SIGNS OF ACTIVITY IN MUSCLE AND NERVE 



435 



D. MECHANICAL WORK 



The amount of mechanical work done by a muscular contraction depends 

 primarily upon the strength of the stimulus, and upon the load. 



1. Effect of the Strength of Stimulus. If a muscle bearing a constant 

 load be stimulated with a graded series of shocks beginning at a very low 

 level and increasing slowly, it is found, both with direct electrical stimulation 

 of the muscle and with mechanical or electrical stimulation of the nerve, that 

 the height of the contractions increases more and more slowly with a uniform 

 increase in the strength of the stimuli, and that it finally approaches its 

 maximum after the manner of an asymptote (Fig. 172). The maximum 

 shortening which can be obtained under the most favorable circumstances 

 with a single contraction is 



about twenty per cent of the 

 natural length of the muscle. 



The muscular tension ob- 

 tained with a maximal stimulus 

 applied to the nerve is consider- 

 ably smaller than that obtained 

 by a maximal stimulus applied 

 directly to the muscle itself 

 (Dean). If this is true of the 

 natural stimulation from the 

 central nervous system also, it 

 means that the muscles are al- 

 ways capable of more work 

 than can ever, under normal cir- 

 cumstances, be obtained from 

 them. 



2. Effect of Load with Con- 

 stant Stimulus. We shall con- 

 sider only the case of a maxi- 

 mal stimulus. 



FIG. 172. Frog's gastrocnemius. Stimulation of the 

 nerve with break-induction shocks ; load constant. 

 The abscissae represent the strength of the stimuli, 

 the ordinates the height of the contractions. 



One can vary the way in 

 which the ' power of the muscle 



is taken up by making the contraction: (1) isotonic i.e., where the load is 

 constant throughout the contraction; (2) auxotonic, where the load increases 

 constantly throughout; and (3) by supporting the load so that it is not lifted 

 until the muscle has contracted a certain distance (after-loading). 



A perfect isotonic contraction is probably never obtained. Even when the 

 mechanical conditions of the experiment fulfill the requirements for isotony 

 as completely as possible, the contraction is retarded at its beginning by the 

 inertia of the masses to be moved, consequently the tension of the muscle is 

 greater at the start than later. 



We designate as auxotonic contractions, first those in which the muscle 

 works against a stiff spring, where the tension naturally increases as long as 

 the muscle continues to contract, and secondly, those contractions in which the 

 tension of the muscle is purposely increased by retardation of the movement at 

 its' beginning. Here belong the so-called simple projectile motion (Helmholtz), 



