AJP - Cell Physiology, Vol 235, Issue 1 20-C24, Copyright © 1978 by American Physiological Society
Dependence of energy output on force generation during muscle contraction
J. A. Rall
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.
