38 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



will be found that, while the height of the successive contractions 

 diminishes as the load increases (see Fig. 17), the work done that 

 is, the product of the load into the lift first increases and then 

 decreases. For example : 



Work Done in Gram-millimeters. 

 Load in Grams. Lift in Millimeters. Load X Lift. 



5 27.6 138.0 



15 25.1 376.5 



25 11.45 286.25 



35 6.3 220.5 



A series of experiments of this kind furnishes data for construct- 

 ing a curve of work by plotting off along the abscissa at equal inter- 

 vals the equal increments in load and erecting over each load an 

 ordinate showing the proportional amount of work done. The 

 curve has the general form indicated in Fig. 18. Three facts are 

 expressed by this curve: First, that if the muscle lifts no weight 

 no work will be done; this follows theoretically from the formula 

 W = L H, in which W represents the work done, L the load, and 

 H the lift. If either L or H is equal to zero the product, of course, 

 is zero; that is, no external work is done; the chemical energy 

 liberated in the contraction takes the form of heat. Under such 

 circumstances the amount of heat given off from the muscle should 

 be greater than when a load is lifted. In accordance with this 

 fact it is found that a muscle lifting a light load gives off more heat 

 during the contraction than when lifting a heavier load. Secondr 

 There is an optimum load for each muscle with which the greatest 

 proportion of work can be obtained. Third. When the load is just 

 sufficient to counteract the contraction of the muscle no work i? 

 done, H in the above formula being zero. This amount of load 

 measures what Weber called the absolute power of the muscle , 

 As will be seen from the above curve, it is measured by the 

 weight which the muscle cannot lift and which, on the other 

 hand, cannot cause any extension of the muscle while contracting. 

 Or, in more general terms (Hermann), the absolute power of a 

 muscle is the maximum of tension which it can reach without 

 alteration of its natural length. This absolute power can be 

 measured for the muscles of different animals and for convenience 

 of comparison can then be expressed in terms of the cross-area 

 of the muscle given in square centimeters. Weber has shown 

 that the absolute power of a muscle varies with the cross-area, since 

 this depends upon the number of constituent fibers whose united 

 contraction makes the contraction of the muscle. Expressed in 

 this way, it is found that the absolute power of human muscle is, 

 size for size, much greater than that of frog's muscle. For in- 

 stance, the absolute power of a frog's muscle of 1 square centimeter 



