178 THE MUSCLE CELLS 



rate of the physical reactions of the contraction phases and the 

 increases in the physical and chemical reactions of the restitution 

 phase. 



Efficiency of a Contraction. The mechanical efficiency of a 

 contraction is the fraction of the total energy expended which can 

 be recovered as external work. 



It is found from the formula 



E, = lx 100, 



where a = actual work done (in cals.) per unit of time. 



b = total energy used (in cals.) ,, „ ,, 



This gives the gross efficiency. The net efficiency is obtained 



by correcting for the energy expended during complete inactivity 



during a similar unit of time. 



a 

 Net efficiency E^ = -, x 100, 



where c = energy expended during inactivity (in cals.) per unit 

 of time. 



As no external work is done during inactivity it is difficult to 

 assess the value of the efficiency of this phase. 



The values obtained for a complete muscular cycle (contraction 

 and recovery) vary somewhat with the animal chosen and with 

 various other factors, such as temperature, rate of work, load, etc. 

 So far, only the efficiency of individual muscles acting isometrically 

 (or of the entire muscular machine (Chap. XXXVIII.) ) have been 

 estimated. The maximum gross efficiency is under 50 per cent. 

 Various attempts have been made to apply findings from experi- 

 ments on isolated muscles to muscles in situ. For example, the 

 biceps brachialis of man pulling against an immovable object for 

 about 1-4 seconds, turns 26 per cent, of the available energy into 

 realisable work and 74 per cent, to domestic purposes and to 

 warming the muscle. If the duration of the contraction is greater 

 or less than 1-4 seconds the efficiency of the biceps falls off. 

 Similarly the optimum load, the optimum rate of contraction, 

 as well as the optimimi duration of the contraction, could be 

 found for any muscle or group of muscles (see Chap. XXXVIII.). 



Training. The regular use of a muscle or group of muscles in a 

 certain way leads to their more efficient use. Repetition not only 

 causes a " warming up " of the muscle, but leads to a decreased 

 shortening viscosity (cf. Expt. 31 (b), p. 531). Other factors, 

 cardiac and nervous, enter into the question of the effectiveness of 

 training as applied to a complete animal. 



