SECTION IV 



THE EFFECT. OF MUSCULAR WORK ON METABOLISM 



WHEN an animal performs muscular work the energy for the work is derived, 

 as we have already seen, from the oxidation directly of the tissues of the 

 body but ultimately of the foodstuffs. This increased oxidation must 

 result in increased respiratory exchanges ; both the intake of oxygen and 

 the output of C0 2 must be raised. It is a matter of common experience 

 that any muscular exertion is attended with deeper and more rapid breath- 

 ing which, if the exertion is severe, may attain to a condition of dyspnoea. 

 In order to determine the quantitative relationship between the increased 

 oxidative processes of the body and the actual work accomplished, we must 

 investigate the respiratory exchanges of an individual by one of the methods 

 described on p. 662, in the first place during complete rest, in the second 

 place while he is doing a measured piece of work. The following Table 

 represents the respiratory exchanges in a trained muscular subject during 

 complete rest and while doing moderate or severe work, viz. riding a bicycle 

 with a brake. Each observation lasted from ten to fifteen minutes. 



The ' severe work ' in this table was carried to exhaustion, so that 

 the respiratory exchanges represent the maximal augmentation of which 

 the individual was capable. This augmentation may amount to between 

 ten and fourteen times the normal resting exchanges. Similar results are 

 obtained in observations extending over longer periods of time. Thus in 

 one experiment the average output of C0 2 during a six hours' period of rest 

 and starvation was 189-6 grm. During a rest experiment with food the 

 average output for the same period was 2304 grm. During work the 

 average output in the same individual during six hours rose to 705 grm. 

 of carbon dioxide on a carbohydrate diet, and to 634-8 grm. on a diet con- 

 taining a large amount of fat. The oxidation of carbon was therefore 

 increased more than threefold as a result of muscular work. 



We can find out the relation between the actual work done and the 

 energy expended on doing the work, either directly by allowing the work to 



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