The inconstant action of muscles

Abstract

The movements of the hind leg of the frog which are generally ascribed to finely adjusted nervous coördination, are in fact largely the result of the mechanical conditions under which the muscles act. These conditions differ with each new position of the bones entering into the joints of the limb, and consequently alter the effects of the contractions of muscles as the positions of the bones change during the course of any given movement. Thus a muscle which in one position of a bone may act as a flexor, in another position may act as an extensor, and a muscle which in one position of a bone may carry it dorsally, in another position may carry it ventrally. Manifestly it is absurd to try to class muscles as flexors and extensors, for example, or to try to name them according to the movement which they are supposed to produce. Nor can one, without qualification, speak of certain muscles as antagonists, when under slightly modified conditions of action they act as synergists. Moreover, it is evident that we can form no estimate of the part played by the central nervous system in coördinated movements of locomotion, for example, until we have ascertained in how far the coördination observed is due to the mechanical conditions under which the muscles are acting. A study of central coördination must, in short, be postponed until the effects of peripheral coordination based on joint and muscle mechanics has been ascertained. These statements are the result of two years of careful study of the effect of mechanical conditions on the action of the separate muscles of the hind leg of the frog, when these muscles have been electrically excited to action, in different positions of the bones.