450 PRINCIPLES OF GENERAL PHYSIOLOGY 



is only to be obtained in optimal conditions, but its importance is obvious. If, 

 on the contrary, we include the heat given off in the restitution phase, as must 

 be done when we consider the muscle as a machine performing work by means 

 of food supplied, the efficiency is only about 50 per cent., .since, in the second 

 phase, the heat given off is about equal to the work of the first phase, while 

 no external work is done (A. V. Hill, 1913, 2, p. 465). But even this compare^ 

 favourably with the most efficient heat engine yet made. 



This high efficiency of striated muscle applies only to the single twitch, or 

 the act of raising a weight as opposed to that of keeping it supported: While 

 the process of raising a weight, that is, the performance of actual external work, is 

 a very economical one, that of keeping it suspended, without performance of 

 further external work, is much less efficient. In the frog, as A. V. Hill has 

 shown (1913, 4, p. 322), to maintain a particular state of tension in the sartorius 

 muscle it is necessary to liberate six or seven times as much energy per second as 

 that required to produce it. This fact suggests that the state of tension must 

 be maintained less wastefully in other forms of muscular structure, and perhaps 

 in striated muscle in natural modes of stimulation, a question to be discussed in 

 Chapter XVIII. on " Tonus." It may be that the state of shortening is kept up 

 without tension. 



This consumption of energy in processes by which no external work is performed 

 renders the calculation of the efficiency of the whole animal as a motor a matter 

 of considerable difficulty. The experiments of Zuntz, Benedict, and others, on 

 the heat and respiratory exchange of men doing measured amounts of work, are 

 beyond the scope of this book. The paper by Macdonald (1913) may be mentioned, 

 together with that by Glazebrook and Dye (1914), in which Macdonald's results 

 are used to obtain mathematical expressions relating to heat and work. In the case 

 of a particular individual, the efficiency comes out as 25 per cent. The detailed re- 

 searches of Benedict and Cathcart (1913) should also be consulted by those interested. 



It will be clear that the calculation of the efficiency of an animal as a motor 

 depends on how this estimation is made. Owing to the low efficiency of the 

 maintenance of tension, it will make considerable difference whether the calcula- 

 tion is made by taking the difference between the heat evolved in maintaining a 

 weight at a constant height, and that evolved in raising it from this height to a 

 further one, and again maintaining it at this position. The tension being the 

 same in the two maintenance positions, the heat production will be the same, and 

 the difference will be that associated with the performance of the external work. 

 In this way a high efficiency is arrived at. Similarly, Zuntz calculates his values 

 on the basis of the difference between the carbon dioxide output when walking on 

 the level and that when walking up hill. The whole question is discussed by 

 Benedict and Cathcart (1913). The efficiency found by them, under most 

 accurate conditions (p. 142), that is, comparing the efficiency obtained under 

 moderate work with that of heavier work with the same apparatus, was as high as 

 33 per cent. In this way an accurate base line for the increased metabolism was 

 obtained. In other words, " the increase in the effective muscular work may be 

 as high as 33 per cent, of the increase in total heat output." 



According to Macdonald (1914), the rate of heat production, Q, associated 

 with cycling at a uniform rate, but with varied performances of mechanical work, 

 is expressed by the formula : 



where x is the heat production associated with uniform rate of movement, and y 

 the rate of performance of work. 



E is found to vary inversely with W', 



x, for a particular subject, was found to be 



43Vi. 



where V is the rate of revolution of the bicycle per minute. This expression is 

 also found to be related to the weight of different subjects thus : 



VWJ 



