MECHANICS OF MUSCLE 



369 



will find that muscle pull can be resolved into two components, a turning com- 

 ponent and a friction or pressure component as shown in Fig. 369. 



D F = the fixed bone from which the muscle takes its origin. 

 D K = the movable bone. 



I = a line from the middle of origin to the middle of insertion. 

 / M = size and direction of the muscle pull. 



If the parallelogram is constructed with / t and M b _j_ to D K, then I i = the 

 turning component and I b = the component which acts against the joint. 



The size of the two components depends upon the insertion angle 0. The smaller 

 this angle the smaller the turning component, and ^he nearer this angle is to 

 90 the larger the turning component. 



1 1 = I M X sin 



1 b = I M X cos 



If 4> = 90 cos = 0, sine = 1 

 hence / b = and 1 1 = I m 



If = cos = 1, sine = 

 hence 16 = 1 and 1 1 = 



With movements of the bone D K the angle of insertion is continually changing, 

 and hence the two components are changing in value. 



FIG. 370 



If, for example, the distance from origin to the joint D is greater than from 



D to I, as in the Brachialis or Biceps muscles, the turning component increases until 



the insertion angle = 90, which is the optimum angle for muscle action, while 



the pressure component gradually decreases. If the movement continues beyond 



24 



