MUSCLE 251 



to 20 per cent, of the energy of the coal in mechanical 

 work. 



In the steam-engine, where heat energy is converted 

 to work, the efficiency depends upon tbe difference of 

 temperature between the cylinder and condenser. The 

 high efficiency of muscle, with an absence of marked 

 temperature variations, shows at once that it is not a heat 

 engine (p. 247). Compared with other energy transformers, 

 muscle does not stand so hiorh. A Diesel eno-ine Q'ives a 

 theoretical efficiency of about 75 per cent, and a practical 

 efficiency of about 50, and an electric battery from 80 to 90 

 per cent. 



It must, however, be recognised that the heat produced 

 by muscle is necessary to keep the temperature of the body 

 at such a level that the chemical changes, which are the 

 basis of life, may go on. 



Probably a return of 30 per cent, is seldom yielded 

 under ordinary working conditions by the whole muscular 

 system. 



AVithin certain wide limits of work done, the efficiency of 

 muscle remains constant, that is, the total energy evolved is 

 directly proportionate to the work done. 



But if excessive work is put upon muscles, or if the work 

 has to be done at a rate greater than the optimum, the 

 mechanical efficiency decreases and the total energy expendi- 

 ture rises out of proportion to the work done, just as, when 

 a steamer is driven above a certain speed, the coal consump- 

 tion is increased out of proportion to the increased speed. 

 This is well illustrated by the increased production of COo, 

 which occurs when the rate of marching is forced from 

 3 to 5 miles an hour (fig. 128). 



Fig. 131 shows the relationship of the work done (effective 

 work performed; to the total output of energy (total heat 

 output). Such a figure indicates very clearly that, while 

 for moderate increments of work the mechanical efficiency of 

 the muscle is fairly constant, with greater increments the 

 efficiency becomes less, i.e. the total expenditure of energy 

 increases out of proportion to the work done. 



