Supplement to '' NatiL7'e^' July 14, 1923 



in fresh air from the outside atmosphere and expire 

 it all automatically into the bag. A sample of the 

 expired air can be collected for any desired interval. 

 An analysis of it^ a measurement of its volume, and 

 a knowledge of the composition of the inspired air, 

 allow a determination of the oxygen taken in and the 



Fig. 2. — Bag, pipe, tap, v.alves, and mouthpiece used to investigate 

 the gaseous exchanges of man during running. 



carbon dioxide produced. From these the amount of 

 energy used by the man during the period in question 

 can be calculated. A point immediately brought out 

 is (as in the isolated muscle) that the oxygen must be 

 regarded, not as being used during the actual exercise 

 itself, but in recovery, each element of the oxygen 



Fit;. 3.— Oxygen intake in recovery after exercise : exercis 

 ends at time o. 



consumption corresponding to recovery from a previous 

 element of the exercise (Fig. 3). 



Many kinds of exercise have been investigated, for 

 example bicycling, swimming, climbing, walking, 

 running, ski-ing, and skating, and even the laborious 

 process of pushing a motor bicycle up a hill ! The 



two main types of muscular exercise are : (a) very 

 violent exercise lasting for a short time, and {b) 

 prolonged exercise of a more moderate kind. 



Violent Exercise. — Let us take first the case of very 

 severe exercise, for example, that of a man running 

 100 yards at top speed. The first personal impression 

 which one forms of such severe exercise is that 

 immediately after it, and often for a comparatively 

 long time after it, panting occurs. The oxygen taken 

 in is used almost entirely in recovery. In one experi- 

 ment a good runner ran 225 yards in 23I seconds, and 

 in the succeeding quarter of an hour recovered from 

 his effort and used an extra 8| litres of oxygen in so 

 doing. Such exercise, if it could be continued in- 

 definitely, would require about 22 litres of oxygen 

 every minute, but from other experiments the subject 

 is known to be incapable of taking in more than about 

 4 litres per minute. Hence, during the most violent 

 effort of which he was capable, he was using energy at 

 about 5 J times the rate that would have been possible 

 had it been necessary for him to depend upon a con- 

 temporary supply of oxygen. 



The " record " is held by a man of 46, who by means 

 of a rapid quarter-of-a-mile run, followed by violent 

 gymnastic exercise for 30 seconds, succeeded in making 

 himself so exhausted that 13J litres of oxygen had to 

 be used in recovery. This amount of oxygen would 

 have maintained him quietly in bed for about an hour ! 

 It is clear that the body can get energy " on credit," 

 which it has to repay after the exercise is over, by 

 taking in later an extra amount of oxygen. It acts in 

 the same manner as an accumulator, which can be run 

 down at a very high rate for a short time and recharged 

 afterwards. The discharge process is the formation of 

 lactic acid from glycogen : in recovery this is reversed, 

 the energy for the reversal being provided by com- 

 bustion. The maximum lactic acid production in the 

 muscle determines the limits of exercise, and the 

 magnitude of the " maximum oxygen debt." 



Prolonged Exercise. — Let us now discuss the case 

 of exercise continued for a long time. By the most 

 extreme effort of the respiratory system, a healthy 

 man can take in about 4 litres of oxygen every minute. 

 Consider, then, the case of a man taking exercise for a 

 long time, say for an hour, during which time he will 

 take in and use anything from 150 to 240 litres of 

 oxygen. An oxygen " credit " even of 13J litres is 

 only a small fraction of the oxygen which he can 

 actually take in during the hour of exercise. Hence, 

 he is limited in such types of exercise, not by the 

 magnitude of the " debt " to which the body can submit, 

 not, that is to say, by the lactic acid maximum of his 

 muscles, but chiefly by the maximum rate at which he 

 can take in oxygen. The oxygen is brought to the 



