The transition from rest to full activity in muscle: the velocity of shortening

The active state of muscle is characterized by two mechanical properties, the capacity to exert or resist a force and the ability to shorten. The former is known to reach its full extent and very quickly after a stimulus; the development of the latter has been examined experimentally. Isotonic twitches of a frog's sartorius were recorded at 0ºC with equipment of high sensitivity and very small inertia. Following the latent period, about 8 msec, was required for the full velocity of shortening to be reached, after which it was maintained for some time. The delay was not due to inertia, but a substantial part of it is probably to be attributed to a statistical distribution of speed of response in individual fibres. In the ultimate contractile elements, therefore, the transition from rest to full activity is extremely rapid.