Muscle tonus and its relation to posture

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.