The Balance of Entropy Underlying Muscle Performance

Abstract

A general equation for muscle energy balance is derived, describing irreversible macroscopic processes of energy transformation in a muscle as a whole, and the interaction of the muscle with its surroundings. The formulation of the equation stems from the balances of mass, mechanical energy, internal energy and entropy. The equation involves isotonic and isometric contractions, as well as energy dissipation stimulated in a non-contracting state. For an isotonic muscle contraction our formulation approximates the Hill equation. An isometric contraction is approximated by the time-dependence of the external muscle force, involving the fluxes of heat and mass. Using the implicit dissipative component of a pressure tensor (sometimes called the "friction clutch" mechanism), the external force can be expressed in units of power, and this can be quantitatively compared with two other muscle loading states. A noncontracting, energy-dissipative muscle is characterized by an uncoupling between the power of chemical reactions and the dissipative part of the tension tensor; the dissipated energy then manifests itself in fluxes of heat and mass. The quantitative estimation of the dissipative power of a muscle under various physiological conditions should clarify the general role of a muscle, including its dissipative non-contracting state.

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