CELLULAR RESPIRATION 133 



incurred during the anaerobic stages and the lactic acid is 

 excreted. This is true offish such as carp. By contrast a trout pays 

 off its oxygen debt when returned to normal environmental O2 

 tensions. 



During exercise the amount of lactic acid formed in man may 

 be quite considerable. For example, 4 grams can be produced in 

 a second, so that 40 grams will be accumulated by a sprinter at 

 full speed over 100 yards. Such an athlete can tolerate the 

 accumulation of about 127 grams of lactic acid in the body, 

 whereas an average healthy man would have to stop when only 

 half this amount had accumulated in his muscles. A sensation of 

 strain or fatigue is closely related to the blood lactic acid con- 

 centration, and lactic acid can be looked upon as a 'fatigue toxin' 

 in many instances. For each 6 to 7 grams of lactic acid, one litre 

 of oxygen is required for the resynthesis of glycogen or glucose. 

 Athletes can use between 4 and 5 litres of oxygen per minute. 

 Long-distance runners inspiring 3 J litres of Og per minute incur 

 no debt at all, and they are apparently in a steady-state condition 

 which can be maintained for 2 J hours or more. Their ability to 

 incur a debt of 16 litres of O 2 is extremely good and 19 litres is 

 probably an absolute limit found in only very few athletes. From 

 figures of this sort and a knowledge of the amount of O2 

 required at different speeds, it is possible to calculate the 

 distances which can be run at certain speeds and consequently 

 the maximum speed expected on such a physiological basis over 

 given distances in track performance. 



The graph showing the decrease in speed attained over 

 different distances (fig. 39) has a break at about 200 metres. 

 Up to this distance the main factor limiting speed is the maximal 

 contractile activity of the muscle. Oxygen uptake is unimportant 

 because there is insufficient time for it to be transported from 

 the lungs to the muscles. Nor over these short distances is the 

 accumulation of lactic acid sufficient to limit performance. In 

 middle-distance running the supply of energy is partly aerobic 

 and partly anaerobic, the accumulation of lactic acid playing an 

 important part in limiting performance. The aerobic and 

 anaerobic mechanisms may well be independent of one another 



