Spread of Activity

The spread of activity to other muscles as the intensity of the movement varies is not restricted to any particular type of movement or group of muscles but is a characteristic of coordination itself. We find it in the spread from finger to hand muscles, as the resistance against the fingers is increased. From the small supinator^[1] to the biceps with an increase of resistance against supination and from the biceps to pectoralis major as resistance to elbow flexion is increased. But we may not conclude from this that the spread takes place only after maximal contraction of the preceding muscle has been completed, because the spread is frequently for purposes of joint-fixation. During the lateral shift along the keyboard, toward its center, that the biceps continued to contract further, after the pectoralis major had begun to contract. That is because the pectoralis has nothing to do directly with flexing the elbow (since its tendon does not cross the elbow-joint), but fixes the position of the humerus, so that, when the biceps contract more strongly, the humerus will not be pulled out of position. The mechanical principle at work here is more carefully analyzed under Action and Reaction. The spread is normally toward the fundamental (larger) muscles, never the reverse.

A coordinated movement, therefore, considered in its force or intensity phase, is one in which the parts of the body involved act at the best mechanical advantage. As the force of the movement increases, the direction-relationships of the various parts of the body change. For example, a hand-position which is adequate and efficient for a soft tone degree, may be entirely inadequate for the production of a much louder tone. In order for the fingertip to exert its full force upon the key, the extensors of the wrist must contract to prevent the upward movement of the wrist. The extent of this wrist-ascent determines the amount of force lost at the finger-tip.

A few examples from general observation may be mentioned.
In pressing against resistance in front of the body, the arms are held straight in front, not bent at the elbows. The resistance thus pushes against a straight line of bone into the shoulder socket. In the tug of war, the body assumes as nearly as possible a straight line position so that the entire skeleton is opposed to the resistance. In "putting the shoulder to the wheel", trunk and legs are in one line, thus gaining maximal mechanical advantage for the skeletal parts. In piano-playing such effects are not so readily observed because the movements are much finer.

This "shifting of human components" in order to secure "maximal mechanical advantage" with minimal muscular expenditure is entirely in agreement with the definition of coordination already given. Conversely, an in-coordinated movement considered in its intensity aspect, is one in which the parts of the body involved do not assume the best mechanical relationships.
As the intensity of a movement increases, therefore, the individual muscles acting do not necessarily do so at their "maximal strength". The change in the spatial position of the bones relieves the muscles of this necessity. If this were not so, then the small finger muscles would have to be able to withstand the effect of the most powerful contraction of the large shoulder muscles. It would then be an application of the principle that a chain is merely as strong as its weakest link. Because whatever force the shoulder muscles create must be transmitted to the piano-key through the fingers, no single joint can be relaxed to a point less than the value of this force.

[1]supinator: The supinator is a broadly shaped muscle in the superior and posterior compartment of the forearm. It curves around the upper third of the radius and consists of two layers of fibers. In between these layers lies the deep branch of the radial nerve.