56 



PHYSIOLOGY OF MUSCLE AND NERVE 



Isotonic and Isometric Myograms. — If a muscle is made to contract 

 after it has been attached to the writing lever, it must suffer an initial 

 stretching and this stretching must be the greater, the heavier the 

 load against which it acts. A certain part of its energy, therefore, must 

 be lost without being able to produce a visible effect. To counteract 

 this distention, it is customary to after-load the muscle with a slight 

 weight which is neither increased nor diminished during the contrac- 

 tion, or to hold the writing lever in a horizontal position by means of 

 a support or a tension spring. While thus subjected, to the least pos- 

 sible tension, it is not hindered in changing its length and in generating 



Fig, 27.— Different Ways of Counterpoising the Writing Lever. 

 A, B and C, isotonic arrangements; D, isometric arrangement; S, spring. 



visible mechanical energy. A myogram obtained under this condition 

 is characterized as isotonic. As far as the adjustment of the muscle 

 and weight is concerned, the latter may be affixed (a) directly 

 beneath the point of attachment of the muscle (method of loading), 

 (6) precisely in the same place with this modification, however, that 

 the lever is held in a horizontal position by a counterpoising load 

 or other appKance (method of after-loading), and (c) to the axis of 

 the lever by means of a pulley. The latter arrangement gives the 

 most perfect isotonicity. 



If the muscle is attached near the fulcrum of the writing lever, 



