The onset of contraction

The early heat production during the onset of a muscle twitch has been determined with the greatest precision possible for comparison with the mechanical response. When allowance is made for the time taken in propagation the heat is found to start off at its maximum rate. Its rate falls gradually to a constant value, while the muscle continues to shorten uniformly then decreases to zero as shortening draws to an end. The heat occurs in two separate processes, those of activation and shortening respectively. The heat of activation has well started before shortening is detected by ordinary methods. The heat of shortening runs parallel to the shortening. There is no sign of negative heat production at any stage of contraction. If endothermic processes occur they are exactly masked by exothermic ones. The latent period of the activation heat is about 10 msec. at 0 degrees C in frog9s muscle, about 25 msec. in toad9s muscle. These, with an ordinary value of $Q_{10}$, would correspond at 20 degrees C to about 2 and 5 msec. respectively. Various physical methods are discussed of examining the rapid processes that occur during contraction. Chemical methods are inadequate in speed and sensitivity to give direct evidence of the nature and sequence of events occurring in a twitch. Theoretical conclusions from experiments on muscle extracts, without critical comparison with the behaviour of living muscle, may lead to confusing results.