ADVANCED EXPERIMENTAL PHYSIOLOGY 293 



V 



From the two curves in Fig. 174 certain rough but important deduc- 

 tions may be made. The initial tension on the muscle in the two cases 

 was not quite the same, but this may be neglected. In the isometric 

 contraction the observed height of the curve is 14 mm., but the magnifica- 

 tion was 15, and therefore the muscle really underwent a shortening 

 of rl = '933 mm. Supposing muscle were a perfectly elastic body the 

 * contractile stress ' or pull exerted by it at any point during a contrac- 

 tion may be calculated by the formula, T = \lw ; where r is the pull, / is 

 the amount of shortening in mm., and w the resistance in grms., 

 whether that of a load or of a spring, which the muscle has overcome. 

 In the isometric curve the maximal pull exerted by the muscle was 

 Jx -933 x 200 = 93-3 grms. In the isotonic curve the load on the 

 muscle was - 5 g- grms. ,- and the pull exerted by the muscle when it had 

 again shortened by '933 mm. was ^ x *933 x - 5 ^- = 7 '7 grms. 



The comparison of an isometric with an isotonic curve, and of 

 isometric curves having different initial tensions, shows really the effect 

 that resistance to contraction has upon the liberation of energy by a 

 muscle ; for it shows that a muscle which has shortened to a given 

 length will be exerting a far greater pull when its effort to shorten has 

 been resisted than when it has reached the same length during 

 an isotonic or unresisted contraction ; and this is especially true of 

 resistance during the first part of the muscle's period of effort. In the 

 body all muscles even at rest are extended by an initial tension, and their 

 efforts to shorten during contraction are more or less resisted ; and we 

 find that this resistance, so far from decreasing, actually increases the 

 efficiency of the muscle for doing work. We further see that the pull 

 exerted by a muscle during its contraction is not determined simply by 

 the strength of the stimulus reaching it, but also by the mechanical 

 conditions of tension and load under which the muscle finds itself before 

 and after it has begun to respond to the stimulus. For, we have found 

 that with the same strength of stimulus a muscle responds with a pull 

 which increases directly as the resistance it has to overcome. This 

 power of skeletal muscles to adjust their liberation of energy to the 

 work-Jwhich they find they have to do, must clearly be an enormous 

 saving to the body. 



