NATURE OF CONTRACTION 47 



tion into work or heat, and the part played by oxygen Ues 

 not in effecting this transformation of energy but in the 

 restoration of the condition of high potential. What is the 

 nature of the oxidation involved in the secondary process 

 of restoring the muscle to the state of high potential ? 



In this process there is no disappearance of sugar, 

 whereas carbohydrate does disappear in the act of con- 

 traction both in the presence and in the absence of oxygen, 

 lactic acid appearing in the latter case but not in the 

 former. The conclusion is therefore drawn that the 

 energy for the recuperation of muscle into its high potential 

 state is derived from the oxidation of lactic acid formed in 

 the act of contraction. 



Can it be shown that the energy thus obtained from the 

 oxidation of lactic acid is sufficient ? One gram of acid 

 on oxidation gives out 3,700 calories, whereas in the process 

 of recovery the utihsation of the same amount of acid 

 corresponds to the production of only 450 calories. The 

 source of energy, therefore, is amply sufficient. 



Returning now to the process of contraction, if this 

 is not produced chemically by oxidation, as in the case of 

 an internal combustion engine, to what is it due ? It 

 cannot be due to the conversion of carbohydrates into 

 lactic acid, for this reaction is practically isothermic. 

 There are, indeed, strong reasons against its being a 

 chemical reaction at all. The mechanical efficiency of the 

 process has been estimated at practically 100 per cent., 

 a degree of efficiency which is not approached by any 

 known form of chemical energy. 



If the energy appears not to be chemical there is some 

 evidence to indicate that it is physical. We have seen in 

 discussing isometric contractions that although there is 

 practically no deformation, there is a very profound change 

 — a change of tension. The degree of tension developed, 

 and therefore of heat evolved, is greater in an isometric 

 than in aii isotonic contraction, and varies directly, not 

 with the volume, but ivith the length of the fibres, that is to 



