MUSCULAR CONTRACTION 9 



production and comparing the effects of stimulation upon heat- 

 production in conditions promoting or impeding the recovery 

 process. 



According to Hill's data three definite conclusions can be 

 dra\vn. Firstly, when a muscle is excited directly or indirectly 

 in oxygen by either a single shock or a short tetanus, the 

 liberation of heat continues for some time after the mechanical 

 response is over. Secondly, the amount of heat so liberated 

 after mechanical response is over is, in oxygen, at least as great 

 as that evolved in the contraction and relaxation itself. But 



Heat production in Ng 



Heat production in O.^ 

 Fig. 2. — Scheme to illustrate modern work on muscular contraction. 

 I. Latent period — concentration of ions at surface of excitation (elec- 

 trical change) . 2. Development of tension — associated with disappearance 

 of glycogen and appearance of lactic acid. 3. Relaxation — lactic acid 

 neutralised by buffer action of muscle proteins. 4. Recovery — lactate ions 

 in part oxidised : some glycogen reappears, (a) Heat of formation of 

 lactic acid from glycogen, {b) Heat of dissociation of muscle proteins. 

 (c) Heat of combustion of part of lactic acid. 



thirdly, when the muscle is stimulated in nitrogen, there is 

 hardly any heat-production after contraction is over, though 

 the normal quantity is evolved when oxygen is again admitted. 

 It seems therefore justifiable to infer that oxygen is used up, 

 and " delayed " heat liberated, in that part of the recovery 

 process which occurs after relaxation is complete. As we have 

 already seen, this part of the recovery process is dependent on 

 the presence of oxygen and cannot occur in pure nitrogen. 



