Oxygen Intake and Oxygen Debt as Limiting Factors in Exercise

The maximum effort which can be exerted over a given period of time is limited by the maximum amount of oxygen which the subject can absorb per minute and by the maximum oxygen debt which he is able to contract. Since these are both measurable quantities, it should be possible to predict the limits of exertion for any individual from the results of laboratory tests (neglecting emotional factors). A welltrained athlete may be able to absorb 4 liters of oxygen per minute and to acquire an oxygen debt of 15 liters. It has been firmly established that, when the maximum oxygen debt has been incurred, the body becomes incapable of further effort. These facts permit one to estimate the duration of exertion which is possible when the oxygen requirement is greater than the maximal oxygen intake. 8 Assume that an athlete is able to absorb 4 liters of oxygen per minute and to incur an oxygen debt of 15 liters. If he runs at a speed requiring 5 liters of oxygen per minute, he must go into debt for oxygen at the rate of 1 liter per minute, and this intensity of exertion could be sustained for 15 minutes. If the speed of running is increased until the oxygen requirement is doubled, the excess of oxygen requirement over oxygen intake is 10--4 = 6 liters per minute, and exhaustion would occur at the end of 15/6 = 2.5 minutes.

In running, the oxygen requirement increases as the square or cube of the speed. Therefore doubling the rate of running increases the oxygen requirement per minute from 4 to 16 times. A man does not have time to incur his maximal oxygen debt in short sprints. It has been estimated that 50 to 55 seconds of running at top speed would be required before the maximal oxygen debt would be reached. Since the maximum amount of exertion which is possible before exhaustion occurs is determined by the upper limits of the oxygen intake and the oxygen debt, the question naturally arises as to the factors which set these limits. The factors limiting oxygen intake have been mentioned earlier in this chapter and will be discussed in detail in later chapters. So far as the oxygen debt is concerned, the practical limit seems to be set by the tissue acidity which results from lactic acid accumulation. Since lactic acid is a strong acid, it cannot exist as such in weakly alkaline media such as the blood and tissue fluids. As soon as lactic acid is formed during exercise, it is neutralized or "buffered" by alkali. As a result, the increase in acidity is much less than it would be if the acid remained in the free form. Presumably, the amount of lactic acid which the body can tolerate will depend largely on the amount of buffer alkali available. If this is large, more lactic acid can accumulate before the tissue acidity rises to all intolerable level. Meyerhof showed that the lactic acid concentration in the exhausted isolated muscle is much greater if the muscle is kept in all alkaline medium. Also in man the values of blood lactic acid following exhaustion are much higher if the man started the work with higher alkali reserve. This may mean that it is possible to increase the capacity for work by increasing the alkali reserve. That this is, indeed, true is indicated by experiments which demonstrated that the maximum blood lactic acid concentration following running to exhaustion on a treadmill was increased by the previous administration of a dose of sodium bicarbonate.