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

Abstract

In order, in a prolonged contraction, to obtain the maximum work from a muscle, the load must be so adjusted that at every stage the muscle is just, and only just, able to overcome it; and the speed of shortening must be as low as possible. Levin and Wyrnan (1), in their work on the "viscosity" of muscles, employed an ergometer which, allowing the muscle to shorten at any described speed, measured the maximum work which it was capable of performing at that speed. Their instrument, which records a tension-length curve on a fixed smoked surface, is very accurate and convenient to use, and it is theoretically inconceivable that greater work—at a given constant speed—could be recorded by any other means. The only way to increase the work is to decrease the speed, in order to reduce the energy wasted in overcoming the internal resistance of the muscle. From the point of view of the mechanical efficiency of muscle ( i. e. , ratio of work done to total energy liberated) prolonged contractions are to be avoided, since they require large amounts of energy to be liberated in maintaining them (7). The matter has been discussed by one of us and his colleagues in several places (2), (3), (4), (5), p. 32, (6), pp. 48 and 81. It is clear that for a high efficiency the contraction must be of comparatively short duration: there is indeed, for human muscles, an optimum duration at which the efficiency is greatest. We were led therefore to a consideration of the maximum work obtainable in the response to a short stimulus, and to an experimental determination of the maximum mechanical efficiency of the frog's isolated muscle. The matter is much more complicated than is the simple case of a prolonged contraction, considered by Levin and Wyman, and it has required the examination of the effects of varying several different factors.