MUSCULAR CYCLE 178 



The net result, heat evolved less the energy absorbed 



= 2873 X 10' — 1008 X 10" ergs 

 = 1865 X 10" ergs = 444 calories. 



Determinations of the heat evolved and the lactic acid liberated 

 (Hill and Hartree) show that on isometric contraction 444 calorics 

 are evolved during oxidative recovery for every gram of lactic 

 acid. Experiment and calculation agree. 



The lactic acid set free in the contraction phase is, in the 

 restitution phase, once more built up into the physico-chemical 

 compound of which it was a part before the arrival of the stimulus 

 provoked a contraction. As A. V. Hill has said, " The lactic 

 acid is part of the machine and not part of the fuel." During 

 contraction it is set free, during restitution it is built up again. 



As 0-19 gram out of every gram of glycogen involved disappears 

 during activity, it nuist be replaced in some way in order to main- 

 tain the glycogen content of muscle. 



There seems to be little doubt about the experimental evidence 

 regarding the utilisation of glucose during restitution. The 

 glucose stored in the muscle furnishes the main reservoir on which 

 the muscles draw for carrying out this work. There is some 

 evidence, not very clear it is true, suggesting that stored fat may 

 also be called on during muscle restitution. Either because 

 carbohydrate is more readily mobilised or because it requires a 

 lower tension of oxygen for disintegrativ e oxidation than fat or for 

 both reasons, muscle utilises carbohydrate in preference to fat. 



The liberation of lactic acid in the first phase of muscular 

 movement produces not only contraction but a whole series of 

 physico-chemical changes which have got to be reversed during 

 restitution. I. As a dissociable acid (Chap. VII.) it will produce 

 an increase in H ions. II. This increase in hydrion reacts on 

 the colloids in suspension in the muscle, causing them to alter in 

 electrical charge (Chaj). VIII.). III. This in turn sets free salts 

 adsorbed to the colloidal surfaces and so produces an increase in 

 osmotic pressure. IV. Further, the membranes will become 

 polarised. V. From III. and IV. will result endosmosis and the 

 water content of muscle will increase. 



Roaf has shown that there are definite alterations in H ion 

 concentration associated with different stages of muscle contrac- 

 tion. Macallum proved that activity caused an alteration in the 

 concentration of salts in muscle, and Fletcher has demonstrated 

 the increase in water content after exercise. 



What is the effect of temperature on the restitution |)hase ? 

 Theoretically, each of the five sequelae to the liberation of lactic 



