The effects of prolonged muscular exercise on the metabolism

Abstract During recent years a great deal of attention has been devoted to the changes in metabolism which result from very hard muscular work, in the main with the idea of correlating the alterations in the respiratory exchange and in the composition of the blood with the chemical changes responsible for the energy of muscular contraction in the isolated muscle under both aerobic and anaerobic conditions, This work, valuable though its results have been, has tended to divert attention away from another aspect of muscular work which is none the less of great practical importance. In everyday life the muscular work which we are called upon to perform is done at a far lower rate than that which is done by the athlete who is pushing himself almost to the limit of his power. It by no means follows that the facts observed during severe exercise, and the deductions made from these, can be applied without alteration when we wish to explain the effects of exercise of far less severity. It is true that there have been in the past many studies of the physiology of moderate muscular work, but when these are examined in detail they prove to be rather incomplete. As a rule those who have undertaken these investigations have devoted themselves to some limited aspect of the problem, such as the efficiency with which the work is performed under a variety of conditions, the influence of alterations of the diet on the capacity of an individual to do muscular work, or the vexed question of the relative importance of carbohydrate and fat as sources of energy during the performance of the work, and whilst much stress has been laid on events during the actual work too little attention has been given to changes which may result from the work and become apparent during a subsequent period of rest. It therefore seemed to us desirable to study in more detail the effects of prolonged though moderate muscular work, in the hope in particular of throwing further light on the significance of changes of the respiratory quotient, the interpretation of which is admittedly always a very difficult and troublesome problem, as may be gathered from the observations of Cathcart and Markowitz (1927) and he recent review of Rapport (1930), although it is one that must be faced if we are to gain an insight into the metabolic changes involved.