Heat production

Our everyday experience demonstrates that muscular activity is accompanied by heat production. It is necessary to take account of this when studying the energy released by muscle. Much of our knowledge is based on that acquired by A. V. Hill and his colleagues from many years' work on isolated frog muscles. In most experiments the muscle is laid over a thermopile — an array of thermocouples arranged in series — so that very small changes in temperature can be measured.

Fig. 9.11. The power output during tetanic isotonic contractions plotted against different loads and velocities.
Fig. 9.12. The rate of heat production of frog sartorius muscle during an isometric tetanus. The muscle was stimulated for a period of 1.2 s. From Hill and Hartree (1920).

During an isometric tetanus, heat is released at a very high rate for the first 50 ms or so (this is usually called the activation heat), falling rapidly to a lower more steady level which is usually called the maintenance heat (Fig. 9.12). If the muscle is allowed to shorten, an extra amount of heat is released during the shortening process. This heat of shortening is roughly proportional to the distance shortened. Further heat appears during relaxation, especially if the load does

Fig. 9.13. How efficiency varies with velocity during isotonic tetanic contractions. From Hill (1950b).

work on the muscle. Finally, after relaxation, there is a prolonged recovery heat as the muscle metabolism restores the chemical situation in the muscle to what it was before the contraction took place.

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