LOCOMOTION 



345 



o 



That is to say, the "motor" moment varies, according to a 

 sinusoidal law, with the inclination 

 of the muscle to the segment which 

 is undergoing displacement. In the 

 course of a complete movement of 

 the forearm the angle a varies from 

 to 180 (from full extension to 

 maximum flexion). When the angle 

 a is zero, sin a = and the moment 

 M is also zero. When a is 30 sin a = 

 \ (vide fig. 245). The position of 

 maximum moment is when the 

 muscle AB is at right angles to the 

 moveable segment. 



In actual fact, since the heads of 

 tjie joints have projections, and the 

 tendons are attached by surfaces, 

 the " motor " moment is never 

 exactly zero at any given position 

 and instant of time. According to 

 Fischer ( l ) the residual value is not 

 negligible. Furthermore the muscle, F ' G - 



even when reduced to a straight line, 'ifferent degrees of flexion 

 does not always lie in the same plane 

 as the member which it is causing 

 to move. If so, the effective component of its force will be its 



projection on the plane of the 

 jjC member (vide fig. 246). The 



F-^ magnitude of this component 



~~7 being proportional to the cosine 

 ' of the angle a. 



A movement, having one 

 degree of liberty, may be due 

 to the action of one muscle only 

 or may be effected by several 

 muscles. We only need, how- 

 ever, to consider their resultant 

 and we have already seen (vide 



15) that any system of forces can be reduced to an equivalent 

 single force and a " couple." This, however, is only true for a 

 single definite position, since the forces change during the dis- 

 placement. 



The movements which have one degree of liberty only are those 

 of certain of the larger joints, joints such as the forearm and 

 the lower leg ( 75), where no rotational movement is possible, 



of the forearm. 



Fie. ?46. 



AB is the component of the 

 muscular force F in the plane of 

 rotation of the limb. 



( l ) Braune and Fischer (loc. cit.}., vol. xv., No. 3, p. 245). 



