The development of the active state of muscle during the latent period

The mechanical response of muscle, as ordinarily recorded, depends on the shortening of its contractile elements, which is a slow process. An applied stretch obviates the necessity of shortening and allows the change of state resulting from stimulation to be examined directly. During the latent period a considerable mechanical change occurs, the muscle rapidly becoming less extensible. This change begins about half-way through the latent period; by its end a very substantial resistance to stretch is encountered. The effect described is much greater than the‘ latency relaxation’, though beginning at about the same moment. The ‘latency relaxation’ is probably due to a slight increase of length accompanying the major physical or molecular change made evident by stretching. The early increase of transparency may be a sign of the same process. The rapid decrease of extensibility has been followed beyond the latent period into the earlier stages of contraction. The decrease of extensibility begins at about the same moment as the heat of activation; the latter is regarded as a product of the chemical process by which the change of mechanical state is effected.