A thermodynamic approach to the compromise between power and efficiency in muscle contraction.

An analysis of muscle contraction based on irreversible thermodynamics is presented in this work. From reported experimental results, it is shown that a muscle, as an energy converter, does not obey linear phenomenological relations. Some more suitable nonlinear phenomenological relations are found and discussed from an evolutionary point of view. In those relations a parameter alpha appears which is related to the nonlinearity of the equations. It is suggested that this parameter determines the compromise between the power output and the efficiency reached by the muscle. By analysing the energetics of some suitable performance regimes, we suggest that the fact that alpha values greater than 0.4 are not found in muscles of higher species, can be interpreted as an evolutionary necessity.