WORK PRODUCTION 555 



the conclusion seems warranted that if any difference exists 

 in the utilization of the energy of fats and of carbohydrates it 

 is too small to be of much practical significance. 



Conditions affecting efficiency 



658. Efficiency varies. As appears from the foregoing 

 summary (656), the net efficiency of the animal body as a 

 motor is comparatively high in the case of the horse, considerably 

 exceeding in most instances 30 per cent. It may be said in a 

 broad general way that with this animal about one-third of the 

 energy metabolized for a specific form of muscular exertion (i.e., 

 in excess of maintenance or of maintenance plus locomotion) is 

 recovered in the mechanical work done. It is also evident, 

 however, that the organism has no one fixed degree of efficiency 

 but that the latter may vary through a somewhat wide range 

 under different conditions. 



659. Forms of work done. The experiments thus far cited 

 refer largely to work done by walking horizontally or up a 

 grade, with or without draft. Of all the forms of work yet in- 

 vestigated, the ascent of a moderate grade or, in other words, 

 the lifting of the body by the legs, appears to be the one which 

 is performed most economically, a net efficiency of over 36 

 per cent being reported both for the horse and for man. This 

 percentage, however, decreases considerably as the angle of 

 ascent is increased. For horizontal locomotion, as already noted 

 (652), Zuntz computes an efficiency of 35 per cent. Draft up a 

 slight grade was performed somewhat less efficiently in the 

 case of the horse, the percentage being approximately 31, 

 while draft up an 8-J per cent grade was done with an efficiency 

 of less than 23 per cent. 



Other forms of work appear to be performed with a less 

 degree of efficiency. Thus experiments on man in which the 

 work was done by turning a crank with the hands have shown 

 decidedly lower efficiencies than those in which the work was 

 done on a treadmill. The same was true in the experiments of 

 Benedict and Cathcart on man, in which the work was done upon 

 a stationary bicycle, the maximum figures computed l for the 

 net efficiency of 6 subjects ranging from 20.4 to 25.2 per cent. 



1 Loc. cit., p. 125. 



