348 THE HUMAN MOTOR 



action, the first helping to carry the arm forward, or backward. 

 To lower the arm, the depressor muscles come into play, not as 

 antagonists of the levators, but as moderators of the downward 

 movement. Again, the brachial biceps is not exclusively a flexor 

 muscle, it is also a supinator, for we can perceive that it is dis- 

 tended when we turn a stiff key. 



The rectus femoris, the straight anterior muscle, is not merely 

 an extensor of the lower leg. It is also a flexor and levator of the 

 thigh and serves to maintain the equilibrium of the hips. 



The modes of muscular action are numerous. It is obvious 

 that an exact summation of the energy expended in these various 

 actions is of prime importance in the determination of what is 

 called the " degree of fatigue." 



In industry' the various movements of the body should be 

 directed towards the attainment of the maximum effect with the 

 minimum expenditure of energy. We shall often find that limbs 

 are moved to an extent which is quite unnecessary. This is to 

 be avoided since muscles, which might otherwise be at rest, are 

 put into action. In various occupations we shall observe that 

 in addition to those movements which are of direct utility there 

 are others which are superfluous, and whose elimination would 

 result in economy. Sports, such as boxing and fencing in parti- 

 cular, are in the same category. Here, however, the useless 

 movements are generally recognized, and are avoided, by the best 

 athletes, who know that such redundant movements are dis- 

 advantageous both to professional reputation, and to health. 



267. Movements of Parts of the Body. Centres of Gravity. 

 Moments of Inertia. Otto Fischer determined the position of 

 the centre of gravity for various parts of the body and the pro- 

 portions of those parts. The upper and lower limbs, the head and 

 the trunk, have centres of gravity which are referable to a common 

 origin, the line of the joints of the shoulders or hips. 



It will be obvious that the numerical values thus obtained are 

 to be taken as average results only, and are not applicable to any 

 particular subject. 



If we consider the trunk as a cylinder and the limbs as frustra 

 of cones we can calculate, approximately, their moments of 

 inertia. 



p 



I = Mp 2 , in which M designates the mass -, and p, the radius 



g 



of gyration (see 31). 



Consider first of all the following data for an adult of 65 

 kilogrammes : 



