220 JOHN C. KOCH 



greater than tlie body weight but never exceeds 44 pounds. 

 Most of this increment of pressure above the body weight must 

 be produced by the muscles of the lower leg and foot and there- 

 fore no account of it need be taken in the analysis of the stresses 

 in the femur due to walking. 



In ruiming there is greater extension of the leg and the forward 

 inclination of the body is greater than in walking. The succes- 

 sion of muscular movements in running is essentially the same as 

 in walking but there is greater intensity of muscle action and 

 the phases differ greatly in relative duration. While the body 

 is suspended in the air between the periods of contact with the 

 ground, both legs swing forward one behind the other. As con- 

 tact is made with the forward foot the other swings past and 

 there succeeds a quick forward thrust of the foot behind and then 

 for a brief interval the body is in the air. The duration of the 

 contact of the foot with the ground is shorter in running than in 

 walking and the greater the force of contact the shorter the time 

 of contact. The length of time the body is in the air is variable, 

 increasing slightly with the speed of running. 



Marey ('82) has shown that the maximum vertical rise in 

 the position of the body occurs just as one foot reaches the 

 ground. When running the leg bearing the weight is flexed 

 about 30 degrees at the knee and the body is flexed at about the 

 same angle at the hip according to the chronophotographic 

 plates of Marey. No exact figures have been thus far published 

 giving the relation of the maximum pressure to the body weight 

 in running, but it is certain that the maximum effect must be 

 produced shortly after contact has been made by one foot. An- 

 other factor to be considered is the tendency to cushion the 

 impact of the foot as it strikes the ground in running. This 

 effect is produced by the flexion at hips and at the knee of the 

 loaded leg as described above, together with the sudden tens- 

 ing of ligaments and muscles as the mipact between foot and 

 ground occurs. The action in some respects is very similar to 

 that of certain types of shock-absorbers used on automobiles. 



In the absence of absolute values of the amount of pressure 

 produced by the sudden contact of the foot in running, it ap- 



