The Contraction of Muscle

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.