38 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



which 1 calorie = 426.5 grammeters of work. The work, 0.4 

 grammeter, supposed to be done in the above experiment, would be 

 equivalent, therefore, to 0.4 -f- 426, or about 0.001 of a calorie. 



The Efficiency of the Muscle as a Work Machine. In any 

 engine constructed to perform external work it is desirable to know 

 what efficiency the engine exhibits, that is to say, what per cent, of 

 the total energy given to the engine or developed in the engine may 

 be obtained as external or mechanical work. If we represent by Q 

 the total energy developed and by W the energy, expressed in the 

 same unit, obtained as work, then the efficiency E is given by the 

 formula 



=-? 



Numerous experiments to determine this value have been made 

 upon the isolated muscles of frogs. These muscles can be taken 

 from the body, be made to contract under different loads by arti- 

 ficial stimuli, and the work done and the heat developed can both 

 be measured with considerable accuracy. The earlier experiments 

 of this kind indicated that the frog's muscle under optimum condi- 

 tions may exhibit an efficiency as high as 25 to 30 per cent., as is 

 shown by the following example taken from Fick's investigations:* 



Load. Heat Developed. Mechanical Work. Therm ^- ^ivalent Efficiency. 



200 gm. 25.6 microcalories. 2905 gm.-mm. 6.83 microcalories. ^f = 26+ %. 



oo 



Experiments have shown that the efficiency of different muscles 

 in the same animal varies or may vary, and likewise the efficiency 

 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. 947). 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 :f 



* Fick, "Pfliiger's Archiv," 16, 58, 1878. 



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

 Body as a Machine," Carnegie Institution, 1913. 



