226 JOHN C. KOCH 



1 )()(!>• being shifted laterally at each step so as to pass through 

 the acetabiiluin and the centers of the knee- and ankle-joints. 

 If the weight of one leg be taken ecjual to 20 per cent of the 

 body weight, the load on each femur-head in the standing posi- 

 tion will be approximately 30 per cent of the body weight. In 

 walking and running the weight transmitted through the loaded 

 femur would be the weight of the trunk (60 per cent) and of the 

 lifted leg (20 per cent), or 80 per cent of the body w^eight. 



The direction of the load transmitted through the femur, as 

 has been shown above, can be taken as the straight line joining 

 the center of the head of the femur to the center of gravity of 

 the lower end of this bone. This line of action of the load is 

 indicated by the line AB in figure 14. The line of action of the 

 load is the line joining the point of application, at w^hich the 

 applied load may be regarded as concentrated, to the point 

 through which the supporting force at the opposite end may be 

 regarded as concentrated. The load is actually transferred 

 from section to section through the bone from the uppermost 

 to the lowermost extremity of the femur. 



4- Assumed load on the femur. All calculations will be based 

 upon a load of 100 pounds applied to the head of the femur 

 and acting in the direction just indicated. This is a convenient 

 basis of making the calculations and it facilitates percentage com- 

 parisons, while the effects produced by any other load may be 

 accurately determined by simple proportion. 



5. Force diagram. Figure 14 is a diagram of the longitudinal 

 section through the axis of the femur showing the outline of the 

 bone and the position of the line of action AB of the external 

 load. This section does not lie strictly in a single plane, but 

 follows the slightly curved axis of the bone which has its convexity 

 directed anteriorly. As the head of the femur is directed about 

 12 degrees forward and medially to the frontal plane, this longi- 

 tudinal section bears the corresponding relation to the frontal 

 plane. 



In figure 14 an axis BCDE is shown which is drawn so as to 

 pass through the center of gravity of successive transverse sec- 

 tions of the femur. It is, therefore, the neutral axis of the 



