The Metabolism of Muscle

When muscles contract, energy is liberated. If a muscle shortens against a load (isotonic contraction) work is done and part of the energy is thus accounted for; the rest appears as heat. If the muscle is unable to shorten (isometric contraction), all the energy which is liberated is eventually dissipated as heat. In either ease, the liberation of energy is the result of certain chemical reactions which occur within the muscle fibers. Since the total energy of contraction appears as heat in an isometric contraction, and since this heat can be measured very accurately, an analysis of the time course of heat liberation in this type of contraction furnishes valuable cities to the nature of the chemical reactions in tile contracting muscle. This is all the more important in view of the fact that the chemical changes themselves are so small and so fleeting that they can be measured accurately only after a tetanus or a series of twitches and little information is ained about tile sequence of these changes. For these reasons, it is well to begin our study of the metabolism of muscle with a brief consideration of the heat liberated during an isometric twitch.

The Heat Production of Muscle

The contraction of muscle is associated with the metabolic breakdown of certain chemical compounds. During recovery these chemical compounds must be rebuilt and waste products removed if the original contractile power of the muscle is to be restored. Both of these processes yield heat. While this heat production serves a useful purpose in maintaining body temperature in cold environments, it is an inevitable by-product of muscular activity which may render exertion in a hot environment disagreeable or even impossible. It represents energy consumed with no mechanical work produced and hence is a measure of the inefficiency of our muscles considered as machines.

When a muscle performs a simple twitch, heat is liberated in two fairly distinct bursts. The first, which is called initial heat, accompanies the development and subsidence of tension. Hartree 1 has shown that the initial heat of a twitch occurs in two phases: (1) the contraction heat, which rises and then falls off during the developmerit of tension, and (2) the relaxation heat, which is liberated during the subsidence of tension. After the contraction is over, there is a second and slower production of heat, the delayed heat, also called the recovery heat since it is due to chemical changes which restore the muscle to the condition in which it was before its response. When a muscle contracts tetanically, there is a further heat production during the maintenance of the contraction.

We usually associate heat production in living tissues with the oxidation of energy-yielding compounds. However, many chemical reactions of a non-oxidative nature also result in the liberation of heat. Heat which is liberated only when oxygen is present must be due to oxidative processes and is frequently called aerobic heat: heat which is liberated in the absence of oxygen must be due Io non-oxidative processes and is referred to as anaerobic heat. A study of the heat production of muscle under both aerobic and anaerobic conditions yields data from which we may draw certain conclusions about the general types of reactions which occur during contraction, relaxalion and recovery in muscle. The lime course and magnitude of the initial heat are essentially the same whether the muscle is contracting in an atmosphere of oxygen or one of nitrogen. Thus, the chemical reactions which are associated with actual contraction and relaxation of the isolated muscle are presumably non-oxidative, or anaerobic. The magnitude of the delayed heat is greatly diminished in the absenee of oxygen, so that oxidation plays an important role in the recovery of muscle from the effects of contraction. Apparently the energy for contraction of muscle is liberated by the "explosive" breakdown of compounds with high potential energy, and oxygen is not necessary for this breakdown. During recovery these compounds must be rebuilt in order that energy may be available for subsequent contractions. This process of rebuilding requires energy which is obtained, at least in part, from oxidations.