EFFICIENCY OF ISOLATED MUSCLE 397 



As regards the work which has been carried out on isolated 

 muscle, the results which have been obtained are of great interest, 

 as they have led to fresh consideration of the nature of the 

 muscular machine. A. V. Hill, in a long series of ingenious and 

 striking experiments, using special methods of his own devising, 

 has shown that the solution of the problem is not quite so simple 

 as it was formerly imagined. Hill found the simple determination 

 of the mechanical efficiency, i.e. W/H, the heat equivalent of the 

 work done, divided by the energy output determined as heat, 

 was of no real importance. The true efficiency of the muscle is 

 the ratio between the " potential energy thrown into an active 

 muscle by excitation " and the " total chemical energy liberated 

 as heat." He found further, that the heat production varied 

 according to whether the muscle was, or was not, allowed to 

 shorten on stimulation. If shortening were permitted the heat 

 output might be 30 per cent, smaller than if the muscle was 

 prevented from shortening. On examination of the potential 

 energy developed by a stimulated muscle not allowed to shorten, 

 it was found to be approximately 1/6 Tl, where T =the maximum 

 tension and I =the length of the muscle. Hill maintains that the 

 true mechanical efficiency can be determined by comparing this 

 quantity with the heat production. This value 1/6 Tl when 

 expressed in heat units is 10~ 4 /4-26 calories. (See Table LXXX.) 

 He found efficiencies approximating 90 per cent, in the initial 

 phases of contraction, and if the whole process, i.e. initial and 

 recovery phases taken together, were assessed, the efficiency, 

 under the conditions of his experiments, was in round figures 

 50 per cent. 



TABLE LXXX. 



EXPT. Length of muscles, 3-3 cm. ; weight of muscles, 0-135 gm. ; 



1 scale division of deflection = 8-32 x 10" 6 cal. Sartorius and isometric 



contractions. 



Duration of excitation : sees. - 0-075 0-075 0-075 



Initial tension : grin. wt. - 10-5 10-5 10-5 



Heat production H : cal. x 1Q- 6 - 574 740 757 



Tension T : grms. wt 44-8 47 47 



Tl/QH 1-01 0-82 0-80 



Incidentally he found that different types of muscle (e.g. semi- 

 membranosous and sartorius) definitely differed in efficiency. He 

 also found that the maximum efficiency was only obtained under 

 very special conditions of initial tension, strength of stimulus and 

 the physiological state of the muscle. 



