344 



THE HUMAN MOTOR 



(c) That the muscular " insertions " may be considered as 

 points so that the muscle can be represented as a straight line 

 (a vector force) in the same plane as the longitudinal axis of the 

 limb. (We shall find that this assumption does not affect the 

 validity of the reasoning, although it is not in precise accordance 

 with the physical facts of the case.) 



(d) That the movement of any articulated osseus segment can 



be studied as if the remainder of 

 the body was a solid and invariable 



mass.f 1 ) 



Let us take, as an example, a 

 movement having only one degree 

 of liberty, e.g., the movement of the 

 forearm in relation to the upper 

 arm. Assume the latter to be 

 rigidly fixed in a vertical position. 



In fig. 244 the biceps muscle is 

 represented by the straight line AB. 

 Tension is exerted at the extremities 

 A and B. A is fixed, and B turns 

 round an axis at o, at the elbow. 

 The moment of rotation of the force 

 F (or AB) in reference to the point 

 o is : 



M = F x od. 



The moving segment (the fore-arm 

 and the hand) is however attracted 

 downward by gravitation. If the 

 weight of the segment (the resistance) 

 is P, acting at its centre of gravity G, 

 then this weight has a moment : 



M' = P x od'. 



Flexion can only take place if the " motor " moment M is 

 greater than the " resistant " moment M 7 . 



The muscular effort needed varies inversely as the length of 

 the lever od. Hence in any given position, the shorter the 

 distance from B to the joint, the greater the force which must 

 be developed. Hence : od oB sin a. 



M = F X oB sin a. 



Morr ent of rotation of 

 the forearm. 



(*) Otto Fischer (Abhandlungen . . . vol. xx., 1893 ; xxii., 1895 ; xxiii., 



