432 MANUAL OF PHYSIOLOGY. 



hindmost limb having the ankle and knee a little bent. By sud- 

 denly straightening these joints it gives a "push off" with the 

 toes and propels the body forward so as to move it around the 

 axis of motion of the fixed or supporting ankle-joint. At the end 

 of the swing the pendulous leg comes to the ground and leaves 

 the other limb in the attitude ready for the push off. Thus on 

 level ground walking is carried on with but small muscular exer- 

 cise ; but in ascending a steep incline or going up stairs, the sup- 

 porting limb has to elevate the body at each step by extending 

 the knee and ankle-joints by the thigh extensors and the calf 

 muscles. 



Running is distinguished from walking by the fact that, while 

 in the latter both feet rest on the ground for the greater part of 

 each pace, in the former the time that either foot rests on the 

 ground is reduced to a minimum, and in fact the supporting limb 

 disappears. The legs are kept in a semi-flexed position ready for 

 the push off or spring, which is so forcibly carried out that the 

 body is propelled through the air without any support. Thus an 

 interval of greater or less duration according to the pace exists 

 during which both the feet are off the ground, as the moment 

 either foot comes to the ground it executes a new spring without 

 waiting for the pendulous swing described in walking. 



