LAWS OF BONE ARCHITECTURE 275 



It has been shown in Part III that in the erect, standing atti- 

 tude the load resting on each femur could be taken at three- 

 tenths the body weight: and in walking the loaded femur carried 

 approximately eight-tenths of the body weight. In walking the 

 load is carried alternately by the femurs, and it may be assumed 

 that the load is cushioned by the padding of the feet and by 

 muscle action so that it acts practically as a load gradually 

 applied and produces the same effect as a static load of the 

 same amount. 



In running, jumping and falling, it is clear that the effect of 

 such suddenly appHed loads on the femur-head is greater than 

 that due to standing or walking. It is a general principle of 

 mechanics that the stresses produced by a suddenly applied load, 

 without impact, are just double those produced by a gradually 

 applied, or a static load of the same amount. Hence, if allow- 

 ance is made for the cushioning effect of the padding of the feet 

 and the resilience of muscles, ligaments, etc., as previously 

 shown in Part III, the stresses produced in running may safely 

 be taken as twice as great as those produced by walking. The 

 femur is therefore considered to be stressed in running, as 

 though it carried double the static load in walking, or a load of 

 1.6 times the body weight. 



In jumping the stresses are still further increased by the 

 impact of the body, which is suddenly brought to a stop. The 

 velocity which the body attains just before striking determines 

 the amount of the impact. The stresses produced by impact, 

 as in jumping or falling, may be enormously greater than in 

 walking or running. Probably few fractures occur without the 

 element of impact being involved to a marked degree. As the 

 mechanical analysis of the stresses due to impact holds only 

 for stresses within the elastic limit, the large stresses due to 

 impact are difficult to analyze. Impact may produce dynamic 

 stresses many times as great as those produced by the static 

 load alone. In the body the effect of impact is often offset to a 

 greater or less degree by the involuntary flexing of the body at 

 hips, knees and ankles as well as by the muscle tonus. For these 

 reasons, probably, relatively few fractures result from the impact 

 due to high jumps and falls. 



