Postural Mechanisms and Their Control

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.