The heat of activation and the heat of shortening in a muscle twitch

Abstract The heat produced in a single muscle twitch is made up of two parts, (1) the heat of activation, and (2) the heat of shortening. In the leg muscles of toad or frog at 0°C the heat per cm. of shortening is about 350 g. cm. (expressed .in mechanical units) per sq. cm. of muscle cross-section. The heat of activation is usually rather less than the maximum heat of shortening and depends little, if at all, on the length at which the stimulus was applied or on changes of length thereafter: it is equal to the heat which would be produced if shortening were altogether obviated, a condition approximately realized in a muscle brought to a very short length by previous stimulation under a very small load. The heat of shortening occurs at the same time as the shortening. The heat of activation has its maximum rate at the start, very soon after the stimulus, and falls off in rate from then onwards. The heat of maintenance in a tetanic contraction is the summated effect of the heats of activation resulting from successive elements of the stimulus. The effect on the heat production of a sudden arrest of an isotonic contraction is described. Under such conditions the contractile elements of a muscle continue to shorten but at a decreasing rate as the tension rises. The complications due to inequalities of length and contractility in the different fibres of a muscle are discussed.