WORK PRODUCTION 545 



work done, deferring to the following section a study of the 

 efficiency of the animal as a converter of feed energy into use- 

 ful work and of the feed requirements of work animals. 



646. Mechanical efficiency of muscle. A muscle may be 

 regarded as a machine for the conversion of chemical energy 

 into mechanical work and one may, therefore, speak of its 

 efficiency in somewhat the same sense as of that of a steam 

 engine or an electric motor. By efficiency in this sense is meant 

 the proportion of the total energy mobilized during a contrac- 

 tion which is recovered in the work done. Thus if an isolated 

 muscle lifts a weight of ten grams through one centimeter, it 

 does 10 gram centimeters of work, equivalent (308) to 

 0.2344 X io~ 4 gram calories. If the increased katabolism caused 

 in the muscle by its contraction were shown to be 0.4688 X io~ 4 

 gram calories, the efficiency of the muscle would be 

 0.2344 -r- 0.4688 = 50 per cent, that is, 50 per cent of the total 

 energy mobilized would be recovered as mechanical work. 



Much experimental work has been devoted to the study of 

 the single muscle as a machine. The subject is a complicated 

 one, and unanimity of views upon it, especially as to the mecha- 

 nism of muscular contraction, has by no means been reached. 

 As regards the efficiency of the muscle as a converter of energy, 

 however, one fact is perfectly well established, viz., that it 

 varies within quite wide limits, depending especially upon the 

 load as related to the capacity of the muscle and upon the de- 

 gree of shortening. 



647. Mechanical efficiency of the body as a whole. If the 

 amount of energy mobilized in each muscle concerned in the 

 performance of a certain form of work were known, it is con- 

 ceivable that, assuming each muscle to act with its maximum 

 efficiency, an average theoretical efficiency might be computed 

 for the whole group of muscles. The conditions for the max- 

 imum efficiency of a muscle, however, seldom or never obtain 

 in the working animal, and consequently this hypothetical 

 efficiency is not attained. Of its many muscles, some serve 

 largely or wholly to maintain the relative positions of the 

 different parts of the body, i.e., their contractions are isometric 

 (629) and consequently have an efficiency approaching zero. 

 Others contract to a varying extent and under loads less than 

 the maximum. Some muscles, owing to their anatomical re- 



2 N 



