Dependence of energy output on force generation during muscle contraction.

It has been proposed that the energy (heat + work) output of an isometric twitch is determined by the force that is generated under conditions of invariant activation, irrespective of muscle length. To test the effect of length and force on total energy output, muscles were stretched by increments beyond the muscle length at which twitch force is maximum (LO) and then stimulated; energy output and force then were measured. These data were compared with isovelocity twitches in which stimulated muscles, initially at different lengths, shortened (near maximum velocity) a constant distance and then redeveloped tension at lengths less than LO. If energy liberation was determined by force generation, plots of energy output versus force produced would be parallel with isovelocity twitches liberating extra energy as shortening heat. As predicted, the ratio of the slopes (n = 13) of these relations, 0.98 +/- 0.02, was not different from 1 and the shortening heat coefficient (alphaF/Pot, measured from the difference in intercepts), 0.15 +/- 0.01, was near to the expected value. Therefore, energy liberation in twitches appears to be uniquely determined by force generation and not by muscle length.