222 PHYSIOLOGY 



by one of the methods described. A. V. Hill has shown that the heat 

 production in a contracting skeletal muscle occurs in two phases, a rapid 

 production of heat which apparently is synchronous with the contraction 

 itself, and a slow production of heat which continues for some time after 

 the muscle has relaxed. The second phase of heat production depends on 

 the presence of oxygen, and is observed at its best when the muscle is kept 

 in pure oxygen. If the muscle be allowed to contract in nitrogen only 

 the initial heat production is observed. The heat production of the second 

 phase is stated by Hill to be approximately equal to that in the first phase. 

 These results have been interpreted as showing that the initial change in 

 muscular contraction is the development of lactic acid. The appearance 

 of this lactic acid in some way changes the muscle and sets up potential 

 energy at the surface of its ultimate fibrils, which will result in a shortening 

 of the muscle if any movement of its ends be allowed. A comparison of 

 the energy of the tension set up with the actual heat evolved in the initial 

 stage when a muscle is not allowed to contract shows that the two quantities 

 are approximately equal. In a series of experiments Hill found that the 

 ratio -J- TZ : H in the sartorius muscle under low initial tensions and in 

 comparatively weak contractions approximated to the value 1, the mean 

 value being -91. Under high initial tensions and in strong contractions 

 of the sartorius muscle, it is lower, being roughly from 04 to 0-6. He 

 concludes from this that under certain conditions the initial process of con- 

 traction consists largely, if not entirely, of the liberation of free potential 

 energy manifested as tension in the muscle. This potential energy may be 

 used for the accomplishment of work or for the production of heat. The 

 efficiency of the initial stage of contraction is therefore almost 100 per cent. 



If, however, a muscle is to go on contracting without rapidly showing 

 signs of fatigue, it must be kept in oxygen, so that the processes of replace- 

 ment or of removal of the lactic acid may take place. Under these circum- 

 stances there is a further evolution of heat after the contraction, equal to 

 that set free during the initial stage. So that the total efficiency of a muscle 

 kept in oxygen would not be more than 50 per cent. 



This is assuming that the process of oxidation of the lactic acid and its replace- 

 ment in whole or in part in the muscle molecule is completely carried out during the 

 time of the observation. It is improbable that such is the case, and it seems possible 

 that the evolution of heat during the so-called recovery stage of the muscle has been 

 under-estimat ed. 



If a series of observations of the heat production and tension developed 

 during isometric contractions be made with varying initial tension on the 

 muscle, it is found that while the ratio of tension developed to heat produced 

 is approximately constant, both these quantities first increase and then 

 finally diminish. The optimum of the heat production in some experiments 

 seems to fall later than the optimum of the tension developed. It seems 

 probable that in this case the essential factor is not so much the tension 

 exerted on the muscle previous to its excitation, but the length of the muscle 

 fibre during the time that it is excited. The longer the muscle fibre, within 



