Muscular exercise, lactic acid, and the supply and utilisation of oxygen. —Part XI. Pulse rate and oxygen intake during the early stages of recovery from severe exercise

Abstract

At the commencement of muscular exercise the oxygen intake and the pulse rate increase rapidly, but soon attain a steady value depending on the severity of the exertion. In strenuous exercise the intake of oxygen, although large, may not be sufficient to effect the oxidative removal of all the lactic acid formed, and the body goes into “oxygen debt.” During the recovery, therefore, from severe exercise, as opposed to that from mild exercise, the rate of fall of the oxygen intake is less rapid than was its rate of increase during the first minute or so of exercise. We find a condition in which the oxygen intake is determined, not by the contemporary requirement of the body in respect of the exercise which it is taking at the moment, but by a “debt” which was incurred during a previous period. At the end of several minutes of violent effort there is a considerable need of oxygen, which is not satisfied completely for a comparatively long time. Thirty seconds after the end of such exercise the need for oxygen is presumably nearly as great as it was during the exertion itself. The fall in the oxygen intake, therefore, which occurs immediately at the end of exercise is determined, not by any appreciable change in the requirement of the body for oxygen, but rather by an alteration in the mechanism by which it can be supplied. The immediate fall, in fact, of the oxygen intake must be credited largely to a change in the circulation rate of the blood, determined may be by the abrupt cessation of bodily movement, and not to any sudden alteration in the need for oxygen. During the first few moments, therefore, after exercise ends, when the oxygen requirement is still high, we may regard the oxygen intake as some measure of the circulation rate of the blood, and it is natural to compare it with the only other factor which can be continuously recorded for the circulation, namely, with the pulse rate. The simultaneous determination of the pulse rate and of the oxygen intake, during the early stages of recovery from severe exercise, has been made in the experiments to be described.