514 PHYSIOLOGICAL PHYSICS. [chap. XLIII. 



force will be the greater, the greater the inclination 

 of the leg behind with the ground. Again, the 

 faster the movement the greater will be the resistance 

 of the air. A forward inclination of the body will act 

 against the resistance, and so aid the progression. 

 Besides, the impulse from behind acting through the 

 centre of gravity will tend to throw the trunk back- 

 wards. The forward inclination neutralises this, and 

 prevents the necessity of muscular action being called 

 in to preserve the equilibrium. 



Besides the forward movement, there is a slight 

 movement of rotation on the head of the femurs, 

 owing to one leg moving forward and the other back- 

 wards. This is to some extent compensated for by 

 the arms, the arm of one side moving in the same 

 direction as the leg of the opposite side. 



The motion of the leg as it leaves the ground 



O 



behind is akin to that of a pendulum. The swing 

 of a pendulum is directly as its length, and the time 

 occupied directly as its swing. In natural walk- 

 ing, therefore, the length of the step will be deter- 

 mined by the length of the leg, and the rapidity of 

 the movement also. There is, therefore, a certain 

 length of step which is least fatiguing to the individual, 

 since it permits the full development of the rhythmic 

 movement suitable to the limb. 



