Economy of Muscular Activity

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.