The factors determining the maximum work and the mechanical efficiency of muscle

Abstract

The work done by frog muscles tetanized for 0.25 or 0.1 sec. in a Levin-Wyman ergometer at various speeds of shortening is in each case a maximum at intermediate speeds. At lower speeds, relaxation begins before the shortening is complete; at higher speeds, the external work is decreased in overcoming "viscosity" of the muscle. This relation holds at various temps., but the speed for maximum work is 21/2 times as great at 14.5[degree] C. as at 0[degree]. The maximum work increases (more rapidly at higher temps.) with duration of the stimulus. The optimal speed is independent of the duration over a narrow range tested, but with stimuli of 1 sec. it may be reduced. It is the same for different muscles. Delaying the start of the ergometer briefly (e. g., 0.04 sec. for 0.1 sec. stimulation) after beginning stimulation slightly but definitely increases the work. Under optimal conditions and with the heat production measured by a thermopile, the mean maximum efficiency, E max., for the initial phase of contraction was found in 56 observations on 21 muscles (with stimuli from 0.03 to 1.0 sec. at 0[degree] C. to 15[degree] C.) to be 26%. The av. deviation from the mean was 2%. E max. is independent of temp., although at 0[degree] C, but at no other temp., there was a very slight tendency for it to be higher with longer stimuli. Equations for human muscle efficiency are restated for these exps. by introducing c, the excess duration of the contraction over that of the stimulus. In Emax. = [1[long dash]k/]/a, in which k is the "viscosity," and a( = 1), a quantity representing the energy required to set up a contraction of unit strength, k/c = 0.74. The max. efficiency is determined by the ratio of the "viscosity" to the time occupied in a single twitch. The great energy cost of maintaining a contraction of frog muscle requires for high efficiency a short contraction, so that "viscosity" markedly reduces the mechanical work. In man, with E max. about 50% (initial phase), the maintenance of a contraction is relatively inexpensive, longer contractions permit high efficiency, and "viscosity" has a smaller reducing effect.