THE BALANCE OF ENTROPY UNDERLYING MUSCLE PERFORMANCE

A general equation for muscle energy balance is derived, describing ir reversible macroscopic processes of energy transformation in a muscle as a whole, and the interaction of the muscle with its surroundings. T he formulation of the equation stems from the balances of mass, mechan ical energy, internal energy and entropy. The equation involves isoton ic and isometric contractions, as well as energy dissipation stimulate d in a non-contracting state. For an isotonic muscle contraction our f ormulation approximates the Hill equation. An isometric contraction is approximated by the time-dependence of the external muscle force, inv olving the fluxes of heat and mass. Using the implicit dissipative com ponent of a pressure tenser (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 sta tes. A noncontracting, energy-dissipative muscle is characterized by a n uncoupling between the power of chemical reactions and the dissipati ve part of the tension tensor; the dissipated energy then manifests it self in fluxes of heat and mass. The quantitative estimation of the di ssipative power of a muscle under various physiological conditions sho uld clarify the general role of a muscle, including its dissipative no n-contracting state.