The dynamics of muscular contraction

Abstract This investigation started with an attempt to verify, on isolated frog’s muscle stimulated directly, the relations established by A. V. Hill (11) and others (17), (12), between the speed of shortening and the work done. If the flexor muscles of the human arm be caused to exert a maximal voluntary contraction, the velocity with which they shorten can be varied by altering the inertia of the load to which they are opposed ; the work done is then found to decrease as the speed of shortening is increased. Since the extent of the movement in these experiments is always the same, if the work varies in any regular manner, so also must the force exerted by the muscle during any given element of the shortening ; hence the force exerted by the muscle decreases as the speed of shortening increases. This variation of work (or force) with speed is an important factor in determining the mechanical efficiency of muscle, as shown theoretically by A. V. Hill (11) and experimentally by Lupton (18), Cathcart (3) and others. Just as important, however, is its bearing upon theories of the nature of muscular contraction, and upon the problem of the adaptation of the muscles of different animals to the speed required of them by the dimensions and habits of their owners. It was possible that the connection between force exerted and speed of shortening might be dependent upon some automatic regulatory mechanism inherent in the nervous system. The rapid shortening of a muscle opposed to a small inertial load might cause a proprioceptive reflex withdrawing, or reducing, the volley of nervous impulses directed at the fibres of the muscle in consequence even of what was intended to be a maximal voluntary effort. This could clearly be decided by repeating the experiments with an isolated muscle stimulated directly. Part I is an account of experiments which prove that the same general type of relation between work and speed of shortening occurs in isolated muscle. Hence the phenomera investigated are not due primarily to an intervention of the nervous system, but to some fundamental character of the muscle fibre itself.