THE PHENOMENON OF CONTRACTION. 



39 



of the same muscle under different nutritive conditions. Many 

 observations of a similar character have been carried out upon man 

 when doing muscular work of various sorts, such as mountain 

 climbing, bicycle riding, etc. In such experiments the total 

 energy given off during the work may be estimated by the methods 

 of direct or of indirect calorimetry (p. 956). In the former case 

 the heat produced in the body is determined directly, in the latter 

 case it is estimated on the basis of the amount of material con- 

 sumed. The body at rest produces constantly a certain amount of 

 heat which can be measured. In determining the efficiency of 

 the human muscles in work the heat production during rest is 

 subtracted from the heat production during work, the difference 

 giving the new energy developed in the muscles during work. 

 Thus, for example, in experiments made at the Nutrition Labora- 

 tory of the Carnegie Institution results of this kind were obtained:* 



The average results of such experiments show an efficiency of 

 about 21 per cent., which compares favorably with the figures ob- 

 tained for mechanical engines, such as the steam engine (15 to 25 

 per cent.). It must not be assumed, however, from this comparison 

 of the muscle with a steam engine that in the former, as in the 

 latter, there is a conversion of part of the heat to mechanical work. 

 This view was held formerly in physiology, but considerations, 

 which are referred to below (p. 71), have convinced most physiol- 

 ogists that the muscle is not a heat engine. As stated in the pre- 

 ceding paragraph, it is the portion of the original chemical potential 

 energy which does not take the form of heat that is utilized in 

 some way by the machinery of the muscle to perform work through 

 the shortening that is produced. 



The Curve of Work and the Absolute Power of a Muscle. — 

 The statements in the preceding paragraph prove that the muscle, 

 judged from the standpoint of a machine to do work, compares most 

 favorably in its efficiency with machinery of human construction. 



* Benedict and Cathcart, "Muscular Work — The Efficiency of the Human 

 Body as a Machine," Carnegie Institution, 1913. 



