140 THE MUSCLE CELLS 



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. 

 To provide energy to build the lactic acid into its old place, 

 i.e. to restore equivalence of internal energies between sarcoplasm 

 and fibril, glucose has to be oxidised to CO 2 and H 2 O. 



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 of 

 energy 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 to supply energy for muscle 

 restitution. Either because carbohydrate is more readily 

 mobilised or because it requires a lower tension of oxygen for 

 disintegrative 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 pH reacts on the 

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

 electrical charge (Chap. 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 con- 

 traction. Macallum proved that activity caused an alteration 

 in the concentration of salts in muscle, and Fletcher has demon- 

 strated the increase in water content after exercise. 



What is the effect of temperature on the restitution phase ? We 

 have seen that the contraction phase has a negative temperature 

 coefficient. Theoretically, each of the five sequelae to the libera- 

 tion of lactic acid as mentioned in the preceding paragraph has 

 a positive temperature coefficient. The building up of lactic acid 

 into a complex would be accelerated by temperature just like any 

 other chemical reaction. 



