MUSCULAR CONTRACTION 7 



The tension developed when the muscle is stimulated is then 

 recorded. This represents the force the muscle exerts when 

 it has contracted through i mm. In a similar way the tension 

 is observed in stages of i mm. till the muscle is so slack that 

 it exerts no tension on the lever, i.e. till the clamp has been 

 raised through a distance corresponding to that through which 

 the muscle contracts in a single twitch. 



The experiment is only valuable as a means of arriving at 

 a simple expression for ETdl. in terms of easily determined 

 quantities. Now the potential energy of an elastic body 

 stretched to a length x from the unextended condition ex is 

 JTa:(i— rx), and we might therefore anticipate that the energy 

 of contraction would be of the general form K.T/, where K 

 is a constant, T the initial tension and / the normal length. 

 According to Hill's determinations the area of the tension- 

 length curve satisfies the relation K=i/6 approximately. 



Thus the potential energy liberated in a single twitch is 

 T//6. In practice all this energy is not realised, because while 

 it has been found possible to devise an apparatus by which the 

 force opposed to the muscle is balanced against the actual 

 tension throughout the act of contraction, we cannot carry 

 out the process so slowly as to avoid degradation of energy 

 through internal friction {i.e. protoplasmic viscosity). For 

 an understanding of the mechanics of muscle we are only 

 concerned with the theoretical value of W, i.e. Tljb. 



(b) Chemical Phenomena in Contraction of Muscle.— The 

 essential facts as regards skeletal muscle are two : first, that 

 the muscle can contract and relax in the total absence of oxygen, 

 though the presence of oxygen delays the onset of fatigue ; 

 secondly, that lactic acid is produced in the process of con- 

 traction, rapidly disappearing in the presence of oxygen, but 

 accumulating if oxygen is excluded. The elucidation of this 

 aspect of the contractile processes is due in the first place 

 to the work of Fletcher and Hopkins. The conclusions which 

 can be drawn from their observations are that the production 

 of lactic acid without utilisation of oxygen is the salient event 

 associated with the tension which results in muscular con- 

 traction ; that the accumulation of lactic acid underlies the 



