Muscle energetics and the Fenn effect.

This study suggests and statistically tests a consistent analytical method for isolating the critical variables governing the Fenn effect in frog sartorius muscle. It demonstrates that when the Hill factor (P(0) - P) is used to normalize the heat of shortening, or the work, or their sum, the enthalpy of shortening, these reduced energy terms are highly linear functions of the time of shortening. Linear correlation of a given form of reduced energy for a given muscle, against the time of shortening yields correlation coefficients of 0.998 or greater. From the regression equations, equations similar in form to the 1938 Hill force-velocity, and 1964 Hill heat of shortening equations, are deduced. The analysis suggests that the efficiency with which the extra energy of shortening is converted into work is nearly constant over most of the range of fractional loads, and it confirms Fenn's observation that the total extra energy of shortening is about 1.3 times the work. Thus it is also consistent with the results of most of the biochemical studies which correlate the extra breakdown of high energy phosphates entirely with the work done, and no component exclusively with the distance shortened. While the analysis successfully identifies the two factors regulating the release of the energy of shortening, time and the fractional load, there was insufficient data in the literature to develop the form of the relation between the size and the geometry of the muscle and the rate of energy release. Therefore until more data are available it seems unjustified to assume that either b/l(0) and a/P(0) (or the equivalent linear regression constants) are independent of these factors.