122 PHYSIOLOGY CHAP. 



Lastly, it should be noted that the torsion of the trunk is 

 always accompanied (particularly in hurried walking and climbing) 

 by a rhythmical forward movement of the trunk and head at each 

 stride. This movement, which overcomes the resistance of the air 

 and economises the power of the limbs by throwing the centre of 

 gravity forward, is probably the effect of the activity of various 

 muscles, especially of the ilio-psoas. 



VI. After this account of the complex mechanism of walking 

 there is little to add in regard to running. As we have already 

 pointed out, the two feet are never on the ground at the same 

 moment in running, and one foot never comes in contact with the 

 ground till the other has been raised from it ; the entire body is 

 consequently suspended for a moment in the air. This is shown 

 by the tracing taken with the exploring shoes (Fig. 81). It can 

 also be seen with instantaneous photographs upon a fixed plate, 



FIG. 81. Curves of running, traced with recording shoes. (Marey.) D, movements of right foot ; 

 S, movements of left foot ; 0, vertical oscillations. The application of the foot to the ground 

 begins at the moment at which the curve rises ; its removal, at the moment at which the 

 curve drops. 



when the exposures occur at a rhythm corresponding with that of 

 the two phases of the step in running (Fig. 82). 



This essential difference between walking and running depends 

 upon the fact that in running the extension of the limb upon the 

 ground and of forward displacement of the body is more marked, 

 so that the body is thrown forward and raised from the ground. 

 During the moment while the body is unsupported in the air the 

 two legs swing forward. The leg which gives the forward impulse 

 is a little behind during the swing, and a little forward while the 

 other leg touches the ground. 



The contact of each foot on the ground is shorter in running 

 than in walking, and its duration is inversely proportional to the 

 force with which each foot is applied to the ground ; this increases 

 with the rate of running. The frequency of contact increases 

 with the pace, but only within certain limits, beyond which the 

 space covered in a certain time depends more on the length of the 

 steps than on their number. 



The absolute duration of the period in which neither foot is on 

 the ground varies very little with the variations of the speed of 

 running; but its relative duration increases considerably, since, 



