The Oxygen Debt of Exercise

The ability of man to go into debt for oxygen is dependent, at least in part, on the formation of lactic acid. Hence the quantitative relation between the oxygen debt and the amount of lactic acid formed during exercise is of importance in an understanding of the limiting factors in exercise. Since a direct estimation of the total amount of lactic acid in the human body at any one time is, of course, impossible, it is necessary to rely partly on animal experiments and partly on inference from the blood lactic acid concentration. It is still uncertain whether blood lactic acid concentration is a true index of the lactic acid concentration in the corresponding tissues.

The equation for the oxidation of lactic acid may be written: C3H6O3 + 3 O2 → 3 CO2 + 3 H2O. Three molecules of oxygen are required for the oxidation of one molecule of lactic acid. In terms of gram equivalents, 3 × 22.4 liters of oxygen will oxidize 90 grams of lactic acid, or each gram of lactic acid requires 746.7 ml. of oxygen. In isolated frog muscle, only 150 ml. of oxygen are consumed for each grant of lactic acid which disappears, or about one-fifth the theoretical amount. This led to the belief that the oxidation of one-fifth of the lactic acid produced furnishes energy for the reconversion of the other four-fifths to glycogen. There is no direct evidence that any of the lactic acid is oxidized during recovery in oxygen.

It is not lactic acid that is oxidized in recovering muscle, but its equivalent in glucose. In this case all of the lactic acid would be reconverted to glycogen, while a quantity of glucose equivalent to approximately onefifth of the lactic acid so removed is oxidized. A portion of the energy thus liberated is used in the resynthesis of glycogen from lactic acid, while the rest appears as heat. In the exercising human subject, the quantitative relation between oxygen debt and lactic acid production is not so simple as it is in isolated frog muscle. Margaria, Edwards and Dill 6 have made a careful study of the relation of oxygen debt to metabolic rate and to lactic acid production. Some of their results necessitate a considerable revision of Hill's original theory. Two of their findings deserve particular emphasis: (1) in moderate exercise, the oxygen debt may reach 3 or 4 liters with no evidence of lactic acid accumulation, and (2) a considerable fraction (about one-third) of the total oxygen debt is repaid very rapidly (within three minutes) after the cessation of exercise, while repayment of the remainder of the debt may require several hours. Oxygen debts greater than 3 or 4 liters there is a linear relation between oxygen debt and blood lactic acid concentration. From these and other facts, the authors drew the following conclusions: The excess oxygen consumption following exercise is really made up of two fractions: (1) a true "oxygen debt" which is used to repay an oxygen deficit incurred during exercise and (2) an increased "basal" oxygen consumption which may last for several hour's or longer, and is not used for the reversal of any of the reactions which occurred during the exercise. The oxygen debt, in turn, consists of two fractions: (a) the "alactacid" debt, which is not related to the accumulation of lactic acid and is repaid within a few minutes after exercise ceases, and (b) the "lactacid" debt which is proportional to the lactic, acid accumulation and may require an hour or longer for repayment. The excess oxygen consumption of the alaclacid phase probably is used in the oxidation of the same fuels used by the resting muscle, and the resulting energy is perhaps used for the resynthesis of phosphocreatine or of adenosine triphosphate. The lactacid phase certainly represents the excess oxygen consumption which furnishes the energy for the reconversion of lactic acid to glycogen. The reason for the long duration of this phase is uncertain. The conversion of lactic acid to glycogen and the oxidation of glucose are both rapid reactions, while the oxidation of lactic acid is a relatively slow process. For this reason, Margaria and his associates suggest that it may be true, as Hill believed, that the energy for resynthesis of glycogen is derived from the oxidation of a portion of the lactic acid, and not from the oxidation of an equivalent amount of glucose, as indicated by other types of evidence.

Finally, from 4 to 5 per cent 7 of the total recovery oxygen is used by the heart and respiratory muscles and hence is not directly related to the recovery process.