Muscle Thermodynamics

Abstract

This chapter explains the physical principles involved in the interpretation of the heat produced by muscle, i.e. the meaning of internal energy, enthalpy, entropy, free energy and efficiency. After a short description of the methods used to measure heat production on the isolated muscle specimen, this is divided into resting heat of the relaxed muscle, and initial heat developed during contraction with its components: (i) activation and maintenance heat during isometric contractions at different muscle’s lengths, (ii) shortening heat with its dependence on the applied load, (iii) the Fenn effect, i.e. the additional energy output during shortening, with its suggestive relationship between thermal and mechanical measurements, and (iv) the heat produced during stretching the contracting muscle at different velocities of lengthening. The heat produced after contraction, the relaxation heat, is shown to evolve on a much slower time scale than the initial heat with a trend that parallels that of oxygen consumption by muscle. Finally, muscular efficiency is defined in physical terms after the contraction-relaxation cycle, and distinguished from the improperly called ‘initial efficiency’ measured during shortening against different loads.