CHAPTER V 



THE UTILISATION OF FOOD AND ENERGY IN MUS- 

 CULAR WORK — LAWS OF PRODUCTION OF ENERGY 



(i) The sources of muscular energy, 

 (a) Protein as a source of muscular energy. 



SOME of the many investigations which have 

 been carried out on dogs and men have 

 shown that even during hard work the quantity 

 of protein metabolised was not much in excess of 

 that during rest. In other cases an increase, which 

 was sometimes quite considerable, was observed 

 during work. Sometimes, too, but this was ex- 

 ceptional, the performance of work resulted in an 

 increased metabolism of nitrogen-free substances. 

 In all these investigations the time during which 

 they were carried out was only short, so that the 

 energy which had been stored up whilst the animal 

 was at rest was possibly sufficient to provide for 

 the extra work. The matter was first placed in a 

 clear light when the work was allowed to go on for 

 a longer period, as was done in the following ex- 



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