Chemical Changes in Muscle During Contraction and Recovery

Muscle may be regarded as a machine which transforms chemical energy into work. The mechanism of this conversion remains obscure despite the enormous amount of research which has been directed toward its solution. Accordingly, the views which are presented here must be regarded as tentative and subject to almost certain modification as new data become available. The difficulty is that, although the time course of tension development and heat production can be followed quite accurately, there are no methods sufficiently accurate and rapid to permit a study of the chemical changes which accompany and follow a single twitch. In fact it is necessary to stimulate a muscle for several seconds involving scores of contractions before the chemical changes become great enough to permit accurate analysis. In the discussion which follows an attempt will be made to correlate some of the more striking chemical changes with the phases of activity and recovery in skeletal muscle under both aerobic and anaerobic conditions.

Carbohydrate Metabolism

The metabolism of carbohydrate in muscle begins with glycogen, a form of animal starch. The exact chemical structure of glycogen is uncertain, but it is known to be built up by the combination of large numbers of glucose molecules with the splitting out of water. Glycogen is the common storage form of carbohydrate in the body, and is found in large amounts particularly in the liver and in skeletal muscles. The lactic acid (an organic acid produced by the partial breakdown of carbohydrate) accumulates in muscles contracting to the point of fatigue and disappears during recovery of the muscles if adequate oxygen is present. Fletcher and Hopkins concluded that lactic acid production (probably from glycogen) is the fundamental chemical reaction producing energy for muscle contraction; they suggested that the development of tension may be due to an action of lactic acid on the colloidal protein contractile substance in muscle. According to this relatively simple theory contraction itself is an anaerobic process, and oxygen is necessary only for the oxidative removal of lactic acid, since its accumulation leads to loss of irritability of muscle.

Much of the lactic acid is formed after contraction and relaxation are over, so that the formation of lactic acid cannot be an immediate source of energy for muscle contraction. Lactic acid production in normal muscle contraction is associated, either directly or indirectly, with the development of tension in anaerobic contractions, but there is no satisfactory evidence that lactic acid is produced under aerobic conditions. Some lactic acid may be formed during short periods of tetanic contraction in intact cat muscle, but there is some doubt whether the oxygen supply was completely adequate in these experiments.