Frog Fast Muscle

This is the authors' abstract. We don't add key points for this paper.

A method is described for producing a family of curves of mechanical power output against tension, with time as parameter, in an isometric twitch. It is found that these differ from corresponding curves produced from the same muscle under isotonic conditions. A method is described of simulating a variable mass against which the muscle can contract. The time course during a twitch of the power expended in moving this mass, and of that expended in stretching the series compliance, is determined. In the early (‘active’) part of the twitch the total power agrees well with that predicted from isotonic measurements, but in the latter part (‘tail’) the muscle’s behaviour abruptly changes : it then resembles, with some differences, a large viscous resistance, in a manner which is compatible with the form of isometric twitches, but has no parallel under isotonic conditions. The power developed in certain phases of the isometric twitch falls short of that predicted from isotonic measurements. It is shown that this can be explained by postulating an upper limit to the rate at which the contractile component can develop tension, and supposing that this limit declines as the twitch proceeds.

Late-twitch viscous resistance in frog muscle cautions against uniform isotonic models; leaves open relevance to mammalian cardiac or skeletal muscle.