INFLUENCE OF MUSCULAR WORK ON METABOLISM 113 



immediately as soon as work begins and has been known to increase from 

 27 to 131 g. per hour. There is no corresponding increase in the excretion 

 of N", as we have seen, hence this excess of C0 2 must represent an increased 

 oxidation of nonnitrogenous food. 



In the experiments of Pettenkoffer and Voit on men, the daily amount of 

 fat destroyed in fasting rose 171 g. as the result of work, and upon an 

 ordinary diet it rose 101 g. The experiments of Atwater have yielded similar 

 results. In one of his subjects the total metabolism during rest amounted 

 to 2,357 Cal., of which 429 came from proteid, and 1,928 came from non- 

 nitrogenous food. With severe muscular work the metabolism rose to the 

 mean value of 5,119 Cal., of which 462 came from proteid, and 4,657 from 

 nonnitrogenous food. 



By parallel experiments, in which on the one hand chiefly fat, and on the 

 other chiefly carbohydrates were fed, Zuntz has shown very definitely that the 

 muscles can use one kind of nonnitrogenous food as well as the other. The same 

 appears also from Atwater's experiments. 



We can say, therefore, that the muscles are able to perform their work at 

 the expense of all three classes of organic foodstuffs, that they prefer the 

 nonnitrogenous substances, and, as it appears, they draw upon the carbohy- 

 drates first. Thus the one group or the other is utilized according to the 

 kind of food eaten. The specifically carnivorous animals perform their mus- 

 cular work at the expense of proteid and fat; the herbivorous animals, espe- 

 cially our domesticated farm animals, at the expense of carbohydrates, and 

 in view of the large quantity of carbohydrates eaten by man the latter is 

 probably true of the human body also. 



It is of special importance for the physiology of general metabolism as 

 well as for the physiology of the muscle itself, to determine how large a part 

 of the increased transformation of energy accompanying muscular activity 

 takes the form of external work. 



For this purpose the respiratory exchange has been determined both during 

 rest and while an accurately measured quantity of work was being performed. 

 Subtraction of the carbon dioxide excretion and the oxygen absorption during 

 rest, from the corresponding factors during work, shows an absolute increase 

 which represents the known quantity of external work. In this way one can 

 readily obtain the amount of metabolism which 1 kg. m. of work represents and 

 these figures can be reduced to heat equivalents. 



Such determinations have been made in Zuntz's laboratory in Berlin, and 

 it has been found that the dog uses 0.007-0.0077 Cal. to do 1 kg. m. of 

 external work and the horse 0.0069 Cal. at the same kind of work (walking 

 uphill). And since theoretically 1 kg. m. = 0.00235 Cal., or 425 kg. m. = 1 

 Cal., we conclude that in these animals about one- third of the actual energy 

 developed in muscular work appears as effective external work. In man the 

 heat equivalent of 1 kg. m. of work with the lower extremities (mountain 

 climbing) is about the same, namely 0.0072 Cal. ; so that one-third of the 

 total energy developed in our own muscles also is utilized as external work. 



Even the slightest muscular movements influence the metabolism per- 



