The application of irreversible thermodynamics to muscular contraction. Comments on a recent theory by S. R. Caplan

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Abstract
In contracting muscle a chemical reaction takes place in such a way that part of the free energy of the reactants can be converted into mechanical work. Recently Caplan (1965, 1966) has made a theoretical study of certain types of energy converters, using the methods of irreversible thermodynamics. He has extended this study to muscle and he makes several predictions of practical importance, notably about the efficiency of muscular contraction. In this paper a simplified summary of Caplan’s theory is given, with the equations rearranged, as far as possible, to provide relations between experimentally observable quantities. Four tests of the theory are then possible, of which the results are: 1. The theory correctly predicts the shape of the force-velocity curve. 2. The theory predicts a relation between the maximum efficiency of energy conversion and the degree of curvature of the force-velocity curve which is probably similar to what is found experimentally. 3. The theory predicts a relation between the force on the muscle and the rate of the driving chemical reaction which is markedly different from that which can be inferred from measurements of heat and work production. 4. The theory requires that there should be large variations in the free energy change of the driving reaction when the force on the muscle changes. It is difficult to see how these could occur in practice. No simple modification of the theory seems to avoid these difficulties. It is concluded that muscle is not an energy converter of the type described by the theory.