identify facial expressions representing certain emotional states improves with age, as has been noted already. Actual objects may be identified as to use by the age of from two and a half to three and a half years. Simple words such as ball, hat, and stove can be defined by the age of five. The understanding of abstract words is a later development. Only at age twelve can the average child define two of four words such as constant, courage, charity, and defend.

Such concepts as playing fairly, being kind, helping others, and the like have their beginnings in the early pre-school years. Normally they are well developed before the advent of puberty. We must not conclude, however, that having an accurate knowledge of certain social concepts insures their operation in the daily lives of children. Often a notable discrepancy is found between knowledge and overt behavior.

Social Contacts and Adjustment of Later Childhood

Entrance upon school, provides stimuli to many social reactions. The first reactions of children to school are not necessarily complete indications of their social development. Thus, a one-year study of forty seven-year-old boys just entering school showed that thirteen were confident, sixteen indifferent, and eleven were shy. The early over-submissiveness to the teacher's authority tended to disappear during the first six weeks. Considerable disobedience was found, even though the authority of the teacher was clearly recognized and not disputed. Often difficulties arose because the boys did not know exactly what was expected of them, or because they misunderstood the meaning of a given command.

Various schemes for classifying children's social contacts have been devised. One type of classification divides social contacts into, five principal classes. (1) Protective contacts are those in which the shy and submissive or uncertain child attaches himself to the selfassertive. If anything happens to break up the association, the timid child seeks someone else to whom he may attach himself. (2) Social contacts may reveal a certain kind of devotion in which a beloved or popular child is the center of a group, not because of any marked leadership, but because of his gentle, friendly, attractive ways which make so many children like him. (3) Social contacts also may be those of the "leader." (4) Sometimes they show the "despot." (5) A fifth type is that of the child who is socially unsuccessful. Social contacts, characterized as a despotism, tend to diminish by the time children finish kindergarten or enter first grade. Force as a controlling element becomes socially unacceptable and many children give it up. The socially unsuccessful child often is the one who has some physical defect or who has been badly neglected, having torn, dirty, or ill-fitting clothing. Often such children become trouble-makers in school. By the beginning of the school years the range of the child's social reactions is vast indeed. By the age of twelve years, nearly every type of social response has appeared, although the range and complexity of the situations evoking them are far from the scope shown in later adolescence or adulthood.

The Relationship of Language to Social Development

2008-03-25T04:10:00.000-07:00

The child's social development is profoundly influenced by his language development. Through language he not only expresses his thoughts and feelings to others, but he understands something of their thoughts, feelings, and desires. Language thus enables the child to understand others and gives him a ready means of influencing their behavior. It helps him in the development of social concepts of more complex and abstract nature. Anyone can observe the great amount of talk or conversation in a group of pre-school children at play, even though much of it may relate to the speaker and his exploits. At the earliest pre-school age, language is not the child's usual means of initiating contacts with others, nor is it the most common means at the age of five or six. Some object or activity of common interest is much more likely to be used to bring about social contacts. Only in later years of childhood and more particularly in adolescence does language hold a relatively important place in making social contacts. It is, however, a very important means of maintaining them, even during the pre-school years. On countless occasions parents and teachers have observed children trying to hold attention by asking questions, by telling about events, or by other conversational efforts.

Social Attitudes of Pre-School Children

Socially indifferent children are rare, however, at the age of four or five years. A few may be observed who seem almost lacking in social perception, and whose behavior is very little influenced by the activities of others. Mental ability has some effect on this characteristic. We have seen feebleminded children ten to twelve years of age who seemed to give no sign of being aware of the presence or activity of other persons. At the very bottom of the scale of intelligence there are always children whose social behavior is practically zero.

Social dependence or social independence may be quite marked in pre-school children, although most youngsters possess this trait in some degree intermediate between the extremes. The social dependence or independence of a child is probably the result of his training, and experience more than of any other factors. We have observed many children in kindergartens of city public schools and in free kindergartens maintained by charitable and philanthropic organizations and have seen timid children lose much of their timidity in groups and develop leadership in group activities. The child on whom satisfactions are bestowed because of his submissiveness, who is threatened into frequent yielding, or who is given no opportunities for spontaneous and voluntary action, is likely to be dependent. When these circumstances operate in the opposite manner, independence results. Since most children receive these stimulations in an intermediate degree, they are neither strikingly dependent nor independent, or else they show these characteristics differently in various particular situations.

The Formation of Friendships During the First Five Years

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As nearly as can be told from the available evidence, children are naturally neither friendly nor unfriendly. Friendliness is the outcome of environmental forces, as is also unfriendliness. The meaning of anything to a child is the result of his experiences with it, determined by what it does to or for him, and by what he can do to it. In this respect, people are merely objects in the child's environment. What they mean to him follows directly from his experiences with them. Under one kind of environment he will become friendly, under another, unfriendly. The majority of children spend their early years in homes in which people care for their needs. They are fed, clothed, and comforted, their pain is relieved, and many other things that make for their well-being, happiness, and contentment are performed. Accordingly, we would expect the majority of young children to be friendly rather than unfriendly. A study of two-year-olds shows this to be the case. They were brought it into a small playroom two at a time, each child being paired with each other child of the group, and their responses were recorded. The results showed that friendliness was far more common than unfriendliness -- in terms of score, 89.5 and 20.5, respectively. If children's early experiences with people were marked by abuse, pain, and the like, unfriendliness undoubtedly would be developed instead.

Obviously, the factors which further the formation of friendships in childhood are those which satisfy, please, comfort, or help the child in some way. Factors which hinder or prevent the formation of friendships are those which irritate, anger, pain, or otherwise displease the child. A very long list of specific conditions tending to develop friendliness could be given, and another list conducive to unfriendliness could be set forth. The close contacts incident to living in the same family may lead to bitter hatred or warm affection. If children interfere with each other in any way continually for a considerable period of time, they are likely to become unfriendly and jealous of each other. With young children proximity is a very common factor in friendship and companionship. The child cannot go far from home. Accordingly, he makes friends with children near at hand. Mere propinquity may not be enough to develop companionship, but if the children are near the same age, they are likely to have enough community of interest to become friends. At all ages, it would seem, community of interest is a very important factor conditioning friendship. Children who like to do the same things are likely to be friends, if they have the opportunity to be together so as to discuss their common likes, provided, of course, that other conditions giving rise to friction are absent.

When pre-school children are allowed to form groups freely, sex plays less of a part than with older children who tend to group themselves according to sex. Children, if left to themselves, form groups largely upon the basis of acceptable behavior and ability to enter into the group activities. When race, color, poverty, or riches are factors in the formation of groups of pre-school children, we may be sure that home pressure or the influence of other adults is likely to be responsible for the social discrimination displayed. Such factors otherwise have little meaning or value for the child of four or five years.

A youngster of three or four years of age often forms a strong attachment for some other child and seeks to be with him as much as possible. Such little chums may be seen going home together from nursery school or kindergarten. They are much together at school and out of school, if opportunity can be found. Such friendships may last for several months, although more commonly they last only a few days or weeks. We have observed many of these among kindergarten children four or five years old. Usually two children are chums. Less frequently three may be chums for a while. In one case we observed three boys who became great chums in kindergarten at the age of five. Their friendship continued through the first and second grades, until one of the boys moved to another city. We also have seen a few cases of three girls being chums in kindergarten for a short time. We have seen the close friendship between two boys or two girls lasting for some time in several cases through kindergarten and several years thereafter. We would expect such cases to be found, because some of the children, having the traits which make for friendship, are likely to develop in ways which will continue their being chums. On the whole, however, pre-school children do not maintain such chumships for a long period of time, as nurseryschool and kindergarten teachers often have observed.

Interpretation of Facial Expressions

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The early recognition of certain facial and vocal expressions, such as smiling or angry looks and kind or scolding tones, found among infants from the fifth month on, is an important step in understanding the behavior of human beings. We find children showing greater skill along this line as they get older.

More than 70 per cent of the kindergarten children recognized laughter, whereas less than 50 per cent of them recognized fear, anger, or pain. By the age of seven years more than half recognized anger, by eleven, more than half recognized surprise. The course of development may be seen also in the fact that the average number of photographs correctly named was 1.5 at the age of three years, and, nearly four and one-half at the age of eleven. Of course, this test is somewhat artificial because actual facial expressions are mobile or changing rather than fixed or static. Accordingly, we must not conclude that children cannot recognize fear, anger, and pain in a familiar person's behavior until the ages shown in the foregoing study. The addition of vocal expressions is a distinct supplementary aid to the child in identifying the emotional states of a familiar person.

Laughter and Crying of the Pre-School Child

The laughter and crying of children under five years of age have significance in studying their social development. Apparently, 8 more of the two-year-old's laughter takes place when he is unaware of another child's presence and is playing by himself. At a later age, however, the most laughter seems to occur when children are in social contact with other children. Boys seem to laugh most and cry most when with boys. In the latter case, the teasing or amusing nature of the social contact may be a contributing factor.

Responses of Infants to Other Infants

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AGE IN MONTHS

Observes other child 4 to 5
Smiles at other child 4 to 5
Cries if other child receives attention 8 to 9
Offers toy to other child 8 to 9
"Lalls" to other child 8 to 9
Imitates movements of another child 9 to 10
Opposes toy being taken away 9 to 10
Organized play activity 10 to 11
Strives for attention by means of "lalling" 10 to 11
Ill-humor if another child moves away 10 to 11
Setting aside toy and turning toward another child 11 to 12

If they noticed them, it was with no interest or emotion. Even the others' movements were of no interest. Such infants played, moved about, cooed, and smiled without any interest or regard for the presence of other infants who were near. Such behavior may be regarded as exhibiting social blindness. The infants who were not socially "blind," that is, those who paid attention to the behavior of others, exhibited varying degrees of independence in social relations. At one extreme some showed a high degree of social dependence. They were much influenced by the presence and activities of other infants. At the other extreme were infants who, equally aware of the presence and activities of others, still showed a great deal of social independence. The socially dependent infant's behavior seemed to be conditioned largely by that of another. He usually copied it or he may merely have watched it. Or if he was not so timid or if his responses were not so readily inhibited, he might go through his little repertory of stunts trying to arouse or please the other. The socially independent infant was aware of the presence of the other and responded to him and his behavior, but did not seem to be dependent upon him. He played with him but was clearly the leader, not being afraid of or intimidated by the other. Children from six months to eighteen months of age possess these characteristics in greater or less degree. Bühler believed that they show them without reference to previous contacts with others, to their being only children, to the home conditions, or to nationality. We may have here early evidence of dominance and submission.

Any reference to infants as socially dependent or socially independent, however, must not be applied too rigidly. We must not think of all children who are not "socially blind" as belonging at either of these other two extremes. Some do belong to the first class and some to the second. Many, however, seem to fall into groupings between the extremes, especially as they pass from infancy to the pre-school and school years. As characterizations of general social attitudes of individuals, Bühler's classes are suggestive and valuable because they throw into clear relief important considerations in the social development of the child. No scheme of classifying children into two or three "types" is satisfactory, however, as is seen in connection with the discussion of child personalities.

Responses of Infants to Adults

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AGE IN MONTHS

Returns glance of adult with smiling 1 to 2
Is quieted by touching 1 to 2
Cries when adult who was attending him leaves 2 to 3
Smiles back at adult 2 to 3
Disturbed when approached 2 to 3
Returns approaching glance with "lalling" 3 to 4
Displeasure when loses glance of adult 3 to 4
Quieted by caressing 4 to 5
Disturbed by the sight of people 4 to 5
Striving for attention by "lalling" 7 to 8
Stretches out hands toward adults 7 to 8
Cries when adult stops talking 7 to 8
Strives for attention by movements 8 to 9
Pulls on the clothes of adult 9 to 10
Offers adult an object 9 to 10
Imitates movements of adult with a plaything 9 to 10
Organized play activity 10 to 11

Other Social Responses During Infancy

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The infant makes other responses which are indicative of very simple social behavior. Upon the approach of a familiar person the fourmonths-old infant very often raises his arms and stiffens his body slightly in anticipation of being lifted. He also shows some delight upon the approach of the familiar person. Of course, some infants make these responses earlier, but by the age of four months the majority are likely to do so. Infants of this age have some remembrance of events and often seem to look for a face that has disappeared, gazing for some little time toward the place they last saw it. Sometimes infants of four months chuckle or laugh when familiar persons resort to certain playful activities. A month or two later many infants show greater development of social behavior, and respond to other social stimulations. With the development of motor skill, adaptive behavior, attention, discrimination, and language, the infant increases not only the scope of his responses to the presence and activities of other persons, but also the number and types of social situations to which he can respond.

How Infants Respond to the Presence of Other Infants

Babies at four and five months of age frequently seem to be unaware of each other's presence, even though near each other. By the age of six months, however, they actively look around and attract each other's attention. Thus, the infant of six or seven months may touch another one who is near, or he may coo, or hinder the other's activities in some way. We have observed a few babies of five to eight months, placing them two at a time near each other on a bed or couch. The older ones showed distinct evidence of being aware of the presence of each other. Thus, an eight-months-old infant when placed near one of five months reached out and stroked the younger one and then took hold of her arm, cooing, gurgling, and smiling. The younger infant had given no previous sign of seeing the other and was cooing contentedly. Upon being touched, she stopped cooing, turned her head, and looked at the other baby. She did not, however, put out her hand and try to touch the older one. Responses to the presence and activities of adults and older children are observable at an earlier age than are responses to other infants of about the same age, undoubtedly because the former provide more adequate stimuli.

Earliest Social Stimulations and Responses

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Since normal infants become able to perceive objects in the environment at about the same time and since all have the fundamental experiences basic to social behavior, a typical sequence of the development of social responses can be described. Some conditioned social responses have been reported during the first month of life. Infants of this age sometimes stop crying when someone speaks, when someone enters the room, or even at the sight of a human face. Although these reactions seem to be responses to the presence of people, caution is necessary in interpreting them in individual instances. Very young infants often stop crying when any stimulus arouses their attention. If something touches him, if any noise or movement occurs, or if he merely relieves his own discomfort by turning to a less cramped position, the same response may be made. Some responses of infants to persons, then, are really social, while in other cases the infant reacts to the person only as a mechanical agent.

By the second month, social responses to persons are more clearly distinguishable. At the age of two full months, many infants will turn the head and eyes toward the sound of a human voice. These responses, of course, do not develop because of any inner social tendency, but are evidences of learned behavior.

The Recognition of Facial and Vocal Expressions

During the first two months of postnatal life the infant's smile at the presence of another human being is not dependent upon the latter's facial expression or tone of voice. The young human is not yet able to differentiate such relatively simple things as smiling and angry facial expressions, or kind and scolding words and tone. If a person bends over the crib of a two-monthsold infant and has a "smiling countenance," the infant may smile in return. But if the adult has an angry expression, the infant also may smile. The, infant even at the age of three or four months probably is unable to differentiate kind and angry looks, tones or gestures, although exceptions occur. From the close of the fourth month, however, infants show distinct signs of differentiating expressions and by the fifth month may cry at the scolding voice and threatening gesture. The child by this time has developed his powers of attention and observation to such an extent that he can watch the face and note any changes in its details. He also is capable of discriminating differences in tone of voice. Perhaps even more important is the fact that he has now learned that kindly expressions typically accompany satisfying ministrations to his needs, while harsh ones imply neglect or even painful punishment. Before the end of the first year he has made a great deal of progress in understanding vocal and facial expressions.

The Learning of Social Behavior

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The newborn child is neither a social nor an unsocial being. He is a highly complex organism equipped to respond to certain sorts of stimulations, as has been seen throughout the discussion of infant behavior. He also possesses a high degree of modifiability and significant potentialities for future growth and development. How soon after birth, then, does the child first show evidences of social behavior, of responding to the presence or activities of other persons? What are some of his earliest social responses? What particular kinds of stimulations evoke them? How do infants respond to the presence of other infants? What development of social behavior occurs during infancy and the pre-school years? During the years from six to twelve? What are the causes and significance of conflicts between children? What factors influence the child's social development? What is the significance of social approval, self-assertion, language development, gregariousness and other elements upon the development of social behavior during the first dozen years of life? These are some of the important problems whose answers now concern us.

Although the infant is non-social at birth, he cannot remain so for a very long time. He lives in a society and is constantly dependent upon other people for his welfare and comforts. Accordingly, the infant soon learns to make responses to other persons. As he grows from infancy to childhood, these social adjustments become even more evident.

The Learning of Social Behavior

The earliest social behavior of infants arises from the care and handling given to them by, adults. When a baby is fed, dried, kept warm or petted, he responds by behavior that may be taken to indicate satisfaction. If he is restless or crying, this agitated behavior ceases. With a little greater maturity, positive evidences of pleasure are apparent in the forms of smiling, gurgling, cooing, laughing, and reaching with the arms. Fundamentally, all social behavior is based on these responses to the satisfaction of the infant's bodily needs.

During the early months of life, learning takes place by which these reactions come to be made to persons, rather than only to the actual bodily stimulations. This learning is an example of the operation of the conditioned reaction. Since the mother or nurse or some other adult is always present when these ameliorative satisfactions are administered, the sight, sound, or other symbol of these persons becomes capable of evoking the response. The beginning of social behavior in infants is dependent upon the development of their abilities of perception and discrimination and upon the occurrence of experiences through which they may learn.

Children's Recreational and Social Interests

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The play activities of children give valuable clues to the child's nature and needs. In infancy his interests are centered upon manipulating the simple toys and other objects in his hands -- squeezing, pushing, pulling, or striking them upon the table, crib, or high-chair, or attempting to put them in his mouth. The diffuse and random nature of such activities has been reported frequently. By the age of three or four years we find more varied play activities. Playing with toys in the sand pile, building houses with blocks, riding kiddie cars, playing with toy automobiles and wagons, playing house, playing train, playing with dolls, riding tricycle, etc., may be observed. At the age of five years play interests still center about games and activities which are largely individual and solitary, and do not require the participation or cooperation of other children. Jumping, climbing, running, sliding, digging, throwing, lifting, and rolling are whole-body activities performed without any motive of doing one's best or of doing better than someone else. The presence of another child may be resented, and if he has some plaything, a struggle for its possession may ensue.

Children at this age may play in small groups, but often they really are not playing in groups, they are merely near each other while playing. The directed play activities of the kindergarten often involve ring games, rhythmic movement, and singing games. The five-year-old is likely to enjoy construction work. If he is given some direction and if tools are available, he will try to make crude objects, using saw and hammer. Play interests at this age are characterized by the child's engaging in the activity from sheer enjoyment of it, and not to acquire any skill. With further increases of age, the nature of his play interests again changes. By the age of ten or eleven marked differences are seen. The free individual play activities without rules and competition have been displaced by games with rules and with some object or goal. Such games are likely to be largely competitive, with much rivalry in the case of boys. Interest centers on skill and excellence. The games run a definite course, that is, they have a beginning and come to an end. Little cooperation is found. In fact the adult who gets together a group of ten-year-old boys for a baseball or football team is likely to have a surplus of would-be pitchers, firstbasemen, quarterbacks, and centers, but a dearth of fielders and players who do not carry the ball. Emphasis upon speed, strength, and accuracy is characteristic of the play activities of the boy at ten. He is much interested in excelling the others in running, throwing, and the like. Using tools to make things, engaging in bicycle-riding, climbing trees, skating, swimming, camping out, and playing with various kinds of mechanical devices are also much enjoyed. Among girls of ten years, doll play has begun to decline and in the next three years will almost disappear. Playing with paper dolls, making clothes for dolls, participating in various kinds of table games, bicycle-riding, playing on the horizontal bar, house-keeping activities, puzzles, dancing, and dramatic games are common among girls of ten or eleven years.

A conspicuous change in recreational activities is suggested by the percentages of children, at each age up to fifteen, attending the movies, climbing trees, porches, and fences, riding bicycles, and playing marbles. At twelve years of age, two-thirds of the town boys and three-fifths of the town girls attend motion pictures. Nearly half of the eight- and nine-year-old town children also attend. These proportions are in distinct contrast with the smaller percentages who engage in climbing, play marbles, skip rope, etc.

The environment facilitates participation in some play activities and limits it in others, as we would expect. In all of these play problems is found the customary wide range of individual differences. Many play activities indicate the development of social interests. Girls' interests include social dancing, picnics, parties, and "dates," while boys' interests are in football, baseball, and basketball, all of which involve cooperation or teamwork.

Relation of Interest to Aptitude and Maturity

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Investigation seems to lend support to the common-sense view that the child having ability in a given thing is likely to have more interest in it than he would in something in which he has little ability, and more interest in it than another child having less ability in it, other things in both cases being equal. Of course, if other things are not equal, they may overcome or outweigh the interest-producing effect of his ability. If, for example, a child does have considerable ability to do something but his experiences with it are loaded negatively with strong emotion, he may have little genuine interest in it. If an incompetent or disagreeable teacher introduces the child to some subject in school, he may have a resultant dislike for it, although he really may have considerable ability in it. Aside from the effect of such irrelevant, unfavorable elements, it seems reasonably well established that interest may be taken as some evidence of ability, provided the child actually has had some vital experience of the thing in question. We do not regard as evidence of interest a mere idle wish to do something, or a desire to do it which is the result of social pressure. The sense in which we use the term requires that it be genuine interest, as defined in the first part of this section -- active, objective, and personal.

Changes in interests are partly dependent upon maturation, as may be seen in the case of children's play interests. The ten-year-old's play activities are different from those of the four-year-old, not necessarily because he has had so much experience with them that he is surfeited by them, but rather because they were suited to a degree of development which he has long since passed. The reading interests of children also vary with age because of differences in, maturity, in intelligence, in outlook, and in experience. Undoubtedly, maturation is a factor in moulding children's interests along many other lines.

Importance of Interests During Childhood

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The value of many wholesome interests in the life of the child can hardly be overestimated. They bring him into vital contact with many activities. They provide a wealth of experiences because a child interested in an activity tends to engage in it. Sampling many lines of activity not only serves an exploratory function but also tends toward breadth of personality because of the wide range of experiences. Under these circumstances little danger exists of developing a narrow, one-sided personality. Out of a wealth of experiences may come a desirable breadth of appreciations. A wealth of interests also is important because it facilitates substitution in case of thwarting and helps the child avoid some conflicts which otherwise might arise. If he has many interests, he can, when blocked in respect to one of them, turn the more readily and with less strain from one interesting activity to some other one. Thus, many-sided interests have mental hygiene value not only during childhood, but also during adolescence and adult life.

As the years of childhood pass and the child comes into adolescence, we normally find some interests more permanent and of greater strength. Efficiency is dependent upon a few intense abiding interests which lead to centering attention and effort along some particular lines. Greatest achievement seems to be dependent upon the individual's having a strong abiding interest in the work he is doing. We need not, however, expect the child to have such a narrowed, intense, and relatively permanent interest along some line. He may show a very intense interest in some one thing for a short time and then turn with equal intensity to something else, or he may show much interest in several things simultaneously. During childhood cultivating and developing a wealth of wholesome interests should be an objective of child guidance and control by both parent and teacher.

What extent do interests motivate the child's behavior?

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What is meant by interest? To what extent do interests motivate the child's behavior? Of what importance are they in his development? How closely are interests related to aptitude and maturity? What fundamental directions do the child's interests take? What are his recreational, social, intellectual, aesthetic, and vocational interests during the first dozen years of his life? What is the significance of his interests in motion pictures? What are the practical guides and effective laws and principles for developing wholesome interests? These are important questions for all who would understand the forces motivating children's behavior.

The Meaning of Interest. Interest has two diverse meanings in psychological usage. It means a condition or cause of attention; it also refers to the feeling of pleasure resulting from giving attention to something or from experiencing something. Webster defines interest as "excitement of feeling accompanying special attention to some object; concern; as, an interest in Botany." Thus, this term has been used to refer both to the cause and to the result of giving attention or of experiencing. Using the word in the first sense, we say the boy gets out his blocks because he is interested in building a house, or the little girl makes a doll dress because she is interested in having lots of clothes for her doll. Using it in the second sense, that of a resultant feeling of pleasure, we say the child is interested in playing at building a house or making doll dresses. These two meanings, while diverse, are closely related in the experiences and behavior of the child. Interest which is a result of the feeling of pleasure attendant upon some event or experience tends to condition the child in such a way that he later on does attend to that experience, object, or event because of the pleasure which resulted from previously experiencing or attending to it. This is merely saying that the result of a previous response may be, and frequently is, a present cause of repeating that response. Similarly, a present result may be a future cause. At any rate, interests are motives and often have very strong activating influence on the child's behavior.

Genuine interest is the accompaniment of the identification, through action, of the self with some object or idea, because of the necessity of that object or idea for the maintenance of a self-initiated activity. Effort, in the sense in which it may be opposed to interest, implies a separation between the self and the fact to be mastered or task to be performed, and sets up an habitual division of activities. Externally, we have mechanical habits with no mental end or value. Internally, we have random energy or mind-wandering, a sequence of ideas with no end at all, because they are not brought to a focus in action. Interest, in the sense in which it is opposed to effort, means simply an excitation of the sense organ to give pleasure, resulting in strain on one side and listlessness on the other.

Genuine interest is active, projective, or propulsive, is objective in that it does not end simply in itself, as some feelings may, but involves some object of regard, and is personal, signifying direct concern. The emotional side of interest is quite as significant as its active and objective sides.

Habits, Purposes, and Ideals as Motives

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Other complex learned motives arise from habit formation and from the acquisition of purposes and ideals. A child who has formed a strong habit of any kind tends to respond with that habitual reaction to the appropriate stimulation. This is one of the greatest values of habit in human behavior. The motivating force of habit is not unrelated to the simpler and more basic forms of motivation, however. To eat at a certain hour is a habit, and an individual becomes restless if his meal is delayed. To eat at some time, of course, is one of the most fundamental of motives, and the particular habit is based on this need. Thus the child may "learn" to like spinach and olives and may have a definite desire for them which he previously did not have. Many more complex forms of motivation shown in social situations have a similar origin. They are based on the original needs of the organism, but are developed through complicated processes of learning.

There is another very important factor in the relationship of habit to motivation. If a child has become accustomed or habituated to an excessive amount of some satisfaction he will be strongly motivated to seek it. The child who has been given candy cries for more. Similarly, the youngster who has been petted and protected in too great a degree is likely to spend the rest of his life looking for sympathy and guidance. To be dependent is satisfying for the child, but to have this satisfaction continued into later life is a social handicap.

Purposes and ideals also are motivating forces in the individual's behavior. A small boy may want a wagon, "skate-omobile," or scooter. He asks his father to get it for him, and the father does so. In this instance, little overt behavior is observable that may be attributed to the drive of his desire, want, or purpose. Suppose, however, that the father cannot afford to buy the scooter. The boy, if suitably trained to depend upon himself for the satisfaction of many of his own wants, may hunt about to find the necessary parts, materials, and tools and set to work making his own "skate-o-mobile" or wagon. For hours he may work at it, making many mistakes, bruising his fingers or getting splinters in them, having to try several times before he succeeds. Or the little girl who makes clothes for her doll has many things to learn and may have such high standards of excellence that she literally will work for hours, asking her mother many questions and doing things over and over, before she is satisfied with the result. Even at a younger age, long before children enter first grade, we see evidence of some purpose or, desire activating their behavior, as, for example, when a child of three or four years of age uses some blocks to build a house. The purpose in mind or incentive is not only a motivating force in the child's behavior, but it is also an organizing force through which his activities are integrated in accomplishing a given goal. The formation of suitable purposes and ideals is one of the most significant processes in the development of effective human personality. Reliance upon self in accomplishing many of his purposes is an excellent thing for the child. Even in infancy it seems desirable that the child relieve some of his own wants. If, for example, he is in an uncomfortable and cramped position and begins to cry, his own wriggling, squirming, and thrashing about are likely to overcome his discomfort. He thus learns to adjust independently, instead of expecting assistance. Purposes and ideals of children are likely to be concrete and relatively simple. They are often the expression or outgrowth of their "interests," which are thus also powerful motivating forces.

Social Motives

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In addition to the motives that serve the individual's physiological needs, other forms of motivated behavior relate to his typical contacts with other people, which might be styled his social needs. These social motives were once thought to be primary and native (as by the "instinct theory") but may now be shown to arise from the simpler tissue needs and emotional states. Social motives are learned forms of behavior, and hence may differ considerably from person to person. In spite of individual differences, however, some patterns of social motivation are sufficiently common to justify enumerating them and indicating their principal sources.

(1) The need for prestige or the desire for social recognition is a very important social motive. To the stimulus of the presence of other people, the individual acts in such a way as to win attention, approval, and recognition. Much of the child's behavior is activated by this powerful motive. If deprived of the attention of a group, the child "shows off" or becomes noisy. Even serious delinquency may be motivated in this way, as when a youngster seeks to be a "tough guy" in the eyes of other children. On the other hand, some may find apparent modesty, docility, or humility a means of gaining recognition, as in the case of Uriah Heep who boasted of his "'umbleness."

The need for recognition and attention is derived from the satisfaction of the individual's physiological needs in infancy. Whenever ameliorative satisfactions such as those resulting from feeding, warming, or petting are administered, some person is present and is giving attention to the infant. Moreover, these services are frequently accompanied by kind words, regards, and gestures. Learning occurs, which causes the attention of human beings to be esteemed and sought throughout the rest of life.

(2) Another important social motive is the need for security. Persons, and especially children, need to feel that they are wanted and loved, and that they will be cared for. The origin of this motive is very similar to that of prestige and recognition. Because his needs are so great, the infant must depend on others for his welfare. Although constantly modified throughout the various stages of life, the security motive remains a strong one.

(3) The need for pre-eminence or mastery is very marked in child and adult behavior. Persons seek to excel, to get ahead, to worst rivals, and to overcome obstructions. If frustrated in the normal attainment of this goal, they often will assume an excessive aggressiveness toward substitute objects, resulting in bullying, quarrelsomeness, and, in some instances, delinquency.

The pre-eminence motive seems to be based on the rage behavior of infancy. If blocked or restrained in activity, the infant will display an intense emotional reaction and strong uncoordinated activity. Later, other situations come to arouse the same intensely motivated response, as when commands, the competition of other children, or material obstructions to be overcome, restrain him. The motivation, under proper training, is redirected from useless responses into persistence and effort. The pre-eminence desire is a very useful one in human, affairs, motivating desirable forms of ambition and labor as well as the less socialized efforts toward mastery.

(4) Persons are usually strongly motivated toward conformity. They desire to do the expected thing, to have the appearance of other people; they fear to be "different" or to receive scorn or blame. In a sense, this is the converse of the prestige motive but is not merely a passive desire to avoid losing approval. The conformity motive is undoubtedly related to the fear reactions of the infancy period. Whenever an adequate stimulus for fear, such as painful punishment by parents or a pummeling by child associates, is accompanied by blame, criticism, or condemnation, a conditioning occurs. Thereafter, attitudes of expressed or implied criticism tend to arouse the original strong, fear-motivated avoidance reactions. In general, conformity motivation is an inferior form of drive, which might well be eliminated. Social control is as adequately and more humanely achieved through the milder motives of prestige, security, and pre-eminence.

Native and Acquired Factors in Motivation

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Just as native or unlearned responses lie at the basis of all behavior, so the fundamental organic drives are the foundations of all motivation. The essential vigor of human responses arises from the internal conditions of the body. The elementary motives that have been discussed so far may be considered as native ones. Most of these, such as hunger, thirst, unfavorable temperature liberation through the skin, rest and sleep, elimination of waste products, certain aspects of sex, and the effects of emotional states, are internal. Some external stimuli also may be regarded as native arousers of behavior. These include tissue injury and other forms of excessively intense stimulation, and those milder skin stimuli leading to the so-called "love" responses. In general, the internal stimuli are originally more important than the external ones. External stimuli may set off responses, but these are directed, facilitated, or inhibited by the inner state. The total organism, with its characteristic structures and functions as organized into a living whole, possesses the energy and the modes of response which the stimulus, only releases. The key to man and to subhuman forms is to be sought more in the enormously complex energy changes going on within him than in the fortuitous play of outside energies working on him.

In the course of the development of motives, external stimuli come to play a larger part, but the essential controlling forces remain those within the organism.

The Modification of Motives

Problems of child behavior are complicated by the inextricable blending of native and acquired elements. Conditioning, learning, and habit formation begin the transformation of native responses immediately after birth. Acquired motives are largely developments from the early native ones, but the origin and development of a motive activating the child at the age of ten or twelve years only rarely can be traced in adequate detail. Habits, purposes, and ideals are still more complex forms of acquired motives, although even in these, the native elements might be found to furnish important parts of the total pattern, if we could only resolve it into its native and acquired components.

We see this process of modifying or conditioning drives in many features of child development, as when some external stimuli are substituted for the earlier internal native ones. Thus, at first the stimuli to play probably come from within, but later external stimulation may set off these activities, as when the child sees other children with whom he plays. The sight or smell of food may come to elicit responses which are aroused originally only by the actual pangs of hunger. Merely thinking about food may start off the flow of saliva which originally was activated, by the sight or smell of food. Thus, organic drives eventuating in positive (or negative) responses may become conditioned to various external stimuli. The sight, taste, smell, color, or merely the idea of an object associated with some unpleasant event may come to arouse the response originally evoked by that event. A child at a very early age is bitten by a large black dog. For years he may be decidedly afraid of dogs, and he may even find himself making incipient negative responses to a wide variety of objects that in some way resemble the black dog. A boy of six stepped on the tail of a sleeping Scotch collie, whereupon the dog lunged for the boy's throat, but jumped too high, sinking his two tusks in the boy's upper lip. For years the boy had a violent fear of large dogs, although he had always played with dogs and continued to play with dogs which were not strange to him. A boy of ten, trying to slide on the ice on a little pond into which refuse from a paper mill was dumped, broke through the thin ice which covered the shallow water and sank in above, his waist. For some minutes he was doubtful of his ability to extricate himself. Finally he got hold of a branch of a small willow tree and pulled himself toward the bank and out of the thick, slimy, vilesmelling, nauseating stuff. He, too, for years, at the sight of any place where the ground was yellowish, which had a small shallow puddle of clear water on part of it, and had a few small straggly willow trees about, not only felt afraid, but also felt strong disgust and some nausea at the anticipated thick, slimy, vile-smelling refuse. Even now, in maturity, although he cannot recall vividly the appearance of the place where he had the unpleasant experience, he occasionally sees a place which he instantly recognizes as like that of the childhood episode, and has some feeling of disgust and aversion.

It is evident from the above instances that the principal mechanism operating in the substitution of motive-stimuli is that of the conditioned reaction. By the simultaneous occurrence of an external stimulus with the original internal or sufficient one, the former becomes capable of evoking the behavior in question. This pattern of learning applies as clearly in the case of motives, likes, aversions, and interests as in simpler muscular or glandular responses.

Another class of stimuli, words, come to act as motives through the operation of the same processes of learning. The development of language permits the visual or auditory symbols of things to function in place of the things themselves. Commands, offers of reward, soothing commendations, all commonly act as motives at the earliest ages in which language is understood. As the child grows older, the motivating power of language symbols increases with his experience with them.

Other Physiological Conditions as Sources of Drives

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When the skin is injured, strong defensive reactions follow, involving the skeletal muscles. If the pain is intense and continues for some time, the smooth musculature also is involved. Gently stroking or patting the skin tends toward relaxation of the muscles. Its therapeutic value in cases of restlessness, nervousness, and insomnia is well known and need not be based upon the psychoanalyst's view of stimulating "erotic zones."

Change or shifting of attention seems also to be a normal human activity, motivated by the boredom which follows a period of attentive regard (which may be too short for fatigue to enter) or by the urge to active seeking of different perceptual or other fields. Whatever the exact nature of this motive may be, we do not doubt its existence. Such a motive or drive is of inestimable value, insuring a wealth of sensory, perceptual, and other experiences.

Emotion as a Source of Motive

Emotional states constitute another important original source of vigorous and directed, or motivated, behavior. In the two preceding chapters, attention was called frequently to the intense character of the emotional response. An emotional state involves an upset condition of the visceral organs, which, in a sense, is not unlike the conditions found in hunger and the other appetitive drives. This visceral state may act as a strong internal stimulus, impelling the individual to activity.

Strong emotional states of the type of fear and rage are likely to stimulate intense physical activities which are, originally, of an uncoordinated nature. A three-year-old who is angered by another child's appropriating a cherished toy may resort to tears, may hold on to the toy, may try to pull it from the other's hands, or may strike, kick, or shove the offender. He selects one response or another according to the circumstances and his own past experiences, but the emotional state is the motivating factor throughout. As children mature, the character of the outward response to emotion usually becomes modified. A young child who is angered may respond by striking. A few years later, when enraged, he may respond only by angry words and facial expression. As he comes into the teens and social pressure is exerting its influence more effectively, he may respond merely by angry looks. As an adult the individual may not change his expression, but may make a remark that does not reveal the inner turmoil that he feels. Even in this last case, however, the stimulating effect of the emotional state may be present, although held in check, and may lead to the highly pertinent reply.

The milder organic states, such as those elicited by a full stomach, by loving attention, or by certain features of enjoying music or art, are sometimes designated as pleasant or relaxed emotions. These also may have an effect on the motivated behavior of the individual, reducing the activity of skeletal muscles and tending toward repose and compliance. Various internal states, therefore, may have the same effects as tissue needs in stimulating the various reactive mechanisms of the individual. The internal drives based on emotion are of great importance in the development of the social motives and in the mental, hygiene of the individual.

Temperature through the skin, Rest and sleep

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The human organism tends to maintain a constant body temperature at around 98.6° F. Regardless of weather or season the skin keeps the bodily temperature very nearly constant. Even a variation from normal of a few degrees is a sign of sickness, and a deviation of as much as eight degrees from the usual standard is evidence of very serious illness indeed.

The combustion processes of the body produce or liberate a great amount of heat. The metabolic functions of the body require only a small part of it. The remainder is given off, chiefly through the skin. With variations in the temperature surrounding the body, in the amount of clothing, and in internal bodily conditions (as in fever), marked changes occur in the rate of temperature release through the skin. An excessive rate of release or a subnormal rate is a powerful stimulus to activity. Many writers have emphasized the rôle of temperature and humidity upon man's mental and bodily efficiency and cultural development. They have shown that to torrid and arctic regions have not developed a high state of civilization and culture, and that science, literature, and other forms of art have been developed chiefly in the more favorable temperature of the temperate zones.

Unfavorable temperature liberation through the skin as one of the tissue needs giving rise to fundamental drives to overt behavior, resembling in this respect the great drives from hunger, thirst, and elimination. He also points out the place of this drive in seeking and developing shelter and clothing, and suggests that "the gregarious form of life among some animals at least is undoubtedly an outgrowth of unfavorable skin conditions," and that "their original 'sociability' is a huddling together of individuals who have been restlessly moving about until the warmth of each other's bodies furnished enough heat to allow the organism's to come to, rest -- as is easily observed in the nestling together of very young animals."

Rest and sleep

When the striped or skeletal muscles are fatigued the individual seeks rest, and when they are rested he again seeks activity. Activity and rest thus alternate with each other in a rhythm analogous to that of hunger and eating. Rest is a normal need of the organism and may be required by other factors than muscular fatigue alone. The hygiene value of rest and sleep is well known. The alternation of activity and rest is best seen in the case of the heart. The brief rest period after each beat occupies approximately one-third of the time, so that the heart muscle has abundant rest, which enables it to do its work for all the years of life.

That maturation plays a part in rest and sleep is shown by the decreasing amount of sleep needed as the child passes through the stages of infancy, early, middle, and late childhood, adolescence, and maturity. Problems of social control relate to training the child to sleep at times that meet his own needs and fit in with the convenience of the other members of the household. To regard sleep as a pleasure, and not to use it as a punishment seems highly desirable.

Sex

One of the most powerful drives is that of sex. During infancy and childhood the drive is manifested largely in connection with stimulating the erogenous zones and autoeroticism. The child's natural curiosity about sex is stimulated by many conditions and events in the environment. Inhibitions and taboos often repel the child's innocent and perfectly natural questions on sex matters as something nasty, unclean, or "bad," and prevent him from receiving adequate training. Accordingly, we reasonably may expect difficulties in the individual's control of this powerful fundamental drive along socially approved lines.

Thirst, Elimination Sources of Simpler Motives

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Thirst

Drives to overt behavior arise also from dryness of the mucous lining of the mouth and throat. Usually this means also a certain amount of water deficiency in the tissues. Saliva usually keeps the mouth and throat moist. As the body tissues become dehydrated, the saliva is no longer secreted in quantity sufficient to supply moisture to the mouth and throat.

As thirst develops the child or adult becomes restless, and, in fact, shows about the same rhythm phases as in hunger: restlessness, satisfaction, and quiescence. The significance and strength of the drive resulting from this tissue need are similar also to those in the case of hunger. The mechanisms involved are ready to function at birth.

Elimination

Many diverse structures and functional processes are involved in the elimination of waste products from the body. Water is eliminated by the lungs, the kidneys, the skin, and the lower intestine. Carbon dioxide is eliminated by the lungs, soluble salts by the kidneys, salts in solution by the skin, and insoluble waste products by the lower intestine. Of psychological importance are the features of these processes which are subject to modification and control for desirable social development. Breathing and the elimination of waste products by the sweat glands of the skin are well established soon after birth and are little modified by child training except in so far as training in breathing is given for singing, sports, and the like. The eliminative functions involving the bladder and lower bowel are of significance in child training because of their bearing on social adjustments. Adequate control of these processes is necessary for the protection of society, that is, for reasons of sanitation. Their control has significance also because of the social taboos developed about them, such as shame and modesty.

Three phases are observed in the rhythms of bladder and lower bowel elimination, corresponding to those in hunger and thirst. First is the period of accumulating pressure which directly necessitates and stimulates elimination. It is followed by that in which some response is made satisfying the drive. Finally ensues the period of relaxation, rest, or quiescence so far as elimination is concerned. Apparently, sensory experiences are involved, both in the increasing pressure of distention and in the eliminative responses, because voluntary control otherwise would be impossible.

During the first few months of post-natal life these processes are involuntary, but with maturation comes the possibility of developing voluntary control. With increasing age the frequency of these two eliminative responses decreases. The eighth month is usually the time to begin training in voluntary bladder control, and by the age of two years the child normally will be able to keep himself dry in the daytime. Similar control while asleep is not developed so soon, but if training is begun shortly after the end of the first year, control may be expected normally to develop by the end of the third year. Training in voluntary bowel control usually may begin advantageously about the end of the first month, and regular habits may be expected to be established by the end of the third or fourth month. The value of shame as an incentive in developing adequate voluntary control is seriously questioned. Intestinal or digestive difficulties, of course, may be expected to interfere with the regular routine being established for these functions.

Hunger: Physiological Conditions as Sources of Simpler Motives

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The simpler types of motivation are based on the fundamental physiological processes of the organism. Although often overlooked or undervalued, these factors are of great importance in human life, and especially so in childhood. Much of the restless activity of the young is motivated by these simple drives, and even some more complex forms of motivation are learned elaborations of them. Among the physiological motives described here are hunger, thirst, elimination, temperature control, rest and sleep, and sex.

Hunger

Hunger is caused by contractions of the smooth muscles of the walls of the stomach, which seem to appear and disappear at regular intervals. That they provide a fundamental drive is well known. The hungry person is restless; the hungry infant likewise is restless and given to much overt activity which tends to subside as hunger is satisfied.

The importance of this drive in human history can hardly be overestimated. Food supply, with "fat" and "lean" years, is a fundamental problem in any national economy. In one primitive group fish was very scarce. Stealing fish was punishable by death, although stealing various other things such as another person's wife was not a capital offense. The hunger drive is equally important in the life of the child, with its implications for overt behavior and general bodily vigor.

The whole mechanism for the intake and digestion of food is ready to function at birth. With growth and maturation come changes, as well as through social control. Sometime around the sixth month the teeth begin to erupt, and when this process has proceeded far enough solid foods are included in the diet. Three phases of the hunger rhythm are found from birth. The period of restlessness, already referred to, is evidence of an organic need. Among infants this may be marked also by crying or other evidences of discomfort. This is followed by the responses of feeding -- sucking and swallowing. Then follows the period of quiescence during which the infant sleeps or is active in some way not related to hunger. In children and adults these three phases also are found. The feeding schedule and other features incident to the satisfaction of hunger (use of spoon, cup, etc.) are arranged not only to meet the child's requirements but also to meet those of society. As children develop, problems of poor adjustment may arise over eating, such as eating between meals, poor or irregular appetite, dislike for or refusal to eat certain foods, refusal of any food, refusal to drink from a cup or to feed self, etc. Lack of regular routine probably is responsible more than anything else for wanting to eat between meals and for poor or irregular appetite. On the whole, maladjustments like these seem to be due primarily to poor training; the child has learned the particular undesirable behavior from the training he has had.

What are the fundamental drives in children?

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What are the fundamental drives in children? What are the important motives directing their conduct? How are these developed or modified? Whence do motives come, or what are their sources? What is the relative importance of original nature and learning in motivation? What part do purposes, attitudes, ideals, and interests play in motivating childhood behavior? In what things are children interested at various ages from birth to maturity? These are important problems to which we must now turn our attention if we would secure a fruitful understanding of child nature.

Teachers, parents, and others who guide and direct the behavior of children can do so to best advantage, only if they really understand them. Obviously, such an understanding is dependent upon knowing why the child at a particular time does what he does.

A nine-year-old girl, large for her age, comes home with her best shoes not only covered with mud, but her stockings or socks are also covered with mud halfway to her knees, and splotches of mud are on her dress. The mother has told her "a thousand times" to keep out of the mud and has scolded and punished her for getting her shoes and other clothes muddy. To her the girl's behavior is perverse indeed, and the child may be punished severely for disobeying her mother's previous ultimatum, especially if the mother is crowded with work, has a headache, or is under some other strain. A boy of eight, going to school, takes a short-cut across the corner of a large newly seeded lawn, although he has been told repeatedly to keep out. He is a quiet, well-behaved boy, but persists in cutting across the lawn despite scoldings and threats to notify his parents and the police. Regardless of what is done to handle these two cases, the mother and the lawnowner probably can secure a more effective modification of behavior if they understand why the girl got so muddy and why the boy uses the much objected-to short-cut. The girl, taunted and teased by a young bully's remarks, finally in desperation gives chase through a muddy garden and settles the matter. While on his way to school when in the first grade the boy had been very badly frightened by a cross dog on the next street. He had cut through a vacant lot, and for two years had been going to school this way. A new house had been built and the lot graded and seeded. The boy was still using the accustomed way although the cross dog no longer lived on the next street.

Rational guidance and control of children are contingent upon understanding the motives which activate them.

Some of the earlier attempts at understanding human behavior postulated a series of entities known as instincts which acted as drives, or springs to action. Thus, an individual was said to have a drive or urge to do those things which led to selfpreservation because he had an instinct or instinctive urge of selfpreservation. He was said to want to be with other persons because of the drive from the gregarious or social instinct. If he tore things to pieces this was because of his instinct of destructiveness, or if he put things together in some way it was because of the constructive instinct! On the whole such attempts to understand human motives have not carried us very far. Part of the diffculty, but not all, lies in the fact that the term instinct has been used with at least two diverse meanings. Sometimes it means stimulus arousal of an activity, and sometimes it means a pattern or kind of activity itself. Thus it is used to mean the motivation of an activity, as in pugnacity, curiosity, gregariousnes, parental love, and the like; whereas in other cases it refers to the kind of activity, as when it was alleged that walking, manipulation, collecting, etc., were instincts.

Trying to understand complex human behavior by postulating such entities seems to break down in the study of actual cases. The discussion of mass activity in several previous chapters should make it clear that definite, clear-cut, specific, fixed-inadvance patterns of behavior are rare indeed, and that diffuse, non-specific mass activity is generally the rule, the former developing after experience, learning, or-habit formation has played its part in the infant's development. The definite patterns of response called for by the instinct theory are not found upon careful observation of infants during the first few days or weeks of post-natal life; neither does it seem fruitful to suppose entities (instincts or springs of action) concealed inside us, which, when stimulated, make us do this, that, or some other thing. Such a description of human motivation has little to commend it aside from tradition.

We require ten inorganic or mineral elements

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Proteins consist of giant molecules which are resolved during digestion into about twenty relatively simple organic compounds known as amino-acids. We know that two of these can be synthesized by the body, and, pending further investigations, we may assume that the remaining eighteen must be supplied by the food. We must have the sugar glucose, which may be taken as such or derived from cane or milk sugar or from the various starches. These are all changed by the organs of digestion so as to introduce only glucose into the blood. We require at least ten inorganic or mineral elements: sodium, potassium, calcium, magnesium, chlorine, iodine, phosphorus, sulphur, iron, and copper. Probably this list of indispensable inorganic elements will eventually be extended to include several others, such as manganese, zinc, silicon, fluorine, and possibly nickel and cobalt, boron, etc. These last appear to be necessary for the development of plants, and may also be necessary for that of animals. At least six nutrient principles, called vitamins A, B, C, D, E, and G, are known to be required for normal nutrition. Among the twenty or more fatty acids known to chemists, at least twelve occur in the fats of our ordinary foods, but it appears from the results of experiment that only one of these, namely, linoleic acid, cannot be synthesized by the body from carbohydrate molecules. From the data available we may say with some assurance that the simplest diet which would furnish everything necessary for normal nutrition must contain appropriate proportions of at least thirty-six simple chemical substances. Actually, we eat daily many times this number in our ordinary foods of animal and vegetable origin.

Methods have been developed by means of which experiments on animals fed a single natural food (wheat, maize, etc.), which does not alone support satisfactory nutrition, supplemented with single or multiple additions of the indispensable nutrients, yield information concerning the nature of their deficiencies. Such experiments may be made nearly quantitative. Through such studies we have secured a considerable body of knowledge concerning the extent to which each of our more important natural foods-cereal grains, tubers, fruits, roots, leaves, meats, milk, eggs, etc.--furnishes the body with the indispensable nutrients. In addition, such experiments have shown us the nature and extent of the deficiencies of the different natural foods. On the basis of such data it is possible to theorize concerning which natural foods, individually lacking or deficient in one or another food principle, should, when combined, supplement each other's deficiencies. Experimental verification of predicted results confirms in a highly satisfactory manner the belief that we understand, to a great extent, the nutritive needs of the body and the dietary properties of most of our natural and manufactured foods.

Since most foods have been shown to be deficient or lacking in one or more of the nutrient principles, it is necessary for us to combine foods of unlike composition so that one will provide what the other lacks. Herein lies the cause of safety in variety in eating. The whole wheat kernel, which many people have long believed to be a complete food, is incapable, when fed as the sole source of nutriment, of supporting growth of the young or prolonged health in the adult. It is deficient in three respects: (1) it lacks sufficient calcium; (2) it lacks sufficient vitamin A; and (3) its proteins require supplementing with proteins from other sources which supply in abundance certain amino-acids which the wheat proteins contain in amounts too small to serve as building stones when food proteins are converted into body proteins. Even though wheat is supplemented with one or two of these substances in which it is deficient, the nutrition of an animal will not be so good as it will if all three are added to the diet.

Forms of muscle irritability, contractility, conductivity, metabolic activity

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All forms of muscle possess the following physiologic properties: irritability, contractility, conductivity, and metabolic activity. The specific function of muscle is to shorten for the purpose of movement, hence, its most specialized property is contractility. This property is most marked in skeletal muscle.

When a muscle responds, certain rather definite changes occur within it, namely, mechanical change, chemical change, thermal change, and the production of electricity and sound.

During contraction muscle changes its shape but its volume remains unchanged.

In response to a single stimulus, a muscle gives a single twitch, the record of which is known as a myogram. The myogram of a frog's muscle shows a latent period of 0.01 second, a contraction phase of 0.04 second, and a relaxation phase of 0.05 second. The various phases of the response of human muscles, although varying somewhat from those of the frog muscles, have about the same relative duration.

Following a series of repeated stimuli (two to five per second), the first few contractions progressively increase in extent to a maximum--treppe-after which the responses soon become less and less effective as fatigue develops. Fatigue prolongs each of the phases of the response--especially the relaxation phase. As the relaxation phase is lengthened the succeeding stimulus begins to reach the muscle before relaxation is completed. The new contraction begins at this point leaving a contraction remainder or contracture.When the stimuli are applied more rapidly the muscle may be held in a state of continuous contraction or complete tetanus. If a partial relaxation is permitted, the tetanus is now incomplete and is known as incomplete tetanus. The importance of tetanus lies in the fact that practically every voluntary and reflex response in man is of this nature. They possess three fundamentally important characteristics, namely, maintenance, fusion of separate contractions, and a summation of single responses.Although the mechanical response is continuous and maintained, it can be shown by the electrical change and sound produced that, within certain limits, the internal response is discontinuous. This discontinuous phase is, however, limited by an inherent period of forced rest imposed upon the muscle due to what is known as the refractory period.Within certain limits, a rise or fall of temperature increases or decreases respectively the rate of response. Again it is the relaxation phase which is especially affected. At a certain low temperature activity is reversibly suspended; at a certain high temperature it is irreversibly lost. These temperatures differ from animal to animal.The skeletal muscle fiber responds as a unit, that is, when stimulated it either gives a maximal contraction or fails to respond at all,--the "all-ornothing" phenomenon. Graded response is a function of the whole muscle, the strength of the response being determined by the number of fibers acting at any given time. Heart muscle in its entirety obeys the same law.

Discontinuous nature of tetanic contractions and refractory period

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Although the mechanical response in complete tetanus is maintained and continuous, that is to say, no sign of relaxation is apparent, the true character of the response to the rapidly repeated stimuli can be shown to be discontinuous. The most striking proof of this fact is the electrical change. Within certain limits, the number of electrical changes corresponds with the rate of stimulation. A further proof of the discontinuous nature of tetanic contractions lies in the sounds produced by contracting muscles. This correspondence between the internal response of muscle and the rate of stimulation will extend from a single stimulus up to about one thousand stimuli per second for the muscles of man and other warmblooded animals. When the rate of stimuli exceeds this higher figure, the muscle fails to respond to each stimulus but responds only to every other one.

This result is due to the fact that immediately following a response to a single stimulus there is a period, as it were, of forced rest or period of adjustment during which the muscle is non-irritable and fails to respond. This brief period is known as the refractory period and is of about 0.005 second duration in skeletal muscle. It serves as a means of further protection to the muscle against a demand which is beyond its capacity. This discontinuous character of the response is to be explained on the basis of the discontinuous nature of the chemical changes occurring in response to the rapidly repeated stimuli. Likewise the refractory period and limits to which these separate changes can take place must find their explanation in the factors controlling the biochemical reactions involved. The refractory period of muscle is so brief, however, that it probably never functions to limit the frequency of the response in man.

The importance of tetanic contractions lies in the fact that this type of contraction is practically the only one to be found in the human body during muscular exercise or activity. This fact, in the intact human body, is due to the manner in which the muscles receive their stimuli and are controlled through the central nervous system. When a muscle or group of muscles is called into action, a shower of impulses is sent from the nerve centers over the motor nerve fibers which innervate these muscles. A few, many or all of the muscle fibers of the muscles involved may be stimulated, the response being graded in proportion.

Compound or tetanic contractions of muscle

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A second method of grading the response of a muscle as a whole is by altering the frequency at which the separate stimuli reach the muscle. When a frog's gastrocnemius muscle is stimulated with a series of rapidly repeated stimuli (twenty or more per second) it remains in a state of more or less maintained contraction as long as the stimuli are continued, or until fatigue develops to terminate the response. This maintained contraction is known as physiological tetanus. When the stimuli follow each other so rapidly as to prevent even partial relaxation, the response is termed complete tetanus. If, on the other hand, the stimuli are less frequent and permit the muscle to partly relax, but not completely, before the following stimulus initiates a new response and the myogram shows a condition of incomplete maintenance of the contractile phase, it is then known as incomplete tetanus. Various grades of incomplete tetanus are possible. Complete tetanus probably involves all of the muscle fibers of which a muscle is composed.

Tetanic contractions present three fundamentally important characteristic, namely: first, maintenance or increased duration over that of the simple twitch, second, the more or less fusion of the separate responses initiated within the muscle by the separate stimuli, and third, the phenomenon of summation of the single responses so that the extent of the contraction in tetanus is much greater than that presented by a simple twitch. It would seem that in a single twitch acting against a load, the contractile stress of the muscle passes off before the load is lifted to the maximum height possible. Summation is probably due to the added mechanical effect of the load since in the second response the lever stands at a higher abscissa. The single twitch is inefficient because it is too brief except for handling very light loads against very low resistances. This principle of summation was inferred in the discussion of fatigue given in a previous paragraph. It depends upon the application of a second stimulus at some time during the contraction or relaxation phase of the response initiated by a previous stimulus.

The greatest summation occurs when the second stimulus is so timed as to reach the muscle just as it reaches the state of greatest tension, and diminishes as it appears earlier or later in the contraction phase. Any conditions which will prolong the contraction and especially the relaxation phase, such as fatigue, diminished temperature, and others, will diminish the number of stimuli necessary to induce a state of tetanus and, therefore, will bring about an exaggeration of the normal response.

The unit of the nervous system is the neurone

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The unit of the nervous system is the neurone,--a nerve cell with all of its processes. The nervous system is composed of myriads of neurones joined into a network by means of synapses.

Each muscle fiber is supplied by at least one motor nerve fiber which ends beneath the sarcolemma in the motor end organ. This constitutes the efferent nerve supply. From the muscle and tendon spindles, muscles make afferent connections with the brain and spinal cord (central nervous system), in part sensory and in part non-sensory.

The simple nervous connections between peripheral receptors to the center (afferent) and the return to the effector (efferent) is known as a reflex arc, and is composed of at least two neurones. More complex arcs are established by means of one or more connector neurones and may extend to the opposite side of the cord, to other levels, and to the cerebrum. In the last case they become sensory fibers since they arouse sensations. Voluntary discharges may issue from the motor area of the cerebrum to the lower reflex mechanisms and elicit voluntary movements.The term reflex action is used to designate involuntary action. Reflexes are dependent upon the integrity of the reflex arc. Pavlov has classified all known reflex actions into unconditioned or inborn reflexes, and conditioned or acquired reflexes.Muscle tonus is that state of partially maintained contraction of muscle by which its length and tension, especially true of antagonistic muscles, are automatically adapted to the new positions taken by parts of the body as in locomotion or of parts of the body to the whole as in posture.

The physical properties of muscle, extensibility and elasticity, are increased or decreased directly as muscle tonus is increased or decreased.Muscle tonus and postural tonus are dependent upon the integrity of the myotatic or monomuscular arc, and upon connections with the semicircular canals, cerebrum, and cerebellum. Loss of any one will alter muscle and postural tonus.The nicety of adjustment between the position of the body to its environment or of the parts of the body to the whole, of muscle tonus, action of antagonists, and in postural tonus is dependent upon the phenomena of reciprocal action and reciprocal innervation. When one group of muscles contract an antagonistic group must relax and vice versa. Movements of locomotion would be impossible without these mechanisms.

Reciprocal action and reciprocal innervation of antagonistic muscles

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In the foregoing survey of the mechanisms involved in muscular activity, it has been pointed out that both reflex and voluntary responses may become very complex yet are coördinated and may be purposeful and protective in character. Such responses cannot be mediated through a single muscle, but by a nicety of balance between the activities of groups of antagonistic muscles. This balance is the resultant of a reciprocal activity existing between muscles which act upon the levers of a certain joint or series of joints in opposite directions. When one group of muscles is stimulated, the other is thrown into a state of inhibition in order that the action may take place without opposition. Such a condition is found at all joints where flexion and extension, abduction and adduction, and rotation take place. Since this power of coördination does not reside in the muscles, but in the central nervous system, a reciprocal innervation of the opposing muscles in any particular instance is required.

The mode of innervation is such that when one group of similarly acting muscles, as the quadriceps in the extension of the leg, is stimulated, the antagonistic flexor group (hamstrings) is inhibited. When the reverse action occurs (flexion) the hamstrings are stimulated while the quadriceps are inhibited. That this actually occurs has been proved experimentally. The nervous mechanism necessary for such an allied or synergic response is known as reciprocal innervation. The flexors (hamstrings) are supplied by their monomuscular arcs from the fifth lumbar to the second sacral nerves. The two sets of arcs are connected by connector neurones within the cord and the number of nerve fibers involved probably amounts to several thousands. Impulses which arise in the voluntary centers of the cerebrum may act through this local mechanism. Similar relationships are to be observed at many other joints and in the muscles of the eyes.

Inhibitory nerves for skeletal muscles have never been demonstrated and it is generally believed that they do not exist. If this is the case the mechanism for reciprocal innervation must then be looked for in the central nervous system. Motor nerve cells in the spinal cord may be either stimulated or inhibited, that is, their activity may be augmented or diminished. Inhibition may be produced by diminishing or blocking the motor discharge issuing from the nerve centers over the efferent nerves and may affect a few or all of the nerve fibers involved. In any given reflex action, one and the same stimulus will increase the activity of the flexor center and simultaneously inhibit that of the extensor center. Conversely, when the stimulus is such as to excite the extensor center the activity of the flexor center is inhibited. Thus the influence on the two centers for antagonistic groups of muscles are of opposite sign.

Reciprocal innervation is not limited to the muscles of a single part, but may extend to other parts as, for example, when two or more parts are employed in progressive movements, such as walking. These mechanisms become more complex. From the same diagram, it may be seen that when the extensors of the supporting limb are excited and enter into contraction those of the opposite limb (progressing) must be inhibited, that is, their activity is of opposite sign. Exactly the opposite order of events is occurring in the flexors, namely, they are inhibited in the supporting and excited in the progressing limbs.

Postural tonus

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The most significant role of skeletal-muscle tonus as a factor in bodily activity is the part it plays in the maintenance of posture. Posture is that nicety of adjustment between the various parts of the body to fit the new position taken as in sitting, standing, running, and reclining. In each position, the muscles must assume a new tonic state (posture) in addition to the more obvious contractions used for the change of position. Smooth continuity between normal movements in all kinds of muscular activity is dependent upon a mechanism which correlates these two types of response. When the position of the body changes, as during exercise, the tonus of its entire musculature must be modified so that each part may make a harmonious contribution to the activity as a whole. The importance of this postural or plastic tonus is indicated by the fact that when the underlying mechanism is disturbed the movements become incoördinated and ataxic.

Postural tonus depends upon the integrity of the reflex arcs and upon a steady but continuous bombardment of impulses into the muscles through the motor nerves from the central nervous system. An increase or decrease in the number of nerve fibers, and consequently the number of muscle fibers involved at any one time, leads to a greater or lesser state of tonic contraction. The afferent impulses upon which the posture of the body as a whole is dependent may arise in the muscles themselves (proprioceptors) or in the semicircular canals or from the cerebrum circuited through the cerebellum. Those from the muscles are influenced by the position of the parts with relation to the body as a whole, those from the semicircular canals by the position of the entire body in space. These are reinforced by the coördinating influence exerted by the cerebellum which also exercises or reinforces a general tonic effect. Thus, with the cord transected (spinal animal), an animal will have lost a large part of its power to maintain postural tonus. Simple myotatic reflexes are still present and are sufficient for certain simple movements of a reflex type. These are not adequate, however, for the maintenance of the postural tonus necessary for standing, walking, running, or support of the body.

There are two types of reaction which, when the proper stimuli arise, are accompanied by changes in postural tonus of the skeletal muscles. If one of a pair of antagonistic muscles normally employed in supporting the weight of the body against gravity is suddenly stretched, it responds with an increased development of tonus and comes to a new and shorter length. Its antagonist, on the other hand, is inhibited and comes to a new and greater length. Reflexes of this type involve the monomuscular are (myotatic reflex). In the other type of reaction the reverse is true. The tonus of the muscle may, from the same stimulus, give way almost suddenly. On releasing the tension applied, the muscle will be found to have taken on a new state of tonus and consequently a new length.

Just what conditions determine which response shall predominate is not known. The tension exerted by the muscles upon their points of insertion is maintained approximately constant. In progressive movements, as walking, postural adjustments must not only be made rapidly but with exactness. The stimulus which sets up a contraction of the extensors will inhibit the tonus existent in the flexors. A shortening reaction in one set of extensors is also associated with a simultaneous extending reaction in those of the opposite side or limb and vice versa. This condition is to be considered as an adaptation to the alternate responses which normally take place, as in walking and running. This kind of coördination is an example of reciprocal inhibition of opposite muscles and limbs.

Muscle tonus and its relation to posture

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Extensibility and elasticity exist in muscle and are commonly known as the physical properties of muscle. By the former is meant that when a muscle is placed upon a stretch it is capable of being extended. This, however, does not obey Hooke's law, that is, the extension is not proportional to the opposing force, but with each increment of weight the extension becomes progressively less until a certain critical point is reached. Beyond this point the extensions become progressively greater with added increments until the muscle ruptures. The property of elasticity is that by which the muscle is capable of regaining its original length after being extended and when the strain is removed. The curve of elasticity is essentially the reverse of that of extension with the exception that the elastic recoil is not one-hundred per cent perfect. Both extensibility and elasticity are inherent properties of all varieties of muscle. These properties are greatly augmented during states of increased muscle tonus and when the muscle is contracting.

The physiological importance of the physical properties of muscle may be briefly summarized. They help to keep the muscles taut and thus prevent slack. This is essential for smoothness and freedom of movement of the bony levers. When a muscle first contracts against a lever, the inertia of that lever, especially if carrying a load, yields relatively slowly as compared to the rapid contractile phase of the muscle. It is obvious that the advantage of an extensible muscle would be to prevent rupture of that organ and to insure greater smoothness of movement. Contrast the result of trying to lift a weight quickly by means of a rigid wire and again by means of a strong but extensible body. Furthermore, in the action of antagonistic muscles, while one set is contracting the other must yield. This condition is made possible, in part, by the physical properties of muscle, but to a greater extent by muscle tonus. On the other hand, an extended antagonistic muscle, by means of its elasticity, may aid mechanically in bringing the moved part back to its original position.

During life the skeletal muscles are normally never in a complete state of rest. Even during sleep there is a slight continuous contraction. This state of tension which is maintained at some point between complete relaxation and complete contraction is known as simple musle tonus. During the waking hours this muscle tonus may be increased, but varies between wide limits from time to time and from muscle to muscle. The muscles may be considered as having taken on new elastic properties and will by virtue of this tend to assume greater or shorter lengths. Characteristic of muscle tonus is the fact that the tension exerted by the muscle on its points of origin and insertion tend to remain unchanged.

This tonus is not inherent in the muscle fibers themselves, but is dependent upon their connections with the central nervous system and reflex arcs. If the reflex are is broken at any point, as by cutting the afferent or efferent fibers of the dorsal and ventral roots, respectively, or by the destruction of the spinal cord, the muscles involved become toneless and flabby. Furthermore, in an animal with the spinal cord cut across, even when the afferent nerves leading from the adjacent muscles and from the skin are cut, thus leaving intact only those which arise in the muscle and tendon spindles (proprioceptive), simple muscle tonus persists. This definite highly localized monomuscular reflex has been clearly demonstrated and described by Sherrington, to which he has given the name myotatic reflex.

The adequate stimulus for this reflex is simple stretching, a stimulus which excites proprioceptors generally. This reflex is very exact and well graded. If only a part of the muscle is stretched that part only responds by a change of its tonus and the extent of this change is proportional to the strength of the stimulus (stretching). This local monomuscular effect may, however, be reinforced by impulses arising from other muscles in various parts of the body, from the semicircular canals, and from the cerebral and cerebellar portions of the brain. Pleasurable mental states as happiness, hope, and anticipation, are associated with a general increased tonus of the skeletal muscles. The jaw is held firm, the head and shoulders are erect, the eye is clear, and the step is firm. When one is mentally depressed, however, as from worry, fear, anxiety, and fatigue, the muscles are more flabby. The expression of the face is heavy, the eyes are dull with drooping eyelids, the facial muscles fall, and we speak of one being "down in the mouth." This condition of toneless muscles is incompatible with good health and proper functioning of the body.

The nature of the mechanism of skeletal-muscle tonus is still a moot question. Some workers hold that in these muscles are two types of muscle fibers, one of which is involved in the specific contractions and the other in the development of tonus. This seems quite unlikely since both the red and white muscle fibers, the two known types of fibers, have been shown to be contractile units and to respond similarly. A second viewpoint is that one and the same muscle fiber has an innervation from both the cerebrospinal and autonomic systems, the former initiating the specific responses, the latter having to do with tonus. The evidence to support this theory, however, all comes from negative experiments and is not at all conclusive. The safest viewpoint to accept seems to be that both specific contractions and tonus contractions are consummated by the same muscle fibers and innervated by the cerebrospinal nervous system only. This partial reaction is due to the fact that only a part of the muscle fibers are active at any one time. This state of activity in one given group of fibers and state of rest in other groups shifts by rotation. Such an explanation is in complete harmony with what we know of muscular contraction. Those fibers which are contracting at any one given time are doing so to the utmost of their ability ("all-ornothing" phenomenon), but they do not become fatigued since the burden is soon taken by another set which in turn is soon relieved, and so on. This would explain the fact that although muscle tonus is a reflex phenomenon it cannot be produced by artificial stimulation of either the afferent or efferent nerve trunks. To maintain muscle tonus, a relatively small expenditure of energy is required and only a small amount of heat is produced. Thus the intrinsic nature of muscle tonus is adapted to the rôle it has to play, namely, maintaining activity over extremely long periods of time.

Cardiac Output in Exercise

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Accurate data on the cardiac output in various types and intensities of exercise are not available, due largely to the very great technical difficulties of measurement. Such data as are available must be accepted with some reservation for the same reason. A great deal is known about the changes in heart rate which accompany exercise, but little is known about the equally important adjustments in the stroke volume.

Even without direct measurements, this could be postulated from the enormous increase in oxygen consumption which occurs. Krogh and Lindhard 3 using an indirect Fick method, found that the cardiac output may exceed 20 liters per minute during heavy exercise. In spite of calculations which indicated a possible total blood flow of 30 to 40 liters per minute during running when the oxygen consumption was 4 liters per minute, it is probable that in the average man the cardiac output during work seldom exceeds 20 liters per minute. An example of the enormous increase in cardiac output which can be achieved by exceptional athletes.

The increase in cardiac output which may be attained in exercise is limited by several factors. First, it is obvious that the heart cannot, for more than a few beats, eject more blood than is returned to it by the systemic veins; in other words, cardiac output cannot exceed the venous return. In numerous conditions, such as shock, hemorrhage, the erect posture with no movement, extreme heat, prolonged bed rest, etc., the cardiac output is reduced below normal by an inadequate venous return. Following strenuous exercise, the sudden withdrawal of the pumping effect of skeletal muscle contractions on venous blood flow results in a sharp fall in cardiac output. On the other hand, the available evidence indicates that the normal healthy adult heart is always able to increase its output sufficiently to handle the greatest amount of blood which can be returned to it in maximal exertion. Additional factors which may limit the increase in cardiac output in exercise are the maximal capacity of the heart for dilating and the reduction in diastolic filling of the heart which results from extremely rapid heart rates.

Finally, such adverse conditions as fatigue, lack of sleep, malnutrition and acute infections may seriously reduce the maximal cardiac output of which the subject would otherwise be capable.

Basal Values, The Effect of Posture

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Basal Values

Under basal conditions, the cardiac output in man averages about 64 ml. per kilogram per minute. 2 It is higher in the adolescent than in the adult and decreases in subjects over forty-five years of age. Sub-basal values are found in the early hours of the morning, during standing and probably also during recovery from heavy exercise. The normal basal cardiac output has an absolute value of 4 liters per minute for an individual of average size and if the pulse rate is 70, the stroke volume is 57 ml.

The Effect of Posture

The cardiac output is usually reduced when the subject stands. Since the pulse rate is usually increased, his must indicate a reduction in stroke volume. This is especially true in prolonged standing without movement and is common after a long period of hot weather. The reduction in stroke volume is due apparently to a decrease in the venous return from the lower part of the body. In the sitting posture, the output is slightly less than when the subject is reclining, but peculiarly enough, the output is not reduced when the subject is in the erect position but leaning against a support.

Miscellaneous Influences

The cardiac output is increased by the taking of food and water and, temporarily, by high altitude. Exposure to cold slows the heart rate but increases the stroke volume, leaving the cardiac output unchanged, unless shivering occurs, when the cardiac output is increased. Exposure to warmth increases resting cardiac output very considerably. The heart rate is increased with a slight reduction in stroke volume. If the heat is severe enough to cause dehydration, cardiac output returns to the basal level. It may fall below the basal level in heat prostration in spite of a rapid heart rate, indicating a reduction in stroke volume due to inadequate venous return to the heart. Cardiac output is diminished by many types of cardiovascular abnormalties.

Heart Murmurs Cardiac Output

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Heart Murmurs

Sometimes, due to congenital malformation or disease, the action of the heart valves is impaired. The valve orifices may be narrowed so that the normal flow of blood is impeded, and the valve leaflets may fail to close completely, allowing a leakage of blood in the reverse direction. This abnormal valve action results in distortion of the normal heart sounds or in the appearance of additional sounds. These abnormal heart sounds are called murmurs. The particular valve involved may be determined from the point on the chest wall at which the murmur is heard most clearly. In the case of so-called "functional" murmurs, there is no structural defect to account for the abnormal heart sounds.

Cardiac Output

From a functional standpoint the most important index of heart function is the cardiac output, that is, the volume of blood pumped by each ventricle per minute. Two factors, each of which may vary within wide limits, determine the size of the cardiac output. These are the heart rate and the stroke volume. The heart rate is very easily determined. The stroke volume, on the other hand, must be calculated from the values of the cardiac output and the heart rate. The fundamental importance of the cardiac output as an index of heart function has led to many attempts to devise a method for its measurement. The cardiac output could be calculated from the experimentally determined values of the oxygen content of the mixed venous blood (that is, the venous blood from all parts of the body after it has been mixed in the right heart), the oxygen content of the arterial blood and the oxygen consumption of the body. Suppose, for example, that the oxygen content of the mixed venous blood is 15 volumes per cent, that of the arterial blood 20 volumes per cent and the oxygen consumption is 250 ml. per minute. It is obvious that each 100 ml. of blood yields 5 ml. of oxygen to the tissues, so that of blood is required to furnish 250 ml. of oxygen to the tissues. Since this amount of blood must be pumped by each ventricle per minute, it represents the cardiac output. Until recently this method has not been used for studies on human subjects because of the difficulty in obtaining samples of mixed venous blood. Various procedures for determining the gaseous content of the mixed venous blood by indirect methods have been devised (for details, see textbooks of physiology) but all are open to criticism, especially when attempts are made to measure the cardiac output during exercise. A plastic catheter is inserted into an arm vein and carefully threaded up the vein and into the right heart. Samples of mixed venous blood are thus obtained and their oxygen content determined. The oxygen content of the arterial blood is determined by analysis of samples obtained by arterial puncture and the oxygen consumption of the body is measured with the ordinary clinical B.M.R. apparatus. From these data the cardiac output is calculated as illustrated above.

The Heart Cardiac Cycle

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The cardiac cycle includes all the events--pressure changes, volume changes and valve action--which occur during one complete period of contraction and relaxation of the heart. Since the complete cycle takes place in a period of one second or less, it is no wonder that early physiologists despaired of ever solving its mysteries. Modern methods of recording rapid changes in volume and pressure have, however, permitted a very exact analysis of events.

A description of the cardiac cycle may begin at any point in the cycle. For convenience, we will start with the phase of diastasis, the period during which the whole heart is completely relaxed. Blood is entering the right auricle from the venae cavae and the left auricle from the pulmonary veins. The auriculo-ventricular (A-V) valves which guard the orifice between the auricle and the ventricle on each side are open and the blood which enters the auricles flows freely through into the relaxed ventricles. The valves leading from the ventricles to the pulmonary artery and the aorta are closed, so that none of the blood entering the ventricles is able to leave. The period of diastasis ends abruptly with the onset of systole (contraction) of the auricles. Filling of the ventricles is already virtually complete when auricular systole occurs, so that it is of minor importance so far as ventricular filling is concerned. Almost immediately after auricular systole is completed, ventricular systole begins. Contraction of the ventricular muscle results in a rapid rise in the pressure of the blood in the ventricle. This very quickly exceeds the auricular pressure (which is always low) and causes a sudden closure of the A-V valves on both sides. The vibrations of these valves as they close set up waves which are transmitted to the surface of the chest, where they may be heard as the first heart sound. As the ventricles continue their contraction the pressure exerted on their contained masses of blood rises steeply, but since all the valves of the heart are closed no blood is pumped out. Since the heart muscle cannot shorten during this period, it is referred to as the isometric phase of systole. As soon as the ventricular pressure rises above the pressure in the pulmonary artery and the aorta, the valves guarding these vessels open and blood is rapidly ejected. During the ejection phase, the ventricular muscle is able to shorten (resulting in a decrease in the size of the ventricular cavities) so that it represents an isotonic contraction.

The termination of ventricular systole marks the onset of ventricular diastole, or relaxation. As the ventricular muscle relaxes, intraventricular pressure falls. When it drops below the pressure in the aorta and the pulmonary artery, the valves guarding their orifices close, giving rise to the second heart sound. The intraventricular pressure continues to fall and eventually drops below intra-auricular pressure, resulting in the opening of the A-V valves. As blood begins to pour from the auricles into the ventricles, the period of diastasis, with which this description of the cardiac cycle began, is reached once more.

A typical cardiac cycle consists, then, of three phases: diastasis (resting period), systole (contraction period) and diastole (relaxation period). Most of the filling of the ventricle occurs during the early part of diastole. This is of importance in the adjustment of heart function in exercise, because the increase in heart rate occurs primarily at the expense of the period of diastasis. When the heart rate becomes excessively high, the filling of the ventricle may be cut short, with a resulting decrease in stroke volume.

General Description of the Heart

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The salient anatomical features of the heart may be reviewed. The heart is a hollow muscular organ, subdivided internally into four chambers: the right and left auricles (or atria) and the right and left ventricles. The auricles are thin-walled collecting chambers. They have little contractile power and serve primarily by storing the blood brought to them by the veins during the contraction, or systole, of the ventricles and then passing this blood on to the ventricles during their period of relaxation, or diastole. The ventricles, on the other hand, are thick-walled muscular chambers which exert considerable force during contraction. This contractile force imparts the necessary kinetic energy to the blood to maintain its circulation.

From a functional standpoint, the heart may be divided into the "right heart" (right auricle and ventricle) and the "left heart" (left auricle and ventricle). The right heart receives venous blood from all the systemic veins of the body and pumps it through the pulmonary arteries into the lungs, where oxygen is absorbed and carbon dioxide eliminated. The oxygenated blood is returned through the pulmonary veins to the left heart which in turn pumps it through the aorta into the systemic arteries of the body. The orifices between the auricles and their corresponding ventricles, and the exit of the pulmonary artery and the aorta from their respective ventricles are guarded by valves which permit the flow of blood in one direction only.

The heart muscle itself is not nourished by the blood contained within its chambers, but is supplied by the coronary arteries which leave the aorta just beyond the aortic valves.

The stimulus which causes contraction of the heart muscle at each beat arises within the heart itself, in a specialized muscle mass known as the sine-auricular (S-A) node or "pacemaker." The impulse is conducted to all parts of the heart muscle by way of a specialized conducting system, the auriculo-ventricular (A-V) node and bundle. The rate of beating of the heart is regulated by two sets of nerves, the vagus nerves which slow the rate and the accelerator nerves which increase the rate.

The heart is enclosed in a fibrous sac called the pericardium. A thin film of fluid (the pericardial fluid) separates the heart from the pericardium and minimizes the friction which otherwise would occur during contraction and relaxation of the heart. The pericardium probably serves a protective function in preventing dangerous overdistention of the heart. It is uncertain to what extent the pericardium limits the stroke volume (amount of blood pumped by the heart at each beat) during exercise. While the pericardium is relatively nonelastic and hence resists attempts at sudden stretching, it can be stretched very slowly to permit the normal increase in heart size often seen in athletes or the pathological dilatation of the heart which occurs in certain types of heart disease.

The Hearth General Features of the Circulation

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The tissues of the body require, for their normal functioning, a reasonable degree of constancy with respect to certain factors; among these factors are: temperature, acidity, food supply, and oxygen. The primary function of the circulation of the blood is to ensure the preservation of this constant internal environment by transporting oxygen, food materials and hormones to the tissue cells and by removing the waste products of activity. The interchange of materials between the blood and the tissues occurs in the thin-walled capillaries. The rest of the circulatory system, including the heart, exists solely for the purpose of maintaining the capillary exchange. The heart is a muscular pump which imparts sufficient kinetic energy to the blood to move it through the capillaries. The arteries conduct the blood from the heart to the capillaries and the veins conduct the blood from the capillaries back to the heart again. As will be seen in later chapters, the arteries and veins are not simply passive conducting tubes, but are also responsible, through alterations in their diameters, for the proper distribution of the blood to various organs and tissues in accordance with their metabolic requirements.

From the standpoint of the physiology of exercise, the heart is primarily a respiratory organ. In the periods of rest between bouts of exercise, the muscles are able to store sufficient food materials to initiate exercise and to maintain it until reserves can be mobilized. There is, however, no mechanism for the storage of oxygen in the tissues. Any increase in the oxygen requirement must be satisfied by a corresponding increase in the transport of oxygen to the tissues. This is accomplished in two ways: (1) by diverting blood to the contracting muscles from less active regions and (2) by increasing the volume of blood pumped by the heart per minute. Not only is the flow of blood to the muscles increased when they become active, but in addition a larger volume of oxygen is removed from each volume of blood. Of these adaptive mechanisms, the increased pumping action of the heart is the most important in terms of the share it contributes to the adjustment of oxygen supply to the requirements of the contracting muscles. It is also the most vulnerable link in the chain; the upper limit to the volume of oxygen which can be delivered to active muscles is almost invariably set by the capacity of the heart to increase its output. For these reasons the heart occupies the key position in any scheme of the physiological adjustments in exercise.

Increase in the maximal oxygen intake

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Oxygen Intake

The practical advantage to the subject is that a greater proportion of the oxygen requirement for a given task is obtained without the accumulation of lactic acid and the acquisition of an oxygen debt. The average individual with a resting oxygen consumption of 250 ml. per minute may increase this to about 1500 ml. per minute during work, an increase of 600 per cent. In the athlete this margin may be raised to 3 or 4 liters per minute, an increase of 1200 to 1600 per cent.

Oxygen Debt

The maximal oxygen debt which can be tolerated by a subject is usually increased by training. This means, primarily, that the trained individual is able to buffer a larger amount of lactic acid since there is no evidence of any difference in ability to tolerate a higher degree of actual tissue acidity. It might be expected, then, that training would increase the body's supply of buffer alkali, and this seems to be true. An average alkali reserve or 72.12 volumes per cent in highly trained athletes as compared with 65.15 volumes per cent in normal but untrained men. A steady rise in the alkali reserve of dogs subjected to regular exercise on a treadmill for a period of 7 to 9 weeks.

It is probable, however, that an increase in the alkali reserve does not invariably occur. Thus, normal alkali reserves in five young athletes of international renown in distance running. Apparently, then, the ability to accumulate a large oxygen debt must depend partly on other factors as yet unidentified.

Influence of Training on Oxygen Requirement, Oxygen Intake and Oxygen Debt.

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The results of training may briefly be summarized as follows:

1. The oxygen requirement for a given task is diminished as a result of more efficient use of muscles and elimination of extraneous movements, and of greater mechanical efficiency of the muscles themselves.

2. The maximal oxygen intake is increased through improved capacity of the heart to pump blood, and through circulatory and respiratory adjustments.

3. The maximal oxygen debt which can be incurred is increased, probably due to an increase in the amount of buffer alkali available for neutralizing lactic acid.

Oxygen Requirement

Training increases the case with which work is performed. It is difficult to determine accurately the relative importance of increased skill and of increased mechanical efficiency of the muscles. In evaluating the influence of training on skill, it is unfair to compare athletes with nonathletes because skill is due partly to inborn aptitude. However, in a given individual, the improvement in skill with training is often striking and may be acquired early in the course of training. Thus in repeated observations on one man the improvement from training appeared to reach a maximum after only a few trial runs scattered over ten days. His net efficiency during this period rose from 17.7 to 20.6 per cent. Improvement in skill and consequent reduction in oxygen requirements are often noted in army recruits learning to march.

Improvement in the mechanical efficiency of muscles is due largely to the increase in diameter of each muscle fiber which results from continued usage. As a result, fewer fibers must contract for the development of a given amount of tension. This change takes place more gradually than does the increase in skill.

Oxygen Intake and Oxygen Debt as Limiting Factors in Exercise

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The maximum effort which can be exerted over a given period of time is limited by the maximum amount of oxygen which the subject can absorb per minute and by the maximum oxygen debt which he is able to contract. Since these are both measurable quantities, it should be possible to predict the limits of exertion for any individual from the results of laboratory tests (neglecting emotional factors). A welltrained athlete may be able to absorb 4 liters of oxygen per minute and to acquire an oxygen debt of 15 liters. It has been firmly established that, when the maximum oxygen debt has been incurred, the body becomes incapable of further effort. These facts permit one to estimate the duration of exertion which is possible when the oxygen requirement is greater than the maximal oxygen intake. 8 Assume that an athlete is able to absorb 4 liters of oxygen per minute and to incur an oxygen debt of 15 liters. If he runs at a speed requiring 5 liters of oxygen per minute, he must go into debt for oxygen at the rate of 1 liter per minute, and this intensity of exertion could be sustained for 15 minutes. If the speed of running is increased until the oxygen requirement is doubled, the excess of oxygen requirement over oxygen intake is 10--4 = 6 liters per minute, and exhaustion would occur at the end of 15/6 = 2.5 minutes.

In running, the oxygen requirement increases as the square or cube of the speed. Therefore doubling the rate of running increases the oxygen requirement per minute from 4 to 16 times. A man does not have time to incur his maximal oxygen debt in short sprints. It has been estimated that 50 to 55 seconds of running at top speed would be required before the maximal oxygen debt would be reached. Since the maximum amount of exertion which is possible before exhaustion occurs is determined by the upper limits of the oxygen intake and the oxygen debt, the question naturally arises as to the factors which set these limits. The factors limiting oxygen intake have been mentioned earlier in this chapter and will be discussed in detail in later chapters. So far as the oxygen debt is concerned, the practical limit seems to be set by the tissue acidity which results from lactic acid accumulation. Since lactic acid is a strong acid, it cannot exist as such in weakly alkaline media such as the blood and tissue fluids. As soon as lactic acid is formed during exercise, it is neutralized or "buffered" by alkali. As a result, the increase in acidity is much less than it would be if the acid remained in the free form. Presumably, the amount of lactic acid which the body can tolerate will depend largely on the amount of buffer alkali available. If this is large, more lactic acid can accumulate before the tissue acidity rises to all intolerable level. Meyerhof showed that the lactic acid concentration in the exhausted isolated muscle is much greater if the muscle is kept in all alkaline medium. Also in man the values of blood lactic acid following exhaustion are much higher if the man started the work with higher alkali reserve. This may mean that it is possible to increase the capacity for work by increasing the alkali reserve. That this is, indeed, true is indicated by experiments which demonstrated that the maximum blood lactic acid concentration following running to exhaustion on a treadmill was increased by the previous administration of a dose of sodium bicarbonate.

The Oxygen Debt of Exercise

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The ability of man to go into debt for oxygen is dependent, at least in part, on the formation of lactic acid. Hence the quantitative relation between the oxygen debt and the amount of lactic acid formed during exercise is of importance in an understanding of the limiting factors in exercise. Since a direct estimation of the total amount of lactic acid in the human body at any one time is, of course, impossible, it is necessary to rely partly on animal experiments and partly on inference from the blood lactic acid concentration. It is still uncertain whether blood lactic acid concentration is a true index of the lactic acid concentration in the corresponding tissues.

The equation for the oxidation of lactic acid may be written: C3H6O3 + 3 O2 → 3 CO2 + 3 H2O. Three molecules of oxygen are required for the oxidation of one molecule of lactic acid. In terms of gram equivalents, 3 × 22.4 liters of oxygen will oxidize 90 grams of lactic acid, or each gram of lactic acid requires 746.7 ml. of oxygen. In isolated frog muscle, only 150 ml. of oxygen are consumed for each grant of lactic acid which disappears, or about one-fifth the theoretical amount. This led to the belief that the oxidation of one-fifth of the lactic acid produced furnishes energy for the reconversion of the other four-fifths to glycogen. There is no direct evidence that any of the lactic acid is oxidized during recovery in oxygen.

It is not lactic acid that is oxidized in recovering muscle, but its equivalent in glucose. In this case all of the lactic acid would be reconverted to glycogen, while a quantity of glucose equivalent to approximately onefifth of the lactic acid so removed is oxidized. A portion of the energy thus liberated is used in the resynthesis of glycogen from lactic acid, while the rest appears as heat. In the exercising human subject, the quantitative relation between oxygen debt and lactic acid production is not so simple as it is in isolated frog muscle. Margaria, Edwards and Dill 6 have made a careful study of the relation of oxygen debt to metabolic rate and to lactic acid production. Some of their results necessitate a considerable revision of Hill's original theory. Two of their findings deserve particular emphasis: (1) in moderate exercise, the oxygen debt may reach 3 or 4 liters with no evidence of lactic acid accumulation, and (2) a considerable fraction (about one-third) of the total oxygen debt is repaid very rapidly (within three minutes) after the cessation of exercise, while repayment of the remainder of the debt may require several hours. Oxygen debts greater than 3 or 4 liters there is a linear relation between oxygen debt and blood lactic acid concentration. From these and other facts, the authors drew the following conclusions: The excess oxygen consumption following exercise is really made up of two fractions: (1) a true "oxygen debt" which is used to repay an oxygen deficit incurred during exercise and (2) an increased "basal" oxygen consumption which may last for several hour's or longer, and is not used for the reversal of any of the reactions which occurred during the exercise. The oxygen debt, in turn, consists of two fractions: (a) the "alactacid" debt, which is not related to the accumulation of lactic acid and is repaid within a few minutes after exercise ceases, and (b) the "lactacid" debt which is proportional to the lactic, acid accumulation and may require an hour or longer for repayment. The excess oxygen consumption of the alaclacid phase probably is used in the oxidation of the same fuels used by the resting muscle, and the resulting energy is perhaps used for the resynthesis of phosphocreatine or of adenosine triphosphate. The lactacid phase certainly represents the excess oxygen consumption which furnishes the energy for the reconversion of lactic acid to glycogen. The reason for the long duration of this phase is uncertain. The conversion of lactic acid to glycogen and the oxidation of glucose are both rapid reactions, while the oxidation of lactic acid is a relatively slow process. For this reason, Margaria and his associates suggest that it may be true, as Hill believed, that the energy for resynthesis of glycogen is derived from the oxidation of a portion of the lactic acid, and not from the oxidation of an equivalent amount of glucose, as indicated by other types of evidence.

Finally, from 4 to 5 per cent 7 of the total recovery oxygen is used by the heart and respiratory muscles and hence is not directly related to the recovery process.

Economy of Muscular Activity

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In activities involving an element of motor skill, the same amount of work is usually performed with a smaller oxygen requirement by the trained athlete. This is due primarily to the fact that the acquisition of motor skill results in the suppression or elimination of extraneous muscular movements which, while contributing nothing to the performance of the task, yet require oxygen.

Environmental Factors

The influence of external temperature and humidity on muscular exercise has been the subject of numerous studies, but the present status of our knowledge leaves much to be desired. The mechanical efficiency of the body is apparently little changed by rather wide changes in external temperature and humidity. Thus, it is found little difference in the oxygen requirement of men working on a bicycle ergometer in the cold room (54°F) and in the warm room (93°F). It is quite possible, however, that in activities requiring skill, high temperatures, by diminishing the accuracy of neuromuscular coordination, might result in a lowered mechanical efficiency. There is no doubt that working capacity (output of work in a given time) is diminished by unfavorable environmental factors, especially by a combination of high temperature and high humidity.

The Steady State

For short periods of time, it is possible to engage in exercise of such severity that the oxygen requirement far exceeds the oxygen intake. In continuous exercise lasting more than a few minutes, however, the oxygen intake must be adequate to meet the oxygen requirement. When this condition exists, the subject is said to be in a "steady state." He is in the state of approximate equilibrium between the processes of breakdown and recovery with respect to his muscle metabolism. When a subject is in genuine steady state, as, for example, at rest or during a long walk at constant speed, the oxygen consumption and carbon dioxide elimination are uniform, the lactate concentration in the blood, the heart rate and respiratory rate and the body temperature are all constant no matter how long, within reasonable limits, the exercise may last. When these "reasonable limits" have been exceeded, exercise may be terminated by accessory factors, such as muscle soreness, blisters or exhaustion of the glycogen reserves.

When a resting subject begins to exercise, the steady state is not achieved immediately. The circulatory and respiratory adjustments which make possible a greater oxygen intake come into play gradually and in heavy work several minutes may be required for the oxygen intake to reach the steady state level. During this preliminary period, a small oxygen debt is incurred which is repaid during the brief recovery period which follows the exercise.

Oxygen Requirement and Oxygen Intake

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The oxygen requirement of a given act is the volume of oxygen necessary for the performance of the act and for recovery. If the exercise is moderate, this requirement may be satisfied by the oxygen intake and recovery keeps pace with activity. If the exertion is severe, this relation breaks down and an oxygen debt is incurred, In this case, the oxygen requirement of the exercise is the sum of the oxygen intake during exercise anti the oxygen debt which is repaid during recovery.

The oxygen requirement of exercise is determined by a combination of factors, among the most important of which are: the severity or intensity of exercise, its duration, its speed, its economy, and certain environmental conditions, notably temperature and humidity.

Intensity of Work

Tile tension exerted by a contracting muscle is dependent on two factors: the number of fibers contracting and the frequency of their contraction. Muscle tone, which is based on the low frequency activation of a small proportion of the total number of muscle fibers, requires a very small oxygen intake. The same is true of a weak voluntary contraction. If a stronger contraction is needed, additional muscle fibers must be brought into activity and the frequency with which each fiber contracts must be increased; both of these adjustments increase the oxygen requirement of the muscle.

Duration of Work

Within certain limits, the oxygen requirement of work is directly proportional to its duration. If, however, the intensity is great enough, or the duration long enough to induce a state of fatigue, the oxygen requirement per unit of time usually begins to increase rapidly. This is easily explained by recalling the shape or the fatigue curve of an isolated muscle. As a muscle begins to tire the tension developed by each fiber is reduced, and hence more fibers must he brought into activity if the same level of work is to be maintained. The oxygen requirement is increased in proportion to the increased member of active muscle fibers.

Rate or Speed of Work

The relation between the oxygen requirement of work and the speed or performance is complex. For many types of work there is an optimal speed at which the oxygen requirement is minimal. If the work is performed at a slower or a faster rate, the mechanical efficiency is diminished and the oxygen requirement increased. The net result of two opposing factors determines the optimal speed of performance: (a) a rapidly contracting muscle has been shown to develop less tension than does a muscle contracting more slowly due to the limited rate at which the chemical changes underlying muscle contraction can occur; (b) a definite amount of energy is required to maintain tension in a muscle, once it is developed, and the slower the contraction, the greater is the proportion of the total energy used for this purpose (that is, tension must be maintained over a longer period of time in accomplishing a given amount of work at a slower rate of contraction). Factor (a) tends to make work more economical (lower oxygen requirement) at low speeds, while factor (b) results in a smaller oxygen requirement at high speeds. The worker, if left to his own devices, usually automatically adopts the optimal rate of working; this is, of course, impossible in assembly-line work.

There is another important type of work in which there is apparently no optimal speed. This is exemplified by the act of running. Table I indicates that the oxygen requirement of a 120 yard dash increases in direct proportion to the speed. Actually, the difference between these two types of activity is more apparent than real. In running, even at low speeds, the subject is usually exceeding the optimal speed for horizontal locomotion. In walking, on the other hand, there is a definite optimal speed of about 100 yards (120 steps) per minute.

Oxygen Requirements of Exercise

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The most fundamental problem confronting any organism, from ameba to man, is that of securing an adequate supply of oxygen. The very existence of living organisms depends on the constant availability of free energy, derived ultimately from oxidations. When any organ or system of the body becomes "active" (as when a resting muscle contracts, or when a gland begins to secrete), its oxygen requirement is proportionately increased. The oxygen consumption of the human body under basal conditions (12 to 14 hours after eating and following a 30 minute period of resting in bed, at a comfortable room temperature) is approximately 200 to 250 ml. per minute. During maximal physical exertion this may rise to 4000 ml. per minute, or 15 to 20 times the basal consumption. This remarkable factor of safety involves the coordinated adjustment of the heart and circulation, the blood and the respiratory system.

While all the tissues of the body require oxygen for their continued activity, some are able to function for short periods at a level of intensity which far exceeds their capacity for obtaining oxygen. They do this by calling on anaerobic sources of energy such as the breakdown of glycogen to lactic acid. This is a wasteful procedure, as compared with oxidation, since only a portion of the energy of the fuel is liberated. Nevertheless, it serves an extremely useful function in enabling the organism to meet emergency situations requiring an energy expenditure in excess of that which can be supplied by oxidations alone. It must be emphasized, however, that anaerobic metabolism is not a substitute for oxidation--it merely postpones it. Following a period of activity powered by anaerobic energy, the anaerobic reactions must be reversed in order to restore the fuel supply and to remove acid metabolites, and this is accomplished by oxidative energy. During the period of exercise the subject has gone into debt for oxygen and this debt must be repaid during recovery.

While some of the organs of the body arc able to meet emergencies by incurring an oxygen debt, others, notably the heart and brain, cannot. These vital organs depend entirely on oxidative energy and their function begins to suffer as soon as their oxygen supply falls short of their requirement. The oxygen requirement of the brain is not particularly affected by exercise, lint that of the heart may be tremendously increased, and it will be demonstrated in a later chaplet that the inability of the heart to obtain its full oxygen requirement is one of the most important factors which limits the intensity and duration of exercise.

Influence of Diet on Muscular Efficiency

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Many attempts have been made to demonstrate a greater mechanical efficiency of the body (less energy expenditure in the performance of a given amount of work) on a diet in which one or another of the major foodstuffs predominates. The modern opinion, is that the efficiency is practically the same on all diets. There is a slight increase in efficiency following a high carbohydrate diet, but probably not more than 5 per cent.

The influence of diet is more clear-cut on another aspect of muscular activity, the working capacity or ability to perform work without early onset of fatigue. Experiments indicated that a high fat diet resulted in greater distress and an earlier onset of fatigue titan did an average mixed diet. Working at an intensity of 7,800 foot-pounds per minute, was aide to continue three times as long on a carbohydrate diet as on a fat diet. This may be associated with the fact that fat metabolism is more prone to result in the production of acid metabolites than is the metabolism of carbohydrate, and a rise in tissue acidity is generally regarded as favoring the onset of fatigue.

Carbohydrate is of primary importance as a fuel for muscular exercise in man. During prolonged work, eventual depletion of the carbohydrate stores (principally liver and muscle glycogen) may force the muscles to obtain a significant proportion of their energy from fat and protein. This is probably an indirect process, involving a preliminary conversion to carbohydrate before oxidation. Muscular efficiency is slightly greater, and the onset of fatigue is postponed, on a high carbohydrate diet. The blood sugar is normal or elevated in light and moderate work and in strenuous work of short duration, so that "priming" with sugar is of no particular advantage. In prolonged strenuous exertion, exhaustion may result from lowering of the blood sugar brought on by depletion of liver glycogen; the administration of sugar is often very definitely beneficial.There is no evidence that hard physical work necessitates an increased intake of protein.

Utilization of Protein and Fat by Muscle

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The metabolism of protein in the body is ordinarily measured by determining the amount of nitrogen excreted in the urine. The fact that exercise increases the excretion of nitrogen has been confirmed by later workers, but the conclusion that this indicates utilization of protein as a fuel by the muscles has been questioned since there is no direct relationship between the amount of nitrogen excreted and the amount of work done.

The possibility that the increased nitrogen excretion may be due to altered kidney function is suggested by the finding of albuminuria following severe exercise. The rise in urinary nitrogen after exercise was abolished by feeding carbohydrate before the exercise. It is possible that in long-continued work, exhaustion of the carbohydrate supplies results in the conversion of protein into carbohydrate, thus making protein an indirect fuel for muscular work.

From a practical viewpoint, the available evidence indicates that protein is of minor importance as a source of muscular energy. There is no scientific support for the idea that athletes and workers doing hard physical labor require a larger protein intake than do sedentary persons. Only when tissues are being built or repaired at an increased rate, as in growing children and convalescent patients, is an increased protein intake necessary. By tile same token, when total muscle mass is being increased by a course of muscle training, there may be some necessity for increasing the protein content of tile diet.

Blood Glucose

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Regardless of the form in which carbohydrate is ingested, it is converted by the digestive enzymes to glucose or similar "simple sugars" and then absorbed into the blood. If there were no provision for rapid storage of glucose in the tissues, its concentration in the blood would rise steeply following a meal, and much of it would be excreted by the kidney and hence lost to the body. Fortunately, much of the glucose absorbed into the blood is removed by the liver and the skeletal muscles and stored as glycogen. The conversion of glucose to glycogen ("glycogenesis") is regulated by the hormone insulin. In the disease diabetes, insulin is deficient or absent and the blood sugar is characteristically high.

The tissues of the body are constantly removing glucose from the blood to use in their metabolic processes and corresponding amounts of glucose must be added to the blood from reserve stores. This is accomplished by the breakdown of glycogen to glucose ("glycogenolysis") under the influence of another hormone, adrenalin. This reaction occurs primarily in the liver and to a lesser extent in the skeletal muscles. The net result is that glucose is stored as glycogen following meals and then reconverted to glucose at a rate sufficient to balance the withdrawal of glucose from the blood by active tissues.

During light exercise, the ordinary rate of delivery of glucose to the blood from the storage depots is adequate to balance the rate of glucose utilization by the muscles, and the blood sugar level is unchanged. As exercise increases in intensity, especially if it is accompanied by emotional excitement, the secretion of adrenalin by the adrenal glands becomes excessive (insofar as blood sugar regulation is concerned) and glucose is added to the blood from the glycogen storage reservoirs at a faster rate than the metabolic activities of the contracting muscles require. The result is a rise in the blood sugar concentration. This effect is more pronounced in intermittent than in continuous exertion. If the exercise is both strenuous and prolonged (as in a marathon race), the blood sugar level often shows a gradual fall, sometimes to half the normal value. This is interpreted as indicating exhaustion of available glycogen stores.

The foregoing discussion has definite implications for the practical management of the diet of athletes. In the first place, the evidence, though conflicting in certain respects, clearly emphasizes the importance of carbohydrate in the energy metabolism of muscle. There is no evidence to support the common belief that candy and other sweets should be restricted during training, unless they diminish the appetite and thus reduce the food intake at the regular meals. On the other hand, since prolonged and exhausting exercise is required to lower the blood sugar level, there would seem to be no practical advantage in the administration of sugar in various forms before or during most types of athletic contests. Any apparent increase in performance which may result is probably psychological in origin. The situation is somewhat different in the case of exhausting exercise which is prolonged over a period of hours. Here the exhaustion of glycogen reserves and the consequent lowering of the blood sugar may be a dominant factor in bringing about complete exhaustion.

It is worth pointing out that the outstanding symptoms of extreme physical exhaustion such as incoordination of movements, collapse and unconsciousness are referable not to the muscles but to the central nervous system. The brain, unlike skeletal muscles, has no available carbohydrate stores and cannot fall back on the metabolism or other substances when its glucose supply is curtailed. It depends, from moment to moment, on the glucose (and lactic acid) brought to it by the blood; when the blood glucose level falls, brain function is depressed and unconsciousness usually occurs when the blood sugar concentration drops below 40 mg. per cent.

Respiratory Quotient

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When foodstuffs are oxidized, oxygen is consumed and carbon dioxide is produced. The ratio of the volume of carbon dioxide produced to the volume of oxygen consumed is called the respiratory quotient (R.Q.); its numerical value varies according to the type of food substance which is being oxidized. Thus, in the oxidation of glucose, represented by the reaction C6H12o6 + 6O2 → 6CO2 + 6H2O, 6 molecules of oxygen are consumed and 6 molecules of carbon dioxide produced, for each molecule of glucose burned. In this case, the ratio =1.00. A similar type of calculation yields values of 0.71 for the R.Q. of fat and of 0.80 for protein. In practice, the expired air of the subject is collected for a given period and the percentage of oxygen and of carbon dioxide determined by analysis. Knowing the composition of the inspired air and the volume and composition of the expired air, the volumes of oxygen consumed and of carbon dioxide produced (and hence the R.Q.) are easily calculated. The grams of protein oxidized are calculated from the urinary nitrogen (1 gram of urinary nitrogen reprethe metabolism of 6.25 grams of protein). The total R.Q. is corrected for protein metabolism by subtracting the oxygen consumed and carbon dioxide produced in the oxidation of protein from the total oxygen consumed and carbon dioxide produced. The resulting ratio of is called the "non-protein" R.Q.; from it the relative proportions of fat and of carbohydrate oxidized are obtained from standard tables. Under resting conditions the R.Q. gives a fairly reliable indication of the relative proportions of carbohydrate, fat, and protein which are being oxidized, although any considerable degree of interconversion of the major foodstuffs will seriously affect the ratio. The respiratory quotient is a reliable index of the fuel used in exercise only when the entire recovery period is included in the study. The reason for this is found in certain of the chemical reactions which accompany exercise and recovery. During moderate to strenuous exercise, more lactic acid is formed in the contracting muscles than can be neutralized by the muscle buffers. Much of the excess lactic acid diffuses into the blood stream where it is buffered by the various blood buffers of which the most abundant is sodium bicarbonate (NaHCO3) according to the reactions:

HL + NaHCO3 → NaL + H2CO3 (L represents the lactate ion)

H2CO3 → C02 + H20

As a result of this reaction, a large amount of carbon dioxide is eliminated in the expired air which did not result directly from oxidation and which, therefore, did not involve a corresponding consumption of oxygen. The ratio will be elevated and is frequently greater than 1.0 (R.Q.'s as high as 2.0 may be obtained during very strenuous exercise).

During recovery the lactate produced during exercise is gradually removed by oxidation, urinary excretion, and reconversion to glycogen. As a result the alkali with which this lactate had been combined is available to combine with carbon dioxide once more, so that much of the carbon dioxide produced by oxidations during the recovery period will be retained in the blood instead of being eliminated in the expired air. Consequently, the ratio will be depressed, and R.Q.'s as low as 0.5 may be obtained. This ratio returns to normal when recovery from exercise is complete. If, however, the expired air is collected from the beginning of exercise to the end of tile recovery period, the excess CO2 elimination during exercise is balanced by a corresponding amount of CO: retention during recovery and the correct R.Q. may be calculated. In practice it is often difficult to determine accurately when recovery has been completed, since the oxygen consumption may remain above the preexercise level for some hours. The adoption of an arbitrary time period of recovery is sometimes necessary, and probably results in only minor inaccuracies in the final calculations.

The Fuel of Muscular Exercise

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There are three major types of foodstuffs which can be oxidized in the body, namely: carbohydrates, fats, and proteins. Our problem is to determine the relative importance of each of these as a source of energy for muscular work of varying degrees of intensity and duration. This problem is complicated by the fact that a considerable amount of interconversion of the major foodstuffs occurs in the body. A common example of this process is the conversion of carbohydrate into fat in the commercial "fattening" of livestock. It is now believed that carbohydrates and fats are mutually interconvertible in the body and that the amino acids which make up the protein molecule are likewise capable, after certain metabolic modifications, of being converted to carbohydrate.

This makes it difficult to draw conclusions about oxidative sources of energy from a simple analysis of the constituents of the diet and necessitates the use of indirect methods of study.

A knowledge of the types of foodstuffs undergoing oxidation during muscular exertion is not only important from a nutritional standpoint, but is essential to a clear understanding of the chemical basis of muscle contraction anti of the resulting energy liberation. The experimental investigation of this fundamental problem is still far from complete and quite divergent results have been obtained by different workers.

Nature of the Shortening of Muscle

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The most obvious feature of the contraction of a muscle is the shortening which occurs. The nature of this shortening has been the subject of many experiments and more speculations, yet a final answer is not available. Recently, a protein called actomyosin, has been isolated from muscle. Threads made from this protein can be made to contract in response to chemical stimulation and it is suggested that this protein may be a prominent constituent of the contractile elements of muscle fibers, the myofibrils.

A relaxed muscle is like a stretched spring, and contracts when stimulated as a spring does when released. Energy must then be used to cause the muscle to relax (i. e., to stretch the spring) in preparation for another contraction. An opposing view is that shortening of the muscle is the active process which requires energy, and that relaxation occurs passively when stimulation ceases.

Chemical Changes in Muscle During Contraction and Recovery

Muscle is essentially a machine for transforming chemical energy into mechanical energy which can do work.

When an isolated frog muscle is stimulated to the point of fatigue, its glycogen (starch) content is greatly diminished and there is a considerable accumulation of lactic acid. If the muscle is then allowed to rest in an atmosphere of oxygen, the lactic acid disappears and the muscle recovers its irritability. It was then postulated that the source(.e of energy for muscle contraction is the breakdown of glycogen to lactic acid. a process which is anaerobic (i. e., does not require oxygen). During the recovery period, oxygen is necessary to burn the lactic acid.

The lactic acid is foxed after contraction has occurred, so that the breakdown of glycogen to lactic acid must provide energy for recovery of muscle, not for its contraction. The ATP molecule contains a large amount of energy which is released when the molecule is split. This released energy, by its effect on actomyosin, is the basic source of energy for muscle contraction. The oxidation of carbohydrate (glycogen and glucose), through a complex series of reactions, provides the energy for rebuilding the high energy ATP molecules. When abundant oxygen is present, carbohydrate breakdown proceeds to its final end products--carbon dioxide and water. If the oxygen supply is not adequate, carbohydrate breakdown proceeds along a different pathway which results in lactic acid formation. If the lactic acid is allowed to accumulate in excessive amounts, the chemical reactions are interfered with and the muscle can no longer be made to contract. In moderate exercise, energy requirements are fully met by oxidations, and no lactic acid is formed.

Chemical Changes in Muscle During Contraction and Recovery

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Muscle may be regarded as a machine which transforms chemical energy into work. The mechanism of this conversion remains obscure despite the enormous amount of research which has been directed toward its solution. Accordingly, the views which are presented here must be regarded as tentative and subject to almost certain modification as new data become available. The difficulty is that, although the time course of tension development and heat production can be followed quite accurately, there are no methods sufficiently accurate and rapid to permit a study of the chemical changes which accompany and follow a single twitch. In fact it is necessary to stimulate a muscle for several seconds involving scores of contractions before the chemical changes become great enough to permit accurate analysis. In the discussion which follows an attempt will be made to correlate some of the more striking chemical changes with the phases of activity and recovery in skeletal muscle under both aerobic and anaerobic conditions.

Carbohydrate Metabolism

The metabolism of carbohydrate in muscle begins with glycogen, a form of animal starch. The exact chemical structure of glycogen is uncertain, but it is known to be built up by the combination of large numbers of glucose molecules with the splitting out of water. Glycogen is the common storage form of carbohydrate in the body, and is found in large amounts particularly in the liver and in skeletal muscles. The lactic acid (an organic acid produced by the partial breakdown of carbohydrate) accumulates in muscles contracting to the point of fatigue and disappears during recovery of the muscles if adequate oxygen is present. Fletcher and Hopkins concluded that lactic acid production (probably from glycogen) is the fundamental chemical reaction producing energy for muscle contraction; they suggested that the development of tension may be due to an action of lactic acid on the colloidal protein contractile substance in muscle. According to this relatively simple theory contraction itself is an anaerobic process, and oxygen is necessary only for the oxidative removal of lactic acid, since its accumulation leads to loss of irritability of muscle.

Much of the lactic acid is formed after contraction and relaxation are over, so that the formation of lactic acid cannot be an immediate source of energy for muscle contraction. Lactic acid production in normal muscle contraction is associated, either directly or indirectly, with the development of tension in anaerobic contractions, but there is no satisfactory evidence that lactic acid is produced under aerobic conditions. Some lactic acid may be formed during short periods of tetanic contraction in intact cat muscle, but there is some doubt whether the oxygen supply was completely adequate in these experiments.

The Metabolism of Muscle

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When muscles contract, energy is liberated. If a muscle shortens against a load (isotonic contraction) work is done and part of the energy is thus accounted for; the rest appears as heat. If the muscle is unable to shorten (isometric contraction), all the energy which is liberated is eventually dissipated as heat. In either ease, the liberation of energy is the result of certain chemical reactions which occur within the muscle fibers. Since the total energy of contraction appears as heat in an isometric contraction, and since this heat can be measured very accurately, an analysis of the time course of heat liberation in this type of contraction furnishes valuable cities to the nature of the chemical reactions in tile contracting muscle. This is all the more important in view of the fact that the chemical changes themselves are so small and so fleeting that they can be measured accurately only after a tetanus or a series of twitches and little information is ained about tile sequence of these changes. For these reasons, it is well to begin our study of the metabolism of muscle with a brief consideration of the heat liberated during an isometric twitch.

The Heat Production of Muscle

The contraction of muscle is associated with the metabolic breakdown of certain chemical compounds. During recovery these chemical compounds must be rebuilt and waste products removed if the original contractile power of the muscle is to be restored. Both of these processes yield heat. While this heat production serves a useful purpose in maintaining body temperature in cold environments, it is an inevitable by-product of muscular activity which may render exertion in a hot environment disagreeable or even impossible. It represents energy consumed with no mechanical work produced and hence is a measure of the inefficiency of our muscles considered as machines.

When a muscle performs a simple twitch, heat is liberated in two fairly distinct bursts. The first, which is called initial heat, accompanies the development and subsidence of tension. Hartree 1 has shown that the initial heat of a twitch occurs in two phases: (1) the contraction heat, which rises and then falls off during the developmerit of tension, and (2) the relaxation heat, which is liberated during the subsidence of tension. After the contraction is over, there is a second and slower production of heat, the delayed heat, also called the recovery heat since it is due to chemical changes which restore the muscle to the condition in which it was before its response. When a muscle contracts tetanically, there is a further heat production during the maintenance of the contraction.

We usually associate heat production in living tissues with the oxidation of energy-yielding compounds. However, many chemical reactions of a non-oxidative nature also result in the liberation of heat. Heat which is liberated only when oxygen is present must be due to oxidative processes and is frequently called aerobic heat: heat which is liberated in the absence of oxygen must be due Io non-oxidative processes and is referred to as anaerobic heat. A study of the heat production of muscle under both aerobic and anaerobic conditions yields data from which we may draw certain conclusions about the general types of reactions which occur during contraction, relaxalion and recovery in muscle. The lime course and magnitude of the initial heat are essentially the same whether the muscle is contracting in an atmosphere of oxygen or one of nitrogen. Thus, the chemical reactions which are associated with actual contraction and relaxation of the isolated muscle are presumably non-oxidative, or anaerobic. The magnitude of the delayed heat is greatly diminished in the absenee of oxygen, so that oxidation plays an important role in the recovery of muscle from the effects of contraction. Apparently the energy for contraction of muscle is liberated by the "explosive" breakdown of compounds with high potential energy, and oxygen is not necessary for this breakdown. During recovery these compounds must be rebuilt in order that energy may be available for subsequent contractions. This process of rebuilding requires energy which is obtained, at least in part, from oxidations.

Phasic Contractions; Reflex and Volitional Movements

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Phasic contractions result in movement; they may be either reflex or volitional in origin. A reflex movement is one which occurs as the result of the stimulation of receptors located at or near the surface of the body. Nerve impulses resulting from stimulation of the receptors travel over sensory nerve fibers to the spinal cord. Here the impulses are transmitted either directly or by way of intermediate neurones to the motor nerve cells. Tire central processes of the sensory nerve fibers terminate as tiny knobs or "end-feet" in contact with the dendrites and eell body of the motor neurone. This junction is called a synapse; its very great susceptibility to adverse conditions, including fatigue, is of particular significance to students of exercise physiology. The motor nerve cell discharges over the motor nerve fiber to the muscle where the impulses must cross another type of junction, the motor end plate, which is a specialized mass of tissue intervening between the nerve fiber and the sarcoplasm of the muscle fiber, lake the synapse, but to a lesser degree, the motor end plate is readily rendered non-functional by fatigue and the action of certain drugs. The simple reflex arc as here analyzed consists of the following structures: receptor, sensory nerve fiber, synapse, motor nerve cell and fiber, motor end plate, muscle fiber. The reflexes which utilize this simple type of circuit are called spinal reflexes, since they are mediated through the spinal cord without the intervention of higher centers.

Spinal reflex activity is primitive and unlearned in spite of its seeming purposiveness. The character of the reflex response to stimulation of any particular area of the body surface is determined largely by the type of receptor stimulated. Thus, painful stimulation of the sole of the foot causes flexion of the leg which withdraws the foot from contact with the injurious agent. On the other hand, gentle pressure applied to the same point elicits extension of the leg due to stimulation of pressure receptors just under the skin (this "extensor thrust" reflex is an integral part of the whole reflex mechanism of walking, and is normally operative on contact of the sole of the foot with the ground).

All reflexes are not of this simple spinal type. Some involve higher brain centers (for example, the instantaneous turning of the head toward the source of a sudden, loud noise). Despite their greater complexity of nervous pathways, however, the general principles are the same as for the simpler types: a reflex is an invariable, predictable response to stimulation of a particular type of receptor, and the response always accomplishes a useful purpose which is related to the nature of the stimulus.

Volitional movements are initiated by impulses which are discharged from certain areas of the cerebral cortex. Cortical (volitional) activity differs from reflex activity in several respects, one of the most important of which is that, unlike spinal activity, it is unpredictable. This results from the fact that it is determined not only by the nature of the immediate stimulus (if any) lint also by the stored memories of past experiences.

The region of the cerebral cortex which discharges the impulses witch bring about voluntary movements is called the motor area; it is a narrow strip of tissue located just in front of the central fissure. The motor impulses arise in the large pyramidal cells and are transmitted along a bundle of nerve fibers, the pyramidal tract, down through the brain stem and spinal cord to terminate by synapsing with ventral horn (motor) cells of the spinal cord. Most of the pyramidal tibet's cross to the opposite side in passing through the brain stem, so that the motor area on the left side controls the muscles on the right side of the body, and vice versa.

Electrical stimulation of the motor area reveals that the muscles of each region of the body have their own distinct controlling areas; stimulation of the arm area o the cortex, for example, results in contraction of the arm muscles of the opposite side of the body. The size of the cortical area which controls the activity of a given group of muscles is determined not by the size of the muscle group, but rather by the complexity of its activity. For example, the cortical area which controls tinge/' movements is much larger than the cortical area for the entire trunk musculature. Another very significant fact revealed by electrical stimulation of the motor area is that this area controls the contractions of single, discrete muscles or small groups of muscles---quite different from the complex group contractions which characterize voluntary activity. If, however, the region just in front of the motor area (known as the "pre-motor area") is stimulated, complex group movements (flexion and extension, pronation, and supination) are elicited. Since this type of response cannot be obtained if the connections between the motor and the pre-motor areas are cut, it is evident that impulses originating in the pre-motor area travel to the motor area and there stimulate the pyramidal cells. What is the significance of this indirect method of control? The answer has been obtained from clinical eases in which the pre-motor area has been destroyed. There is no paralysis of voluntary muscles, but the ability to execute complex, learned types of skilled activities, such as playing the piano, is lost. We are now in a position to analyze the way in which motor skills are acquired by practice. In the beginning, performance of the component parts of the activity requires constant attention; it is controlled by the motor area. Gradually, smoothness and accuracy of performance are developed and constant attention is no longer required; control of the activity has now been transferred to the pre-motor area.

While the pre-motor area of the cerebral cortex thus initiates the muscle contractions which make up skilled activities and ensures that they are performed in proper sequence, other portions of the nervous system are required to adjust the strength, duration and range of muscle movements. The cerebellum, through its connections with the motor areas or' the cortex on the one hand, and with the proprioceptors of the muscles and joints on the other, is the key structure in this coordination. As a muscle begins to contract its muscle spindles and tendon organs are stimulated, and some of the impulses are transmitted up the spinal cord to the cerebellum. In this way the cerebellum is kept constantly informed of the strength of muscle contractions and of the range of movement at the joints. Through its connections with the motor arcas of the cortex, it is then able to increase or decrease cortical motor activity and thus adjust the strength, dural ion and range of muscle movements to the requirements of the act. With training visual memory may largely supplant tile more primitive cerebral mechanism.

Adjustment of the range or extent of movement necessary to accomplish a given act is largely a matter of experience. We learn to correlate our visual impressions of the necessary range of movemerit with the corresponding proprioceptive impulses from the muscle spindles, so that eventually we are able to make this adjustment in the absence of visual stimulation. For example, in learning to type, we must guide the movement of our fingers to the appropriate keys by sight. With practice proprioceptive impulses are sufficient to guide our movements. If the cerebellum is injured proprioceptive information is faulty, and our tendency is to over-reach or underreach the keys; constant visual guidance becomes necessary.

There is evidence that the sensory receptors in the muscles and joints may suffer fatigue in exhausting exercise. This may partially account for the faulty neuromuscular coordination which is often associated with extreme fatigue.

Postural Mechanisms and Their Control

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Skeletal muscles normally display a firmness which is due to a slight sustained contraction of a fraction of the muscle fibers (muscle lone or tonus). This tone is most pronounced in those muscles which keep the head erect and the jaw closed, and which prevent the body from sagging at the hip and knee joints, that is, in those muscles which maintain the body in the erect position against the force of gravity. Hence, they are commonly referred to as the antigravity muscles.

Muscle tone is reflex in nature. The sensory side of the reflex has its origin in receptors (specialized sensory nerve endings) located in the muscle itself (muscle spindles) and in the tendon which the muscle attaches to bone (tendon organs). Sensory receptors of this type, which are stimulated tension or pressure, are called proprioceptors. When a muscle is stretched, certain of these receptors are stimulated and nerve impulses are transmitted to the central nervous system where they stimulate the motor nerve cells which supply this same muscle. The discharge of these nerve cells results in contraction of the muscle, the strength of the contraction being proportional to the degree of stretching of the muscle. This stretch reflex is one of the basic elements in the origin of muscle tone. Since it is especially well developed in the extensor (antigravity) muscles, it plays an important part in the maintenance of body posture. For example, if the body is to be held erect, the tendency to flex the hip and knee joints under the influence of gravity must be counteracted. If the knee joint begins to buckle, the extensor muscles of the knee joint are stretched and their proprioceptors are stimulated. The resulting reflex contraction of the extensor muscles then straightens the knee joint and preserves the upright posture.

While muscle tone and the basic patterns of posture involve local reflexes, they are definitely under the influence of higher portions of the nervous system. If man is deprived of these higher centers he is unable to maintain normal posture, even though the basic reflex mechanisms are still intact. Also, loss of consciousness, as in fainting or in sleep, abolishes the normal control of posture and the body crumples under its own weight. A detailed analysis of the control of posture by the higher nervous centers is beyond the scope of this book. Suffice it to say that some of these influences increase muscle tone while others diminish it and that many portions of the brain are involved, among them being the cerebral cortex, the cerebellum and the proprioceptors of the inner ear.

Another group of postural reactions which is of paramount importance in sports is concerned with the maintenance of equilibrium or balance. In complex motor skill activities it is essential that the body should be in the correct posture for the performance of the necessary movements. A boxer who is staggered by a blow, or a football player who stumbles while running, "automatically" makes compensatory movements which tend to restore the normal erect posture. These movements are not "thought out"; they can occur in the absence of the cerebral cortex so that they must be considered as complex reflex patterns.

There are three major sources of sensory impulses which initiate these reflex movements: (1) visual stimulation, (2) proprioceptors in the inner ear (the semicircular canals and the otolith organs) and (3) stretch receptors (muscle spindles) in the neck muscles. The example of the football player who stumbles and begins to fall may clarify the operation of the balance mechanisms. The abnormal position of the head in space results in stimulation of nerve endings in the retina of the eye and in the otolith organs of the inner ear. The reflex thus initiated tends to restore the head to its normal position in space through contraction of the appropriate neck muscles. Contraction of the neck muscles causes stimulation of the muscle spindles in these muscles and initiates reflex movements of the arms, the trunk and the legs which serve to restore the rest of the body to its normal upright position. The abnormal movement of the head during falling and also during the performance of gymnastic maneuvers, such as turning somersaults or cartwheels, initiates reflexes originating in the semicircular canals of the inner ear which produce the same types of corrective movements as those described above.

Postural tone is a characteristic of muscles which are engaged in maintaining the body in some static position or attitude. When movement occurs, this posture must temporarily give way, else it might retard the prompt execution of the movement. Accordingly it is found that phasic contractions (those which result in movement as opposed to posture) are accompanied by a temporary decrease in the tone of the antagonistic muscles. This phenomenon is known as reciprocal inhibition of antagonistic muscles. For perfect neuromuscular coordination it is essential that the speed and degree of relaxation of muscles opposing a movement be accurately adjusted to the speed and range of contraction of the muscles effecting a movement. A failure of antagonistic muscles to relax promptly is one of the factors which results in poor performance in exercise which is not preceded by a "warming up" period.

Warming Up Performance is improved

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Warming Up

Performance is improved if the muscles have been slightly warmed up just before the activity. Most baseball pitchers, for example, work better on warm days. Failure to warm up before vigorous activity may lead to an actual tearing loose of muscle fibers from their tendinous attachments.

Observations on the contraction of isolated muscles provide a clue to the nature of the warming up process. If the muscle is warmed, the speed with which the muscle contracts and relaxes and the force of contraction are all increased. If a previously inactive muscle is stimulated repeatedly, the first few contractions are often small and irregular and relaxation is incomplete. After this, the contractions become stronger and relaxation is complete. It is probable that warming up is due in part to these changes in the muscle itself, involving a local rise in temperature and the accumulation of metabolic products. It is possible that the viscosity of the muscle is thereby decreased, allowing contraction and relaxation to occur with greater promptness. In the body these same factors also increase the local blood flow through the muscle by dilating the small blood vessels. This improves the functional condition of the muscle by increasing its oxygen supply.

The muscles most frequently torn during strenuous activity which has not been preceded by a warming up period are the antagonists to the strong contracting muscles. These "cold" antagonistic muscles relax slowly and incompletely when the agonists contract and thus retard free movement and accurate coordination. At the same time, the force of contraction of the agonists and the momentum of the moving part exert a terrific strain on the unyielding antagonists with consequent tearing of the muscle fibers or their tendinous attachments.

Muscular Pain, Soreness and Stiffness

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During and following strenuous muscular exercise, particularly in untrained objects, there may be muscular pain, soreness and stiffness.

Muscular pain commonly occurs during exercise, while soreness and stiffness usually appear some hours later. It is well known that when muscles are forced to work without adequate blood supply (for example, rapid flexion and extension of the fingers with the circulation occluded by a blood pressure cuff) severe pain results. The inadequate blood flow results in failure of complete removal of the products of muscle metabolism, and it is probable that the pain of strenuous exercise is due to accumulation of acid metabolites which irritate the receptor organs of pain located in the muscles.

Fluid collects in muscles during activity and a number of hours may be required for its reabsorption into the blood stream. The resulting swelling of the muscle causes it to become shorter and thicker and more resistant to stretching. This gives rise to a sensation of stiffness when the muscle is stretched during the contraction of antagonistic muscles.

The cause of muscle soreness is not completely understood. Two types of muscle soreness have been postulated:

(1) general soreness due to the presence of diffusible metabolic waste products which usually disappears within three or four hours after the cessation of exercise, and (2) localized soreness or lameness which appears eight to twenty-four hours after exercise and may persist for several days. The second type of soreness is probably due to the rupture of muscle fibers or of the sarcolemma which transmits the contraction to the tendon. The less frequently used fibers and the sarcolemma covering them are probably more susceptible to strain than are the fibers more frequently used in ordinary contractions. The generalized type of soreness is alleviated by light work which hastens the circulatory removal of the metabolic waste products, while the localized lameness, which is due to actual injury, needs rest with heat and only enough exercise to prevent adhesions between the injured muscle fibers.

Muscle Tone, Muscle Fatigue, Cause of Fatigue

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Muscles in the body are normally firm to the touch. This is due to the continuous slight contraction of a small fraction of the component fibers producing what is known as muscle tone. This tone disappears when the nerve supply to the muscle is destroyed, as occurs in poliomyelitis, so that it is clearly due to the constant arrival of low frequency, asynchronous impulses from the spinal cord. This concept of the nature of muscle tone has been questioned because no electrical potentials could be recorded from electrodes thrust into resting muscles.

Muscle Fatigue

When an excised muscle is stimulated repeatedly at a frequency of about once per second, the height of each contraction eventually begins to decrease. Not only is the amount of shortening diminished, but also the relaxation becomes slower and incomplete (contracture). Finally the muscle fails to respond even to the strongest stimulation, that is, its irritability is completely lost. This diminished capacity for response which results from previous activity is called fatigue.

Cause of Fatigue

If a fatigue excised muscle is cut across and the cut surface tested with litmus paper, it is found that the interior of the muscle is acid. Since the normal muscle gives an alkaline reaction with litmus, it is apparent that fatigue is associated with an accumulation of acid. Chemical analysis reveals that the amount of glycogen (energy-yielding carbohydrate) is less in the fatigued muscle than in the normal muscle. These experiments suggest that fatigue may be due to the accumulation of acid waste products which decrease the irritability of the muscle or to exhaustion of stored fuel supplies. The accumulation of acid waste products (largely lactic acid) in the excised muscle is due in large part to the absence of a normal circulation of blood. As a result the amount of oxygen supplied to the muscle is not sufficient to oxidize the lactic acid nor can it be removed from the muscle by diffusion into the circulating blood. Conditions, are, of course different in the case of muscles in the body. Here the fuel is constantly being replenished by way of the blood; the oxygen supply is adequate to oxidize most, if not all, of the lactic acid produced and much of that which is not oxidized nor reconverted to glycogen diffuses into the blood and is carried away from the muscle. As a result, muscles in the body can perform large amounts of work before their capacity for response is abolished by fatigue. In fact, it is doubtful whether the muscle fatigue of this degree ever occurs in normal exercise.

Site of Fatigue

If the motor nerve of a muscle-nerve preparation is stimulated repeatedly, the muscle eventually fails to contract. If the stimulating electrode is now placed directly on the muscle, contractions of almost normal height are obtained showing that the muscle itself is not fatigued. Since nerve fibers have been shown to be practically nonfatiguable, the fatigue in this case must be in the junction between the nerve and the muscle, the neuromuscular junction or motor end plate. This junction is also the site of action of numerous drugs and chemical agents which may influence muscular work in the body.

If a muscle in the body is voluntarily fatigued by repeated contraction and the motor nerve then stimulated electrically through the skin, strong contractions of the "fatigued" muscle may be obtained. In this case the fatigue cannot have been in the muscle, its nerve or the neuromuscular junction, but rather in the brain or in the spinal cord from which the motor nerve arises.

Summation of Contractions Tetanus

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When a muscle is stimulated twice in such rapid succession that the second stimulus falls during the response to the first, the tension developed is greater than in the single twitch. In some way, as yet not clearly understood, the tension developed as a result of the second stimulus adds to the tension remaining from the first stimulus. If a continuous series of rapidly repeated stimuli is sent into a muscle there is not sufficient time for relaxation between successive contractions and the result is a steady, prolonged contraction known as tetanus. The tension developed during tetanus may be three or four times that of a simple twitch in the muscle. If the rate of stimulation is not rapid enough to produce complete tetanus, there may be partial relaxation between contractions The result is a jerky type of contraction known as incomplete tetanus.

The All-or-none Law and the Motor Unit

If a muscle is stimulated with gradually increasing strengths of current, the tension developed increases progressively up to a certain point; beyond this point there is no increase in tension with further increase in stimulus strength. With increasing strength of stimulation the tension developed by each muscle fiber remains constant, but more fibers are activated. When a muscle is stimulated each fiber contracts maximally or not at all (the "all-or-none law"). It must be remembered, however, that the amount of tension developed by a maximal contraction will vary with such factors as fatigue and training.

It was mentioned earlier that in the body muscles contract only in response to nerve impulses from motor nerve cells in the spinal cord, and further that each motor nerve cell, through terminal branching of its nerve fiber (axon), supplies a large number (100 to 150) of muscle fibers; the motor nerve cell with its nerve fiber and the group of muscle cells supplied by its branches comprise the motor unit. The group of nerve cells in the spinal cord which together give rise to the motor nerve fibers to an entire skeletal muscle is called the motor pool. If the motor pool of a given muscle consists, for example, of 300 nerve cells, the tension developed by the muscle during contraction may theoretically be increased by 300 steps, each representing the activation of an additional motor unit.

Since the tension developed by each motor unit also varies according to the frequency of stimulation, which determines whether the response will be either a series of simple twitches, an incomplete tetanus or a complete tetanus, we have an additional means of varying the tension developed by the muscle. In brief, then, the strength of contraction of a muscle in the body can be varied by two means: (1) variation in the number of motor units activated and (2) variation in the frequency of stimulation of each active motor unit.

One additional point requires consideration. If an isolated muscle is stimulated maximally (that is, with a current sufficiently strong to stimulate every fiber in the muscle) but at a low frequency, the resulting contraction will be jerky because of the partial relaxation between contractions. Since voluntary and reflex contractions in the body are normally smooth, it was thought at one time that they must be invariably tetanic in nature. It is now known, however, that complete tetanus occurs in very powerful contractions and that most contractions involve incomplete tetanus in a variable proportion of the motor units. The smoothness of the contraction is due to the fact that the different motor units are not activated simultaneously; at any instant, the muscle fibers of some units are contracting while those of other units are relaxing. This asynchronous activity of the motor units results in smooth contractions even when the force of contraction is weak. If, as happens in certain nervous disorders, the contractions of the different motor units become synchronized, a jerky type of contraction known as a tremor results.

We are now in a position to understand the mechanism of adjustment of the strength of contraction of muscles to the task required. For example, it is obvious that a more powerful contraction is required to lift a 50 pound weight than to lift a 1 pound weight. A weak voluntary (or reflex) contraction is the result of partial tetanus in a fraction of the motor units responding asynchronously. A very powerful contraction is the result of more nearly complete tetanus in a larger fraction of the motor units of a muscle.

The Contraction of Muscle

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An intact skeletal muscle in the body normally contracts only in response to stimuli (nerve impulses) reaching it from the central nervous system. This stimulus-response mechanism is highly complex and is more easily understood after a consideration of the simpler response if an isolated muscle to direct electrical stimulation.

The Simple Muscle Twitch

If the gastrocnemius muscle of a frog is isolated, one end secured in a rigid clamp and the other end attached to a muscle lever, the response of the muscle to stimulation may be recorded on a moving strip of smoked paper. Electrical shocks are used for stimulation, because of the ease of adjustment of strength, duration and frequency, and because they do not injure the muscle.

If a single shock is applied to the muscle a simple muscle twitch results. Although it is unlikely that this simple type of contraction ever occurs in the body, it is none-the-less deserving of study, because it is the building stone of more complex reactions. Inspection of a typical record shows that the twitch may be divided into three intervals: the latent period, the period of contraction, and the period of relaxation.

The latent period is the time which elapses between the application of the stimulus and the first appearance of shortening (or the exertion of tension in the case of an isometric twitch). Most of the latent period recorded in ordinary laboratory experiments is due to instrumental inertia. With highly accurate techniques the true latent period is less than 1 millisecond (0.001 second). However, the actual latent period of muscle contractions in the body is considerably longer, since bony levers with a high degree of inertia must be activated.

The rising phase of the contraction curve represents shortening of the muscle and the more prolonged falling phase indicates the slower relaxation process. Both the relative and the absolute durations of the different phases vary with certain factors, such as fatigue and temperature changes. While the form of the contraction curve is similar for all skeletal muscles, the total duration of the twitch is very different for different types of muscles. Thus the extraocular muscles which move the eyeball are extremely rapid (twitch duration = 7.5 msec.), while at the other extreme the soleus, a red muscle, has a very long twitch duration (94 to 120 msec.).

There is a clear relation between the speed of contraction and the function of a muscle. The pale (white) muscles contract and relax rapidly and are ordinarily involved in acts requiring rapid movement. The red muscles, on the other hand, contract and relax slowly so that the tension developed in a single twitch is maintained over a longer period of time; this makes them well adapted for their role in maintaining posture with minimal expenditure of energy. Flexor muscles contain predominantly pale fibers which are designed for speed and are easily fatigued. Extensor muscles contain predominantly red fibers which are slower in their action but are capable of greater endurance. Because of their slower relaxation time the extensor muscles are frequently pulled in exercise, especially when not properly warmed. Thus, warming up should precede strenuous physical activity. Fast movements of light objects are most easily made with flexors. Slower or sustained activities are most easily performed with the extensors.

Organization of Muscle Fibers in a Muscle

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Groups of 100 to 150 muscle fibers are bound together with connective tissue to form a unit known as a fasciculus. Groups of fasciculi are bound together into still larger units which in turn are bound together to form the muscle itself; it is likewise invested with a connective tissue sheath. At each end of the muscle this connective tissue merges with tile tendon bundles which attach to the bone.

Nerve and Blood Supply of Skeletal Muscles

The sarcolemma of each muscle fiber insulates it from adjacent fibers, so that excitalion of one fiber does not directly affect neighboring fibers. Hence, each muscle fiber must be supplied with a separate motor nerve twig. A motor nerve supplying a muscle is composed of numerous nerve fibers, each originating from a separate nerve cell in the spinal cord. In the substance of the muscle each nerve fiber breaks up into many branches and each branch penetrates the sarcolemma of a single muscle fiber to terminate in a specialized mass of protoplasm known as a motor end plate. When a single motor nerve cell in the spinal cord discharges impulses to a muscle, all the muscle fibers supplied by branches of tile corresponding nerve fiber are stimulated and contract together. This group of muscle fibers, which forms the smallest functional unit under normal conditions, is believed to correspond to the fasciculus mentioned above. A single motor nerve cell in the spinal cord together with its nerve fiber and the group of muscle fibers supplied by its branches form the basic neuromuscular unit commonly referred to as the motor unit.

In addition to the motor nerve supply, skeletal muscles are also supplied with sensory nerve endings, some of which lie between the groups of muscle fibers while others are associated with muscle tendons. These sensory nerve endings are stimulated by changes in tension in the muscle (contraction, relaxation, stretching) and send impulses to the central nervous system. These impulses play an important role in maintaining muscle tone, in informing the central nervous system of the position of parts of the body with respect to one another and in adjusting the rate and extent of muscle movements.

Each muscle receives blood through one or more arteries. These arteries break up into profuse capillary networks in the connective tissue surrounding each muscle fiber. Under resting conditions many of these capillaries are closed, but they open when the muscle undergoes vigorous contraction. In this way the blood supply to a muscle is adjusted in accordance with the degree of muscular activity.

Types of Contractile Tissues in the Body

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The contractile tissues of the body arc composed of cells which are able to exert tension by decreasing their length. In the human body there are three principal types of contractile tissue: skeletal muscle, heart muscle and smooth muscle, each of which has certain distinct structural and functional features. The characteristic result of the shortening of muscle is movement, either of parts of the skeleton (skeletal muscle), or of the contents of hollow organs (heart muscle and smooth muscle). Skeletal muscle is characterized structurally by the presence of distinct cross striations; hence, it is frequently called striated muscle. It contracts and relaxes much more rapidly than do the other types of muscle and normally becomes active only in response to stimulation from the central nervous system. Smooth muscle has no cross striations; it contracts and relaxes very sluggishly and is capable of contracting in the absence of stimulation from the central nervous system, although its activity may be increased or decreased by extraneous nerves. It is usually found in the walls of hollow organs or tubes such as the digestive tract, the blood vessels, the ureters and urinary bladder, and the uterus. When the muscle cells contract, the volume of these hollow organs is decreased and their contents are moved onward. Heart muscle is intermediate between skeletal muscle and smooth muscle, both structurally and functionally; like skeletal muscle its cells are cross striated, and like smooth muscle it is capable of contracting in the absence of extraneous nervous stimulation. Unlike skeletal or smooth muscle, heart muscle cells are not separated from one another, but form a continuous network or syncytium, so that all its fibers contract at each beat of the heart.

Structure of Skeletal Muscle

A skeletal muscle, such as the biceps, is composed of thousands of muscle fibers bound together with connective tissue. Each fiber is an elongated cell varying in length from 1 to 40 mm. The thickness of the fibers varies from 10 to 100 μ or more (1 μ=0.001 mm.); apparently the thickness depends not on the length of the fiber, but on the type of animal and the particular muscle. In a given animal the more primitive muscles, such as those of the eye, have thinner fibers. Fibers of varying diameter may be found in the same muscle, perhaps indicating different amounts of usage since the thickness of fibers is known to increase under the influence of strenuous muscular activity.

Each muscle fiber is covered by a thin structureless membrane, the sarcolemma. The fiber itself consists of two parts: (1) a protoplasmic mass, the sarcoplasm, and (2) very thin cross-striated fibrils, the myofibrils, which are arranged parallel to one another in the sarcoplasm. A great deal of discussion has centered around the existence of myofibrils in living muscle cells (many workers claiming that they arc artifacts resulting from killing and staining procedures) and their possible role in the contraction process. Their existence in living muscle cells is now apparently well established and most authorities attribute to them a fundamental role in the contraction process. If a single fiber from a fresh muscle is examined under the microscope both longitudinal and transverse striations are seen. The transverse striation of the muscle fiber results from the fact that each myofibril consists of alternate light and dark segments and the corresponding segments of adjacent fibrils lie at the same level, forming light and dark bands passing completely across the fiber. During contraction complex changes occur in the position and relative thickness of these light and dark bands. These changes, representing reorientation of molecules in the myofibrils, are believed to be the fundamental basis of contraction.

The cytoplasm of the muscle fiber which fills the spaces between the myofibrils is called sarcoplasm. The relative amount of sarcoplasm in the fiber varies. In some fibers it is more abundant and contains pigment granules (muscle hemoglobin) which give it a reddish appearance (red muscle), while in others it is less abundant and the fiber is paler (white muscle). Red muscles are capable of slow, powerful contractions and are not easily fatigued. The diaphragm and the extensor (postural) muscles are composed predominantly of red fibers. White fibers are specialized for speed rather than strength of contraction and predominate in the flexor muscles. In man both types of fibers enter into the composition of every muscle; the relative proportions varying according to the function of the particular muscle. Recently it has been found that if the tendon of a red muscle is cut and then sewed to the tendon stump of a pale muscle (i.e., forced to take over the function of a pale muscle), its hemoglobin content and resistance to fatigue gradually diminish. This indicates that the appearance and endurance of a muscle are largely the result of the type of work it must perform.

Heart Size, Negro vs. White, Superior ability of Negroes

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Heart Size

Heart size is related to constitutional type and thoracic circumference. Measurements of heart size in 233 athletes by orthodiagrams indicate moderate hypertrophy as compared with Hodges-Eyster standards. Such hypertrophy is judged to be normal for athletes. 30 If the larger than average heart is associated with a high vital capacity, the athlete usually also has a slower than average pulse rate and low blood pressure and possesses a large capacity for performance of work.

Negro vs. White

Negroes show a higher rate of sickness absenteeism in industry, but as the occupations and socio-economic status of the Negro and white males become more nearly alike the excess of the frequency rate of disabilities among Negroes tends to decrease, if not to disappear entirely. Negroes can stand humid heat better than white workers. The rate of sickness absenteeism in very humid climates is lower in Negroes than in whites.

The superior ability of Negroes to work in the heat is due in part to greater efficiency, a higher ratio of body surface to weight, the secretion of a more dilute sweat and a greater water intake. Negro and white sharecroppers and a group of northern whites were tested during a summer in Mississippi. Subjects walked for 2 hours on a motor driven treadmill at a rate which elevated the metabolism to 7 times basal. Room temperature averaged 88° F. and humidity 79 per cent. This work raised the body temperature of the partially acclimatized northern white men to intolerable levels, increased their heart rates to nearly maximal values and forced most of the men to discontinue the walk before the end of the two hours. Negro sharecroppers who were acclimatized by field work performed the walk without marked elevation of body temperature or heart rate. They perspired less but drank more water than did the northern whites. White sharecroppers, also accustomed to field work, were intermediate between the other groups in adaptability to the conditions of the experiment. The superiority of the Negro sharecroppers was related to lower energy requirements in performing the walk. Negro servants were no more successful than northern whites in regulating body temperature, but were able to sweat at much higher rates. Most of the Negroes reached an equilibrium with a rectal temperature of about 101° F. and a pulse rate of 150 as compared with about 102° F. and 170 in the white workers. Sweating was of the order of one to two liters an hour. The major item was superior cardiovascular condition. Final mechanical efficiencies were 25.6 per cent in Negroes and 27.5 per cent in whites. Of 23 Negroes, two were below the white average, and of 7 whites, one was better than the average Negro.

In maximal work Negroes reach a greater blood lactate concentration, have a slightly lower maximum oxygen consumption and breathe more rapidly, thus using a smaller fraction of the vital capacity as tidal air. The higher blood lactate indicates greater motivation. Psychological explanation on a similar basis is advanced for the frequent supremacy of the Negro in athletic competition.

Structural Differences Affecting Performance

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Variations in the position of the muscular attachments, the structure of the joints and the lengths of the bony levers all affect the performance of physical activity. Usually when the anatomical structure is particularly advantageous for strength, it is disadvantageous for speed. Large joints are strong, but they also may limit motion. Long distance runners are observed to have the flattest longitudinal arches and sprinters have smaller feet and higher arches.

Although physique plays an important part in severe exercise it plays only a minor role in determining a person's response to moderate exercise. Severe exercise is best performed by those with a slim body build. In general, those with thin legs have better endurance. Those with a normal or stocky build have, on the average, the greatest speed, the greatest strength and the greatest ability to sustain prolonged moderate muscular effort. Good muscular development is required for success in activities requiring speed and strength.

Size and Energy Exchange During Work

Differences in body size affect the energy exchange during physical activity. Even at rest a large man may consume as much as 58 per cent more oxygen than a small man. During light work (simulating operation of an airplane), the large man may require 42 per cent more oxygen than the small man. Among men the correlation coefficient between the oxygen consumption and body surface was plus 0.76 at rest and plus 0.82 at work. Because of the larger energy requirement during work the large man accumulates heat under conditions in which the smaller man attains a heat balance. The ratio of heat production to surface area in a 218 pound man was observed to be 20 per cent greater than in a 97 pound man.

Body Type, Feminine Body Build, Structure and Work

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Body Type

Structure and Work

There exists a mutual interrelation between body structure and heavy physical work. First, activity modifies structure. The result of many years of using a shovel or performing on a particular piece of gymnastic apparatus is a change in physique which characterizes the worker or the athlete. Second, structure modifies activity. The differences in inherent structure affect the performance of work and sport. The linear individual is at a disadvanrage in work and sport requiring the carrying of heavy loads or the reception of body blows. The lateral type yields to the linear in activities involving a wide range of movement.

A demonstration that activity modifies structure is provided in the asymmetry of strongly right or left handed individuals. There is a marked difference in size between the right and left upper extremities. Measurements repeated from time to time show that the superiority of the right upper extremity over the left increases with age, demonstrating the effects of development through use. Asymmetry in somatic development is induced by certain types of sports and gymnastics. Swimming gives the most symmetrical development.

Feminine Body Build

The fear among young women that vigorous physical activity may result in masculinity of build is unfounded. Hard physical labor during youth does not tend toward the development of a significant degree of masculinity in the build of young American Negro women. Young women who already have masculine builds have some advantage in athletic ability, but muscular strength and power are of much greater importance to athletic performance than is body build. Adolescent girls with broad, lateral builds possess a physical capacity superior to girls with the linear type of body build.

Response to Exercise

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The physiologic responses of young women to moderate exercise do not differ sufficiently from those of men to necessitate wide differentiation of physical performance standards. The only difference in the response to moderate exercise is a more rapid and a greater increase of the pulse rate in women. When the work is strenuous, more significant differences in response appear. The endurance in running of women is only half that of men. The maximum pulmonary ventilation, oxygen intake and respiratory quotient arc higher in men. In general, the sex differences are similar to those found between men of low and high fitness for exercise. There is considerable overlapping between the groups and the performance of the best women is comparable to that of the poorest men. In another study, Ceylonese girls gave consistently lower mean fitness indices at all ages in both moderate and severe exercise.

The difference in performance ability between the two sexes is not apparent until after puberty. At puberty, development of performance ability stops or even declines in females, while it continues to advance at a slightly retarded rate in males. At puberty, boys begin to excel in strength and endurance. Maximum endurance declines rapidly in girls after 14 years of age.

Male and female are equally efficient at all ages in skilled work of mild intensity. No significant sex differences in work decrement are found when women and men work together at routine tasks. Women, in general, tend to drop in the level of production sooner, and men have a slightly higher level of performance except in small manipulative operations such as handwriting. Out of nineteen hundred different operations in twenty-one industries, only 331 are unfit for women.

Physical Capacity of Adults

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Men in the seventh or eighth decades have lost one-half of their capacity for transforming energy aerobically. The ability to supply blood to active tissue has decreased and skeletal muscles are weaker. Nevertheless, when working within the limit of their ability, many men well advanced in years can carry on long-continued work with a lower heart rate and less evidence of fatigue than that exhibited by young men. The capacity for anaerobic work appears to be maximal in the early twenties and to be small in young boys and old men. The highest lactate levels are reached in the twenties, while there is only a slight lactate increase in the hardest work attempted by young boys or old men. Work or sport activities with the emphasis on speed and strength are best suited to young men. Older men are best fitted for jobs or sports requiring skill, coordination, and endurance.

Sickness Absenteeism in Industry

Absenteeism increases with age so that the total rate is about twice as high in the age group 50 to 59 as in the age group 20 to 40. Age increases the average number of days lost per absence (severity rate) more than the average number of absences per one thousand workers (frequency rate). The increase of absenteeism with age is most pronounced in work performed under conditions of high humidity. It appears that an age over 35 years is a definite handicap for work in high humidity.

The increase of absenteeism with age is more pronounced with men than with women, so that the discrepancy between men and women is greatest in the younger age groups. The difference in the sickness rate between unemployed men and women is even higher than that between employed men and women workers. Tile greater rate of absenteeism among women is an inherent characteristic and accumulated fatigue from a greater amount of extra work is a secondary factor.

Limits of physical ability among individuals

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Limits of physical ability among individuals are determined in part by body structure. Variations due to structure are exhibited when well-trained athletes perform ill competition. In the competitive situation the athletes have received motivation and training sufficient to enable them to approach the physiological limits of function of the critical organs. The difference ill performance then affected by differences in structure. Variations in structure which affect performance are found among different age groups, between the two sexes, among different body types and among different racial groups.

Individuals with wide differences in structure may all be able to execute the same kind of movement, but the difference in the quality of execution of the movement shown by the individuals results largely from variations in the amount of strength, speed, skill and endurance which is possessed by each. Furthermore, a piece of work which requires only a low grade of organic function does not differentiate individuals in a population. It is not until the work increased in intensity or complexity, requiring coordinate increases in strength, speed, skill or endurance that differences among individuals become apparent.

Responses of Young Ohildren to Exercise

Physiological studies of the responses of young children to exhaustive exercise are understandingly difficult to perform. Special exercise equipment which includes motivational devices must be constructed 1 since young children are usually unwilling to exert maximal effort. Difficulties in taking blood samples and fitting face masks for respiratory and metabolic determinations also complicate the collection of data. Furthermore, children are usually not readily available to physiological laboratories. The great mass of physiological data available to date has been collected predominantly from adult laboratory workers, medical and physical education students and athletes. At present, data on young children are insufficient to describe their physiological responses to strenuous exercise with a satisfactory degree of accuracy or completeness.

The physiological systems of younger children are apparently not so sufficiently developed to meet the demands of strenuous exercise as they become when puberty is reached. Children under 12 years of age possess a highly active sympathetic nervous system which predisposes to a high heart rate and an easily depleted capacity for endurance activities such as running. Children under 12 do not have the capacity to utilize oxygen that older boys do because of a relatively smaller stroke volume of the heart and a consequent smaller capacity for increased circulation of blood through the lungs. The younger boys also possess a lesser supply of carbohydrate fuel.

Special Foods, Sugar, Gelatin (glycine), Salt, Potassium

2008-02-27T00:39:00.000-08:00

Sugar

The athlete in training should have extra carbohydrate in his diet. An additional quantity of sugar fed to an athlete who is to perform only a short bout of work will not increase his efficiency nor forestall early exhaustion. The efficiency of 10 minute work periods on a bicycle, ergometer is not affected by feeding glucose or sucrose. In a series of weight lifting tests the ingestion of glucose had no appreciable effect.

The administration of sugar may improve performance in athletic events which are so strenous and prolonged as to result in hypoglycemia and extreme fatigue. Thus, subjects who worked to the point of exhaustion on a bicycle ergometer were able to continue their exertion for an additional hour if hypoglycemia was prevented. A phenomenal continuous work output was recorded in a dog who was given glucose at frequent intervals. Glucose has been shown to have very beneficial effects in marathon races.

In certain circumstances a large intake of glucose can limit physical output. If physical work is started after the ingestion of a considerable quantity of glucose, at the moment when tile post-prandial blood sugar curve begins to fall, a sudden and considerable drop in the sugar content of the blood may sometimes hamper the performance of the work.

Gelatin (glycine)

The theory that glycine, which constitutes about 25 per cent of gelatin, enhances the supply of energy-yielding phosphocreatine by promoting the formation of creatine in the muscles has led to the belief that gelatin may increase muscular power and endurance. Gelatin administration, however, is entirely without effect on the composition of muscle, not only with respect to creatine but also as regards creatine phosphate, hexose phosphate, total nitrogen, glycogen and adenosine triphosphate. 7 A critical review of conflicting reports on this subject 8 indicates that if an effect of glycine exists it must be temperamental in its occurrence.

Salt

Heavy physical effort in great heat entails a loss of salt, especially sodium chloride, of such magnitude an to cause a complex of symptoms variously described as miner's cramp or heat cramp. In such cases, an additional ration of salt becomes essential. In prolonged athletic contests in which excitement and increased body temperature result in a loss of body salt along with water, sodium chloride given with water during the contest will prevent a salt deficiency and the consequent reduction in standards of performance. Salt feeding during a contest is especially indicated if the salt content of the participant's diet has previously been cut down in order to prevent thirst. In short sporting events doses of sodium chloride arc seldom of advantage. Salt in concentrated closes may cause discomfort, nausea, and vomiting.

Potassium

Exercised muscles lose potassium as fatigue develops. Injection of potassium chloride delays muscular fatigue and restores the working capacity of a fatigued muscle. Execess potassium is rapidly excreted by the kidneys, so that an attempt to store reserves of potassium by massive doses is useless. Instead, ample potassium should be provided in the diet in order to replace losses during hard muscular work.

Energy Foods, Performance Assay

2008-02-27T00:37:00.000-08:00

Energy Foods

From time to time special foodstuffs are advertised as having special qualities of providing "extra" energy or o f otherwise increasing the strength, endurance, speed or skill of museular work. These are, at best, only calorics in an expensive form.

A wholesome diet which contains adequate amounts of the essential foodstuffs needs no supplements for most physical labor and sports competition. If the labor is very heavy or if the competitive event is prolonged, a bar of candy and a sweetened beverage taken at intervals of about one and one-half hours are usually sufficient to restore energy substances in the body to a normal level.

Performance Assay

An analysis of experiments which purport to demonstrate improved performance due to drugs or special food preparations in excess of normal requirements has failed to confirm the claims in most cases. Changes in performance attributed to these foods and drugs are often due to training or to practice of the event. In some studies the work was not measured exactly and experimental conditions were otherwise unsatisfactory. In studies in which the experimental substance was not masked, the improved performance may very well have been due entirely to the suggestive influence of the advertised beneficial properties of the substance. As the result of such analysis claims for the beneficial effects of the following substances upon physical performance have been discounted: lecithin in eggs and soya-beans, vitamin B1 in yeast, vitamin C, liver preparations, phosphates, calcium, alkalis, oxygen, ultra-violet radiation, digitalis, Coramine, Cardiazol and adrenocortical hormone.

A discussion of the action of the various foods and drugs which have been shown to affect work performance is presented no that their use may be evaluated.

Special foods and drugs have been used in work and sport

2008-02-27T00:35:00.000-08:00

Special foods and drugs have been used in work and sport in order to combat the fatigue which limits performance. In prolonged hard work supplies of nutrients normally stored in the muscles, blood and liver are consumed and work capacity is gradually diminished until exhaustion occurs. Addition of food substances before critical levels are reached enables activity to be extended.Drugs may act to improve performance or to hasten recovery from fatigue in many ways:

1. Mobilize stored energy substances.

2. Increase the strength of contraction of skeletal muscle fibers.

3. Increase the power of the heart to eject blood.

4. Increase circulation and facilitate the transport of fuel and waste.

5. Counteract depressant action of fatigue substances.

6. Stimulate the central nervous system so as to forestall the sensation of fatigue.

In connection with the last item, the sensation of fatigue is a normal safeguard which serves to prevent severe displacement of the homeostatic equilibria and to guard against injury to the organs which are under the stress of the activity. Drugs which delay the sensation of fatigue, therefore, are potent tally dangerous and a re justified only in extreme emergencies or while the individual is under the care of a physician. Certainly no athletic performance falls under this emergency classification.

Pre-game Meal

2008-02-27T00:33:00.000-08:00

The pre-game meal should be eaten at least two hours before performance begins so that the major portion of the food has been emptied from the stomach. When vigorous exercise is performed while the stomach is distended by food, the inspiratory descent of the diaphragm may be impaired. This perhaps explains why a heavy meal seems to "cut the wind." It is conceivable also that a stomach distended by food may encroach upon the action of the heart during heavy exercise, especially restricting filling. The consequent restriction of blood flow through the heart would ultimately affect endurance. Exercise probably does not affect the rate of digestion unless the activity is associated with psychic stress.

The meal should consist mainly of carbohydrates, the remainder including a moderate quantity of protein (lean meat), and a very small amount of easily digested fat (butter). The items of food should not include those which have been observed by the athlete to lie gasforming or in any other way distressing. The food should be well prepared and attractively served. A half hour before tile performanee the athlete may enjoy a chocolate bar. At half time or between events he may refresh himself with a soft drink and another piece of candy. If a glucose preparation, honey, mr any other uncommon food is to be used at the time of competition, it should he first introduced during training as it may be found to cause nausea or other discomfort. It should be pointed out that any benefit resulting from the eating of candy or other carbohydrate preparations during a moderately short contest is probably entirely psychological, since the blood sugar level is already higher than normal.

Alkalinization, Distribution of Meals

2008-02-27T00:32:00.000-08:00

Alkalinization

The fact that severe muscular work produces fixed acids which lower both pH and alkali reserve indicates that increasing the alkali reserve might benefit the performance of exercise. The maximal accumulation of lactate in the blood is greater when the alkali reserve is high. The extra alkali reserve allows more prompt and complete neutralization of lactic acid as it is formed and hence the higher level of blood lactate may indicate a better neutralization. Changes in the alkali reserve as a result of dietary alterations probably are not large enough to affect the ability of normal persons to perform muscular work.

Distribution of Meals

The effect of various distributions of meals throughout the day upon work performance indicates that frequent feeding is desirable. Three meals a day are superior to two meals a day with respect to physical efficiency. Production in industry increases when midforenoon and midafternoon snacks are provided. From the point of view of industrial efficiency workers should be allowed a reasonable freedom of choice of what they eat and drink during the rest periods.

A breakfast or coffee alone is insufficient to support a full morning of activity. A light beakfast consisting of fruit, one slice of buttered toast, milk, and coffee' if desired, will increase the level of performance in maximum work output and choice reaction time, and also decrease tremor magnitude during the latter part of the morning. A heavy breakfast consisting of fruit, cereal and cream, egg, bacon, toast and jam, milk, and coffee, if desired, is also better than coffee alone. It is doubtful that the physiologic responses after a heavy breakfast are better than after a light breakfast, especially if the morning's work is not strenuous.

Vitamin A, B, C, D Deficiencies

2008-02-27T00:29:00.000-08:00

Vitamin A Deficiency

In men maintained for six months on less than 100 international units of vitamin A daily after a preliminary massive dose period of thirty days there was no change in plasma vitamin A levels, visual thresholds or scores in moderate and exhausting work tests. The fact that vitamin A did not disappear from the plasma in these subjects indicates that their reserves were not exhausted. The depletion of vitamin A stores in the body lakes a very long time so that there is a substantial margin of safety in most cases.

Performance may be restricted by lack of vitamin A because of the effects on vision. People subsisting for long periods on diets very low in vitamin A develop night blindness. Vitamin A intake may affect color vision. The daily administration of carotene to factory workers was followed by a 75 per cent reduction in the number of rejections for off-color parts of stoves assembled by them. Among normal persons subsisting on diets not markedly deficient in vitamin A there is little relation between vitamin A intake, blood vitamin A concerttration and dark adaptation.

Vitamin B Deficiency

Diets containing about one-third of the recommended optimum intake of vitamin B complex do not lead to deterioration in physical capacity within two weeks if the physical and environmental stresses are not great. Subjects put on a vitamin deficient diet containing 0.16 mg. thiamine, 0.15 mg. riboflavin and 1.8 mg. niacin per 1,000 Calories displayed no change in work capacity, psychomotor test scores or clinical condition during the two weeks' deficiency period. 22 A similar lack of effect was observed when riboflavin only was limited to 0.31 mg. per 1,000 Calories.

Thiamine is concerned in carbohydrate metabolism. When thiamine is deficient, lactic and pyruvic acids accumulate in abnormal amounts and muscular activity is depressed. Ten healthy men placed on a more severe thiamine restriction, doing harder physical labor and in a colder environmental temperature, showed a clear diminution in physical test scores in one week. A trained subject who was fed a diet deficient in thiamine and riboflavin during a hot summer showed a significant decrease in performance on a bicycle ergometer.

Vitamin B deficiency over a long period produces a deterioration in fitness for exercise even in the absence of hard physical labor or environmental stress. Subjects on a reduced thiamine, riboflavin and ascorbic acid intake during a twelve-week period experienced a reduction in work performance.

The requirements for thiamine, riboflavin and niacin are probably proportional to the intensity of the metabolism. It is justifiable to advise an increased intake of B vitamins when work is increased and when the stress of the environment is great. With a reasonably good diet this is guaranteed by simply increasing the total food intake.

Vitamin C Deficiency

Low levels of vitamin C intake do not measurably reduce work performance. Men receiving only 40 mg. of ascorbic acid daily did not differ significantly in fitness for exercise, nor in susceptibility to heat exhaustion, from men receiving 540 mg. daily.

Vitamin E Deficiency

Rats on diets deficient in vitamin E show degenerative changes in muscles, and diminished capacity for treadmill running. Whether man shows similar responses to vitamin E deficiency has not yet been determined.

Vitamins in the normal diet are essential for good performance

2008-02-27T00:27:00.002-08:00

Vitamins

Vitamins in the normal diet are essential for good performance in work and sport. Following the principle that if a little of something is good, then a large amount of the same thing should be better, the discovery and identification of vitamins led to the administration of large doses of these substances in the hope that such a superdiet would result in superperformance. Where vitamins were lacking in the diet these superdiets did improve performance but extra vitamins added to normal diets did not improve performance.

Healthy young men expending an average of 3700 to 4200 Calories per day are not benefitted by a daily dietary supply in excess of 1.7 mg. thiamin chloride, 2.4 mg. riboflavin and 70 mg. ascorbic acid. Fortification of normal diets with vitamins does not improve fitness for work and exercise. This has been shown in static and dynamic work tests, muscular endurance tests, work in heavy industry and treadmill running tests. In neither brief, exhausting exercise nor in prolonged, severe exercise are there any indications of benefit due to dietary supplements of thiamin chloride, cocarboxylase, riboflavin, nicotinic acid, pyridoxin, pantothenic acid, and ascorbic acid or to vitamin B complex given intravenously.

Vitamins are stored in varying amounts in the body. When diets are deficient in vitamins, the stores are depleted and the consequent deterioration in physical condition results in a reduction in the capacity for muscular work. The effects of deficiencies of the various vitamins are discussed separately because of the specific torture of the results in each case.

Fats are usually reduced in the diet

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Fats

Fats are usually reduced in the diet of athletes in training. One reason for reducing the proportion of fat in the training diet is that fat is not directly used as a fuel for supporting muscular exercise. Even after fasting, the stimulated gastrocnemii of rats show no appreciable change in fat content whereas carbohydrate stores are much reduced. It is not until men undergo severe and protracted work leading to hypoglycemia that there is an increased utilization of fat. Subjects maintained on a fat-rich diet showed a smaller net muscular efficiency than when they were on a carbohydrate-rich diet.

By plotting R.Q. against net efficiency, extrapolation to pure fat metabolism (R.Q. of 0.70) indicated 11 per cent greater energy expenditure per unit of work than for pure carbohydrate metabolism (R.Q. of 1.0).

Even though fat-rich diets are associated with reduced endurance and muscular efficiency, the exclusion of all fat from the diet is not recommended. Aside from the important vitamins and minerals and certain essential fatty acids in fats, there appears to be a fatcontained factor that is necessary for the normal metabolism of carbohydrate. A disturbed carbohydrate metabolism has been observed in rats maintained more than a year on a fat-deficient diet. In the light of the above considerations it would seem that the amount of fat in the diet which will maintain a good nutritional condition and yet not materially reduce endurance or muscular efficiency is about 100 grams daily. Normal intake of fat should not supply more than 45 per cent of the total calories. The fat content of the diet during training for an athletic event in which particularly high levels of performance are desired may be reduced to 75 grams or 25 per cent of the total calories per day. The proteins and carbohydrates in this training diet would be approximately 15 per cent and 60 per cent, respectively, of the total calories per day.

Protein in the form of meat

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Protein

Protein in the form of meat is consumed in large quantities by athletes and men doing hard physical work. The quantity of protein needed for bringing about the highest capacity for work is probably much less than the amounts ordinarily consumed by active men. A long distance racing cyclist who was a vegetarian performed with higher gross efficiencies on a low protein diet, but had better endurance on high protein diets. 8 Another man lived for years on a diet that provided a daily intake of only about thirty grams of protein. His physical efficiency was high and severe exercise could be carried out without increasing the nitrogen excretion.

A carefully controlled study of the effect of restricted protein on men working in the heat revealed no essential change in fitness for work in either temperate or tropical environments when dietary protein was varied from 76 to 105 and 149 grams per day, although a 4 per cent higher work metabolism was associated with the highest protein intake.

To summarize, the dietary protein intake of the athlete may vary widely (75 to 150 grams daily) without any noticeable effect upon performance of intermittent (speed) activities. Where work is prolonged (endurance), higher intakes of dietary protein accompanying higher calorie diets may be beneficial.

Dietary Ratio of Carbohydrate, Fat and Protein

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As a result of observations of changes in performance records of highly trained athletes on different diets containing various proportions of the food principles, the following generalization has been made: a diet of 150 grams of fat, 700 grams of carbohydrate and 100 grams of protein plus adequate minerals and vitamins daily was recommended for distance events, and 100 to 130 grams of fat, 350 to 400 grams of carbohydrate and 210 grams of protein, mostly meat, was recommended for speed events. 5 These figures must, at best, be considered as representing the order of magnitude only. Further evidence is needed as a basis for the construction of diets with a view toward providing the best nutrition for muscular activity. To this end, the various food principles will be discussed separately.

Carbohydrates

Carbohydrates are essential dietary constituents but are not the exclusive fuel for muscle. The amounts of carbohydrate used are related to the work level. The depletion of blood sugar levels below 70 mg. per cent results in exhaustion. A low blood sugar may interfere with the metabolism of the central nervous system to such an extent that a mild degree of oxygen want brings on symptoms similar to those obtained under conditions of extreme anoxia. This indicates that the fatigue of protracted exhaustive exercise has a cerebral origin. Associated with the fatigue of prolonged work is a loss in mechanical efficiency due to faulty coordination. Theoretically, a diet rich in carbohydrates should contribute to the maintenance of normal blood sugar concentration for longer periods during prolonged physical work. Even though this is not always actually the case and exhaustion often occurs with a high blood sugar concentration and at high R. Q.'s, the theory is supported by observations in which industrial workers are found to work more efficiently on a carbohydrate-rich than on a fat-rich diet. Evidently the blood sugar level does not always reflect the nutritional condition of the muscles, and also conditions other than the type of fuel affect the R. Q. Although the mechanisms of carbohydrate utilization are not entirely clear, there is sufficient evidence to support the principle that the athlete in training should have extra carbohydrate in his diet.

An exception to the superiority of high carbohydrate meals is observed in workers performing strenuous visual tasks requiring the recognition of fine details moving on a conveyor belt. For this type of work either a balanced standard meal or a high fat meal was preferable.

Dietary Notions Among Athletes

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There are many dietary notions among athletes and athletes have turned in championship performances after training on a diet which in the light of accepted dietary standards would be considered inadequate in many respects. Many champions are vegetarians, others eat large quantities of meat, some of it raw.

Most athletes reduce the intake of fat in their training diets but others make no changes whatsoever and eat what they please or what is served to them with no regard to the possible effect of nutrition upon performance. These experiences are in line with recent experimental evidence that the ratio of fat, carbohydrate and protein in the diet may be altered considerably before changes in work performance appear.

Within wide limits there appear to be certain dietary principles which are associated with improved performance in work and sport. Positive evidence for these principles is accumulating, but a wellcontrolled experiment has yet to be performed.

Weight Loss in Athletics, Making Weight, Semistarvation

2008-02-27T00:22:00.000-08:00

Making Weight

Making weight" by wrestlers, boxers, weight lifters and jockeys is accomplished by profuse sweating and by abstaining from food and liquids for a few hours before weighing in. Weight loss up to 10 pounds (representing 5 per cent of the body weight) was accomplished without any measurable deleterious effects in six wrestlers. 2 Weight loss was induced by a procedure considered orthodox by wrestling coaches. Food and water were withheld. The wrestlers, wearing heavy sweat suits, worked on the mats and alternated this with sitting in a heated cabinet. This abnormal loss of weight is regained in part after weighing in by eating and drinking. More than 5 pounds may be regained in a few hours.

Weight Loss in Athletics

Losses of weight exceeding 10 pounds in one and one-half hours have been reported during football games at the beginning of training. During especially exciting games a substitute who has been "sweating out" the entire game on the bench has been observed to lose 5 pounds. This weight is entirely and rapidly regained after eating and drinking.

Semistarvation

A diet which is inadequate to support the needs of the body will soon result in a deterioration in fitness for exercise. When the daily caloric intake is reduced, the day to day output of voluntary work is reduced proportionately. A substandard diet causes a reduction in the performance of the physiological systems, especially the oxygen delivery and metabolic waste removal mechanisms.

Weight Reduction, Weight Gaining, Body Fat

2008-02-27T00:19:00.000-08:00

Weight Reduction

The obese individual may reduce his weight by limiting the amount of fat and high caloric carbohydrate foods in his diet. The protein foods need not be diminished because of their property of specific dynamic action (stimulation of general metabolic processes). An increased amount of daily physical activity increases the katabolic processes and aids in weight reduction.

Weight Gaining

The underweight individual may increase his weight by increasing the amount of fat and high caloric carbohydrate foods in his diet. More frequent feeding may be desirable, but excessive fat taken between meals may decrease the appetite due to its slow rate of digestion. Additional hours of sleep, perhaps including an afternoon nap may be helpful. A pleasant environment and a serene mental attitude are also important in a weight gaining program.

Body Fat

Various types of competitive athletics require different proportions of fat to muscle for maximum performance. A minimum amount of fat is desirable in the distance runner, the high jumper and the gymnast. These athletes must move their own weight in a highly economical fashion and any added weight taxes the strength and endurance. Distance swimmers need a certain amount of fat distributed near the skin surface to diminish the heat loss to the water, Football players, especially linemen, employ the fat portion of their mass in achieving momentum and also as a cushion to absorb the shocks of repeated contact.

In severe athletic training the normal amount of fat is reduced to a minimum because this tissue is considered an incumbrance. It is an extra load to be carried and it impedes the violent contraction of muscles, In this highly trained slate man is best fitted for framing, jumping, climbing and fighting. As stated before, however, he is not in the best condition to resist an exhausting infection like typhoid fever where an individual's fat stores are a valuable asset.

During a season of athletic training a slight loss in body weight may be expected at first due to a loss of fat. The initial loss is followed by a slow gain attributable to increased muscular development. One evidence of staleness or overtraining is a gradual loss of weight, i.e., the weight lost during exercise is not regained as it should be. Rest and feeding are indicated when the weight continues to decline.

Diet, Quantity of Food, The Weight Chart

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The increased energy metabolism in muscular exercise must be supported by an equivalent increase in energy supply. If the demand exceeds the supply, body tissues are consumed in the course of the activity. The depletion of body tissues impairs the functions of the organs of which these tissues are a part. Furthermore, physical efficiency is lower when body tissues are used as fuels for activity than when the fuels are supplied by the foodstuffs in an adequate diet.

Quantity of Food

The quantity of food required by active men varies to such an extent among the individuals in a group and also in a single individual from day to day that the establishment of a standard dietary requirement for a group or an individual would be meaningless. The total daily energy requirement for an active man may range from 3000 to 8000 Calories, depending upon his size and physical condition and the severity of the work performed each day. An experiment on rats 1 demonstrates some of the variables which affect the daily energy requirement. Twelve female albino rats from four to twelve months old were observed while quantatitive variations in activity, food intake and environmental temperature were induced. When activity was increased, the weight decreased if food intake was constant. When the food intake was increased, there was an increase in body weight when activity was constant. When environmental temperature was increased, there was a decrease in body weight when activity was constant. The quantity of food required to support increased physical activity and environmental stress is soon indicated by alterations in body weight.

The Weight Chart

The weight chart is very useful in estimating the caloric requirement of a season of athletic training. It may also be used effectively in programs of weight control in which an individual is attempting to gain or lose weight in order to attain a more desirable proportion of fat to muscle. In all of these charts, nude weights are recorded every day before breakfast. Weights are measured in the nude to exclude variations in the weight of the clothing. They are recorded every, day before breakfast so that the individual may reduce or increase his food intake during that day in accordance with the posttion of the weight he has plotted on the chart. This act also serves as a frequent and timely reminder of the task at hand. After only a few days the individual keeping the weight chart will observe the rate of change in weight as a result of changes in food intake. He then evaluates each item of food in relation to the caloric requirement for maintaining the daily amount of activity. A sensible program of weight control will plan for an increase or decrease of about one pound weekly. Extremely overweight and underweight individuals may plan for slightly greater rates of change, but weight loss should not exceed two pounds per week.

Classification of Enzymes

2007-12-05T17:14:00.000-08:00

For many years it was the custom to classify enzymes solely by the reactions which they catalyzed. As already noted there is within living systems an amazing variety of chemical reactions and these many diverse reactions are specifically catalyzed by individual enzymes. However, as the chemical composition of enzymes becomes better understood, there is an increasing tendency to classify enzymes not merely in terms of what they do but also in terms of their true chemical composition.

In the first place it is possible to characterize a large group of enzymes as hydrolyzing enzymes or hydrolases. The hydrolases are concerned primarily with the breaking down and the building up of proteins, carbohydrates and lipids. It is a characteristic of these essential organic materials that their large molecules are built up by a progressive subtraction of water molecules. When amino acids unite, the acid group of one amino acid combines with the basic amino group of a second, and as in typical acid-base reactions, water is split off. In this way protein molecules are built up, and if they are to be broken down to amino acids again, water molecules must be added. Similarly, when an alcohol unites with an organic acid to form an ester, water is split off; and in the reverse process by the addition of water an ester may be converted into an acid and an alcohol. Thus by the addition of water, a fat may be hydrolyzed to fatty acid and the trivalent alcohol glycerol. When two molecules of a simple sugar such as dextrose unite to form a disaccharide, then also water is split off, and with the addition of water the double sugar may be broken into simple sugars again. Similarly starch is formed from sugar by a series of reactions in which water is removed. All of these reactions in which water is removed or added are catalyzed by the hydrolases. Proteolytic enzymes break down proteins to polypeptids and amino acids (or build up proteins from such compounds), lipases split fats into fatty acids and glycerol (or synthesize the fats from the fatty acids and glycerol), amylase acts on starch, sucrase (often called invertase) acts on sucrose, and there are other specific enzymes which act on the various isomers of sucrose. The hydrolases also include the lecithinases which split off fatty acids from the rest of the lecithin molecule, and the phosphatases. The latter split off phosphate from organic combination. Thus one phosphatase breaks glycerol phosphate into glycerol and phosphate; another group of phosphatase enzymes, the nucleotidases, break nucleotides into nucleosides and phosphoric acid. Most hydrolases are concerned primarily with the breakdown of large molecules as it occurs in digestion and with the synthesis of the breakdown products to form large molecules again in the cells. Such reactions are of primary importance in nutrition, but they yield little energy.

In addition to the hydrolases concerned primarily with nutrition, there are some hydrolases which play a part in other vital phenomena. Thus the enzyme cholinesterase, which hydrolyzes acetylcholine into choline and acetic acid, is believed to have great importance for nervous function. Another hydrolase enzyme which has excited great interest is hyaluronidase. This breaks down hyaluronic acid, a polysaccharide that tends to hold cells together. The enzyme is found in various bacteria; also in testis extracts. When skin is treated with hyaluronidase, the cells separate sufficiently to permit the spreading between them of substances like India ink. Heparin, like hyaluronic acid, is a mucopolysaccharide. There is apparently a heparinase which breaks up heparin or at least destroys its activity. Mention has already been made of the enzymes which specifically attack ribonucleic acid and desoxyribonucleic acid. These likewise are hydrolases.

The enzymes which cause coagulation or clotting, thrombin and rennin, may also be hydrolases. Thrombin causes clotting of vertebrate blood and rennin clots milk. They may both produce this effect as a result of proteolytic action.

Similar to the hydrolases are the phosphorylases. These enzymes, which should not be confused with the phosphatases, add phosphoric acid instead of water. They are very important in carbohydrate metabolism and will be discussed later in this connection.

Enzymes which act on inorganic pyrophosphates and hydrolyze them to orthophosphates are called pyrophosphatases. (This is a reaction which occurs even without the aid of enzymes.) Pyrophosphatases are found in many different types of living material.

In addition to the hydrolases there are various other categories of enzymes. There are enzymes which transfer a radical or group from one substance, the donor, to another, the acceptor. These have been called transferases, and they include the transaminases, which transfer amino groups; and the phosphokinases, which transfer phosphate groups.

An interesting enzyme found especially in the red blood cells of higher animals, is carbonic anhydrase. This catalyzes the reaction in which carbonic acid is split into carbon dioxide and water. Actually, the reaction is speeded in both directions, so that carbonic acid can be formed more rapidly as well as broken down. The reaction needs to be speeded in order to hasten the gain or loss of carbon dioxide from the blood.

Definition of Enzymes

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The living machine is essentially a chemical engine, dependent for its growth, maintenance and energy on chemical reactions. These reactions are controlled by catalysts. One of the most striking achievements of modern biology and biological chemistry is the isolation of more and more of these catalysts so important for the vital process. Once isolated, it is possible to study their behavior in test tubes or other suitable containers. In other words, one can detach from the living material certain non-living substances capable of causing or promoting the complex chemical transformations which constitute a major part of the mystery of protoplasm.

Originally, enzymes were defined as "catalytic substances produced by living cells." With increase in knowledge and understanding, it has become necessary to restrict this definition. The original concept was that enzymes were not only produced by living things but were peculiar to them. There are some rather simple substances in protoplasm which can act catalytically; so for example, ascorbic acid, glutathione or as a matter of fact, even the hydrogen ion. The term enzyme was never meant to apply to these. Hence in modern usage an enzyme is defined as a "catalyst of biological origin, possessing a high molecular weight." In the light of what we now know, it is generally believed that all enzymes are proteins.

In as much as enzymes are catalysts, it is important to remember that they hasten rather than initiate reactions, and that when they act on reversible reactions they accelerate the progress of the reaction in either direction. Thus the same enzyme may synthesize as well as break down a complex chemical compound, and the very enzyme which digests proteins or fats or carbohydrates also serves, under the proper conditions, to build up these compounds from their breakdown products.

Digestion--Enzymes

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Whenever an organism consumes food in the solid state, this must be brought into solution before it can be utilized. It is necessary, therefore, that such solid food be digested. In some organisms, as pointed out in the last chapter, digestion may occur outside of the body, and this may constitute an important adaptation for those animals which are in the habit of eating animals larger than themselves. Ordinarily, however, food is taken into the body and digested there. Digestion may occur in cavities of special organs such as the stomach or intestine, or it may occur within the protoplasm of cells. The latter type of digestion obviously takes place in protozoa. In organisms such as paramecium or ameba the ingested food is enclosed in a food vacuole, which serves the same purpose as the stomach or intestine of a complicated metazoan. Within the food vacuole the solid food particle is brought into solution. It must not be thought, however, that intracellular digestion is confined to protozoa. In sponges, coelenterates, and flatworms, much of the solid food taken into the body is ingested by ameboid cells lining the walls of the digestive tract, and digestion takes place within the protoplasm of these cells rather than in the lumen of the digestive tube or cavity. Moreover, in some animals rather higher in the evolutionary scale, there is also a certain amount of intracellular digestion. In spiders and in arachnids generally, the digestion of protein, begun either outside the animal or in the digestive tract, is finally completed within the cells lining the tract. In clams and other lamellibranchs, the digestion of protein and fat has been thought to occur exclusively within cells. Both in lamellibranchs and echinoderms, amebocyte cells play an important role. These phagocytic cells even enter into the lumen of the stomach or intestine, ingest particles of food there and then carry these food particles back into the tissues and digest them there. Such intracellular digestion, however, is a primitive character and it does not occur to any extent in higher animals such as the insects and vertebrates.

In the conversion of solid food to a state of solution, enzymes play the leading role. It will be necessary, therefore, to consider the subject of enzymes and the nature of enzyme action. It should be strongly emphasized, however, that enzymes are not concerned only with digestion, but that they are essential factors in all of the chemical activities of the organism. Our discussion at this point will be somewhat parenthetical. It will also be brief; indeed, it would scarcely be possible to present an up to date summary of enzyme chemistry in the space of a single chapter. The subject has grown so rapidly that it has become a science by itself (enzymology), and the modern books on enzymes are heavy with information.

Metazoa

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Most many-celled animals take solid food into some cavity within their bodies, in which it is dissolved or digested. This is not always the case, however. There are quite a few organisms which are able to digest their food outside of their bodies. Thus a starfish or sea urchin may pour digestive juices out over some fish or other large animal which it has captured. It is sometimes possible for an animal to eat other animals larger than itself. When a spider captures a fly, it pours digestive juices into the body of its prey and then sucks out the dissolved interior. Many parasitic animals require no digestive apparatus. In the case of animals such as tapeworms, there is no need for the ingestion of solid food, and the dissolved food substances in the alimentary canal of the host pass directly through the body wall of the animal. Other worms derive food in the same manner from the blood of their hosts.

Some authors have tried to classify animals on the basis of their methods of ingestion. Thus there have been distinctions between whirlers (ciliary feeders), snarers, scrapers and suckers. Small particles are often pulled into the body by ciliary action. This occurs not only in the protozoa, but also in practically every other phylum, except the arthropoda. Ciliary mechanisms for feeding are most highly developed in the mollusca. Tentacles and setae are also used in obtaining small particles. The animals that feed on large particles or masses have mechanisms for swallowing inactive food, for seizing prey, or for scraping and boring.

The usual form of ingestion in metazoa involves some sort of passage into the mouth. Jaws and teeth of various types may aid in the capture and in the subdivision of the food. The earthworm has a pharynx which is pulled on by muscles attached to its outer wall so that it can behave as a suction pump. Marine (polychaete) worms also may have sucking pumps. Thus in Autolytus the proventriculus, by its pulsations (120 per minute), produces a strong inward current of water and food. Sucking pumps also occur in insects of various orders, and these pumps may have a rather complicated structure. Even in the case of those animals which ingest food through the mouth there is sometimes a possibility that food may be taken in through the body wall.

Feeding habits of protozoa

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Feeding habits of protozoa vary widely. Some contain chlorophyll and behave essentially like plants. Thus the various species of Euglena are able to manufacture starch from simple inorganic materials. These forms and also ciliates without mouths, such as the Opalinidae, are able to absorb food materials through their surfaces. In all of the sporozoa, moreover, food enters the organism through the body wall. These forms are parasites and live on the organic materials provided by their hosts.

Most ciliates and flagellates have definite mouths, but it does not follow that all of the food taken up by these organisms passes through the mouth opening. In flagellates, especially, it seems probable that much of the food enters the cell by diffusion through the body wall. Ciliates like paramecium sweep food particles into their gullet by the beat of their cilia. Although various useless materials may enter the gullet and pass into the cell, there is apparently some power of discrimination.

The ingestion of food by ameba is essentially the same process as that which occurs when leukocytes or other ameboid cells take up foreign particles. Such phagocytosis occurs in practically all metazoa, and is of great importance both in the normal life of animals and in the resistance of animals to disease. Phagocytosis plays an essential role in metamorphosis and, both in insects and in amphibia, larval organs are destroyed bit by bit by the ameboid cells which ingest them. Because of its importance as a protection against disease, phagocytosis has been very widely studied by bacteriologists, pathologists, and students of medical sciences generally. Just as amebae select their food, so do leukocytes exercise a selection of the particles they ingest. The rate of ingestion of bacteria by leukocytes has often been studied under a variety of conditions. Bacteriologists have tried to obtain quantitative data by determining either the number of bacilli taken up per leukocyte or the percentage of leukocytes participating. Important factors (not always properly considered) are the absolute and relative concentrations of bacteria and leukocytes. Phagocytosis increases with increase in the number of bacteria, but the rate of ingestion does not keep pace with the increasing possibilities for collision between bacteria and leukocytes. Various authors have studied the effect of temperature on phagocytosis; curves are obtained like those found for ingestion of food by ameba. Any explanation of the mechanism of phagocytosis could at the same time serve as an explanation of the ingestion of food by ameba. At the present time, however, none of the theories of phagocytosis seems very satisfactory.

In higher animals, phagocytosis is not restricted to leukocytes. Scattered through various tissues and organs of the animal are numerous cells capable of ingesting solid objects. Of these, the macrophages move somewhat more freely than the histiocytes, and there are also morphological differences between the two types of cells. Together, they constitute the reticuloendothelial system.

Vitamins of the B Complex

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B 13 has been described as a growth factor for rats and for pigs. There is also a description of a vitamin B 14, extracted from urine.

Over the years, there has been a large and controversial literature about what has been called vitamin P. Substances with vitamin P activity are supposed to maintain proper permeability of the blood capillaries of higher animals and to prevent excessive loss of fluid from them. (The letter P is due to this permeability action.) There is now general agreement that substances with P activity are not to be considered as vitamins. At least this is the judicial opinion of the American Society of Biological Chemists.

Para-aminobenzoic acid (often called paba). This relatively simple compound is an important growth factor for many bacteria. As previously noted, it forms part of the folic acid molecule. Para-aminobenzoic acid and sulfanilamide are very close chemically, as is apparent from their structural formulae:

Thus sulfanilamide is an antagonistic analogue of para-aminobenzoic acid, and its antibacterial action is due to this similarity in structure. Choline. This substance, the basic constituent of lecithin, is believed by many to be a vitamin. Rats deprived of choline develop fatty livers and other types of hepatic injury. They may also show degenerative changes in the kidney. Choline deficiency is also a factor in the development of a leg deficiency (perosis) in chicks and turkeys.

It should be remembered that choline is a constituent not only of lecithin, but also of the important humoral substance, acetylcholine. Inositol. Inositol is a derivative of benzene. It can be prepared from hexahydroxy-benzene by reduction, in the course of which 6 hydrogen atoms are added. Thus inositol is hexahydroxy-hexahydro-benzene. It is an optically active substance found in yeast, muscle, and in various other types of animal and plant tissue.

Inositol is important for the growth of yeasts, and is indispensable for some types of yeast. It also promotes the growth of some fungi. In the presence of inositol, chicks grow more rapidly than when it is absent. However, rats may be bred for three generations without inositol, and György was led to conclude that "the facts presented do not, at the present time, warrant the identification of inositol as a primary and really essential vitamin; they rather favor the assumption that it is a supporting and at least not always specific vitamin." a-Lipoic Acid. This substance, also called protogen, is probably to be included among the vitamins of the B group, for it is water soluble. It is important for the growth of some bacteria and for the protozoan Tetrahymena, and it is involved in the oxidation of the important keto acid, pyruvic acid (for this reason it was formerly called pyruvate oxidase factor). Many types of living material contain a-lipoic acid. It is now known to be a derivative of octanoic (caprylic) acid and it contains sulfur in the form of an S-S group.

Throughout the course of vitamin study, there have indeed been many extravagant claims and many false leads. The vast literature is often contradictory and confused. But in spite of this, in the course of time the truth has gradually emerged and the main outlines of the picture are now clear. All sorts of organisms, from bacteria to man, require certain specific types of organic molecules. These can be synthesized by green plants and by many lower organisms. Higher forms have frequently lost the power for such synthesis, and unless, like the ruminants, they provide themselves with huge storehouses for bacteria, they must depend for their existence on obtaining the vitamins with their food.

From the knowledge of the nature of the vitamins, we can begin to develop a chemical anatomy of living systems. The vitamins represent indispensable organic molecules and apparently the same molecules are almost universally needed. Moreover, we are beginning to discover why the vitamins are needed and what work they do in the living cell.

Much of our information is still fragmentary. The vitamin requirements for the lower vertebrates and for the hosts of invertebrate animals have scarcely been investigated at all. Moreover, in some instances we are completely at a loss to account for the fundamental necessity for a given vitamin. Such superficial disturbances as a reddening of the eye or a scaliness of the skin give no direct indication of the part a vitamin may play in protoplasmic activity.

Vitamin B 12

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Year by year, the number of B vitamins increases. B 10 and B 11 are required by chicks, the former for proper feathering, the latter for growth. From liver it has been possible to isolate a pink crystalline substance which can be used to cure pernicious anemia. Folic acid is useful in this disease, for it has a hematopoietic effect (that is to say, it favors increased production of blood cells), but it does not correct the nervous symptoms of the disease. These depend on degenerative changes in the nervous system. Vitamin B 12 in tiny amounts has been thought to correct diseases of both blood and nervous systems, but apparently if the nervous system is badly affected, it can not be cured. The vitamin has been crystallized. It contains 4-4.5 per cent cobalt and also phosphorus; the molecular weight is approximately 1500. (Estimates vary from 1300 to 1600.) In liver extracts there are really two active substances with slightly different absorption spectra. One of these is the definitive B 12, the other is B 12b. (B 12a is another similar substance derived from B 12 by hydrogenation.)

Vitamin B 12 is apparently identical with what has been called "animal protein factor." This was originally obtained from such diverse sources as milk, muscle, liver and cow manure. Although called animal protein factor, it can likewise be obtained from plant sources. Both animal protein factor and vitamin B 12 promote growth in some bacteria, in chickens, rats and mice. The effect on growth may be due to the fact that the vitamin seems to increase the utilization of amino acids. At any rate, this has been reported for chicks.

Folic acid

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This vitamin occurs in green leaves; hence the name, which is taken from the Latin word for leaf. Folic acid also occurs in mushrooms and yeast; and in animal tissues, such as liver and kidney. The vitamin has been called by many different names, and it exists in various forms. These have different potencies for different types of organisms.

Chemically, folic acid consists of pteroic acid combined with the common amino acid, glutamic acid. Pteroic acid itself is a combination of para-aminobenzoic acid and a pteridine base. Pteridine bases are pyrimidine derivatives related to the pyrimidine bases thymine and uracil found in the nucleic acids. The formula of folic acid is given below:

The compound shown above is pteroylglutamic acid, sometimes called PGA. Pteroic acid may also combine with 3 glutamic acid groups to form pteroyltriglutarnic acid (P3GA) or with 7 molecules of glutamic acid to form pteroylheptaglutamic acid (P7GA). All of these compounds are found naturally and all have vitamin activity for certain types of living materials.

Folic acid is required by some bacteria, by certain ciliate protozoal, by some insects and by various birds and mammals. The bacteria in the rat intestine normally manufacture a sufficient quantity of the vitamin, and treatment with antibiotics such as sulfa drugs is necessary to cause folic acid deficiency. Folic acid is commonly an essential growth factor, not only for bacteria but for higher organisms as well. Lack of the vitamin in mammals results in various ailments, but especially in a type of anemia characterized by a small number of red blood cells of large size. This type of anemia, called macrocytic anemia, occurs in the tropical disease sprue, and sprue can be cured either by liver extract (which contains folic acid) or by folic acid itself. Folic acid is apparently important both for growth and cell division. When chick embryos are in contact with antagonistic analogues such as 4amino folic acid, their growth is greatly retarded. Similar results have been obtained in numerous other studies with folic acid antagonists. Thus 4-amino folic acid has a retarding effect on tumor growth.

A substance related to folic acid is the so-called "citrovorum factor," 45 which is apparently identical with "folinic acid." This factor favors growth of the bacterium, Leuconostoc citrovorum. The citrovorum factor may be obtained from liver extracts; also it is excreted into the urine of rats or men fed folic acid.

Biotin

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As long ago as 1898 it was shown that if dogs were fed raw egg white, diarrhea resulted. Following this it was shown that other mammals also suffered from egg white feeding, and that in addition to the diarrhea, skin ailments and nervous symptoms appeared. The phenomenon became known as egg white injury. Cooking the egg white prevented the injury. György proposed the term vitamin H for a factor which prevented egg white injury. Later, it was shown that this factor is identical with biotin, a vitamin known to be important for yeasts and microorganisms. The raw egg white unites with the biotin and prevents its utilization by mammals or birds.

Chemically, biotin is rather complicated. It has in its molecule a urea ring containing one sulfur atom; the ring is combined with a valeric acid group. The presence of sulfur in the molecule is not important for the vitamin activity, for if the sulfur is replaced by oxygen, the vitamin activity is retained. The formula of biotin is given below:

Biotin acts in extremely low concentration, and it is thus one of the most potent of the vitamins. Many organisms do not grow unless biotin is available for them. Usually in higher organisms it is synthesized in sufficient quantity, but biotin deficiency can occur naturally in birds and in insects. Lack of sufficient biotin has the same effect as egg white injury--diarrhea, disturbances in the skin and nervous system, in rats loss of hair about the eyes. It can also prevent the birth of young in rats. The protein in egg white which binds biotin is called avidin; it is a basic, carbohydrate-containing protein and it occurs not only in the eggs of birds but also in frog eggs.

There is now strong evidence that biotin is important in a process which is known as carbon dioxide fixation. Many bacteria, and higher organisms also, are able to utilize carbon dioxide and form organic compounds from it. This process is interfered with if biotin is lacking.

Pantothenic acid

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Pantothenic acid is found in many diverse types of living materials. Chemically, it consists of ß-alanine in a peptid linkage with a dihydroxy acid. (ß-alanine differs from ordinary alanine in being ß-aminopropionic acid instead of α-aminopropionic acid.) The formula of pantothenic acid is given below:

The acid is unstable and for this reason is sold as a calcium salt. This salt is a white powder, readily soluble in water but not in alcohol.

Many organisms from bacteria and protozoa to higher mammals require pantothenic acid. In its absence these organisms do not grow properly and they may show various ill effects. Symptoms of pantothenic acid deficiency include dermatitis (inflammation of the skin) in rats, deficient feathering in chicks, graying of hair in rats, degeneration of the nervous system in pigs, hemorrhages in the adrenal gland and general disturbance of function of this gland, etc.

The vitamin constitutes part of an enzyme system. Combined with various substances including phosphate, adenylic acid, and a sulfhydryl group, it is coenzyme A, and this in combination with a protein is an enzyme capable of acetylating various organic substances such as choline and sulfanilamide. The acetylation of choline may be very important physiologically. How the acetylation, or rather the lack of it, can produce all the pathological conditions noted above is not clear, although in the case of the nervous system, any alteration of acetylcholine metabolism would certainly be harmful.