248 THE PHYSIOLOGY OF MUSCULAR WORK 



THE INFLUENCE OF MUSCULAR WORK UPON THE EXCHANGE 

 OF MATERIAL 



It has already been pointed out that the law of the conser- 

 vation of energy applies to the performance of muscular work, 1 

 and data have been given to prove the great increase in the intake 

 of oxygen, the output of carbon dioxide and the production of 

 heat. The respiratory exchange represents only a portion of the 

 total exchange which must be determined in any experiment upon 

 the total income and output. The nitrogenous exchange must be 

 estimated from the total nitrogen of the food ingested and of the 

 urine and faeces excreted. In addition, the composition of the 

 food in terms of carbohydrate and fat as well as of protein should 

 be known, and also the amount of water taken in by, and discharged 

 from, the body. 



It is impossible in this article to consider this subject fully, 

 but attention may be directed to the chief results of experiments. 

 The nitrogenous output is dependent upon the intake of nitro- 

 genous food, not upon the performance of muscular work ; the 

 intake of oxygen and the output of carbon dioxide are immediately 

 raised by muscular exercise, and under normal conditions are pro- 

 portional to the work done. It will be understood that the term 

 work is here used in the sense of physiological work which may 

 or may not be immediately apparent as physical work expressed 

 in foot-tons or kilogrammetres. The first time a man performs 

 work, to which he is unaccustomed, he expends more energy than 

 he will do in performing the same work after, frequent practice. 

 One of the most important effects of training is the economical 

 performance of work. 



The fact that the nitrogenous output is not increased by work 

 would suggest that the energy is supplied by carbohydrates and 

 fats. It is necessary to give briefly the evidence upon these points. 

 In the first place the experiments of Pettenkofer and Voit ( 47 ) show 

 that the excretion of nitrogen in the urine is the same whether a 

 man does work or is at rest ; the output of nitrogen is determined 

 by the amount of that substance taken in the food. In their first 

 series of experiments the mean excretion of nitrogen in the urine 

 during hunger was 12-4 grms. during a day of rest and 11'8 grms. 



1 See Atwater, Ergbenisse der Physiol., 3ter Jahrgang, Abtheilung 1, 1904,8. 497. 



