8 : 3/ Muscles 



143 



AL = AL max (1 - - ) 



-*max 



where AL is the contraction and F is the load. The work W done on 

 the load is 



W = FAZ max (1 - £ ) 



"max 



This work W is a maximum when 



dW 



dF 



that is, when F = IF, 



2 *■ max 



Striated muscles, in general, can develop large forces against a given 

 load but even in tetany can contract only a small amount. In the 



Figure 4. (a) Isotonic contraction. Change in length is 

 plotted as a function of load for a muscle supporting a fixed 

 load (isotonic). The straight line is an approximation only. 

 (b) Isometric tension. Maximum tension developed is plotted 

 as a function of length for a muscle held at various fixed lengths 

 (isometric). 



vertebrate body, the skeletal muscles all develop far larger forces than 

 the loads they move. However, the load moves more than the muscle 

 contracts. This is accomplished by the lever action of the muscles and 

 bones with the joints serving as pivots. As shown in Figure 5, the 

 mechanical advantage is considerably less than one, that is, force of the 

 muscle is much greater than load, and the muscle motion is much less 

 than load motion. 



To study muscular contraction, it would appear desirable to work 

 with single muscle fibers. However, these are difficult to obtain and 

 few people have succeeded in preparing them. Most experiments have 

 been done on whole muscle. 



