Muscular exercise, lactic acid, and the supply and utilisation of oxygen.—Part XIII. The gaseous exchanges of restricted muscular exercise in man

The type of exercise studied in former papers of this series involves the activity of the body as a whole. The characteristic of all such forms of exercise is the free and vigorous movement of nearly all the muscles in the body. Since nearly all the muscles were in activity and behaving in a similar way it was more easy to compare their behaviour with that of an isolated muscle, and in previous papers it has been shown how closely the phenomena of muscular exercise in the body as a whole resemble those accompanying severe exercise in the isolated muscle. As far as concerns the observations described in the succeeding pages, the most pertinent conclusions of the former papers are as follows: (1) Provided that the exercise was not too severe, there occurred what has been called a steady state, in which recovery balanced breakdown in a manner analogous to that shown by Fletcher to occur in isolated frog’s muscle; (2) in severe exercise a considerable proportion of the energy employed is derived, not from contemporary oxidation, but by lactic acid formation on what may be called a “credit” of oxygen secured on the oxidation occurring in the recovery process later; (3) the most severe exercise can be maintained only for about 30 seconds, which corresponds to the time when the lactic acid concentration in the active muscle, as measured by the magnitude of the oxygen debt, may reach a value of about 0·3 per cent., the maximum value found to occur in the isolated frog’s muscle.