Anaerobic and aerobic activity in isolated muscle

It is well known that a muscle excited anaerobically fails to contract any more when the concentration of lactic acid inside it rises to about 0·3 per cent. In the presence of oxygen the lactic acid disappears and the muscle recovers, but this does not prove that recovery is due solely to the removal of the acid; it might be argued that some other mechanism also needs restoration before the muscle can function normally again. Now it is possible, by suspending a sufficiently thin muscle in oxygen-free Ringer's solution and exciting it sufficiently slowly, to allow diffusion to remove the lactic acid as it is formed. Under such conditions the muscle is found to liberate 2 to 3 times as much energy as it would it diffusion were not permitted. It appears to continue working until all its carbohydrate is used up. Since, therefore, nothing is known to pass into the Ringer's solution except lactic acid, we may, until some other substance is found, attribute the absence of fatigue to the absence of lactic acid in the muscle, and conclude that when diffusion is not allowed fatigue is due primarily to lactic acid. In a recent paper (1, p. 51) the case was considered of a muscle excited by single shocks at regular intervals in oxygen-free Ringer's solution, attaining a "steady state" in which outward diffusion of lactic acid balances new formation. If a plane sheet of muscle of thickness 2 b , exposed on both sides to Ringer's solution, produce α gm. of lactic acid per c. c. per minute, and if k be the diffusion constant, the greatest concentration attained anywhere inside it, viz., in its middle plane, is (in gms. per c. c.) α b 2 /2 k .