156 MOVEMENT 



photographs (Fig. 105), it is easy to predict that the 

 same method might be applicable in the case of various 

 kinds of paces. 



So far physicists have only calculated the amount 

 of energy expended or gained by a man in walking 

 along an inclined plane, in terms of the body weight 

 lifted or lowered such and such a height. The first is a 

 case of positive energy, and the second one of negative 

 energy. The amount of energy expended in different 

 directions is the product of the body weight and the 

 height of lift, an amount which is expressed in kilo- 

 grammetres. Looking at the matter in this light, 

 progress along level ground would require no apparent 

 expenditure of energy, and yet it is accompanied with 

 muscular exertion and consequent fatigue. 



We thought it was possible to estimate approxi- 

 mately the mechanical energy expended in walking 

 or running along horizontal ground by observing the 

 movements transmitted in various directions to the 

 body by the muscular actions involved. If the move- 

 ments of the centre of gravity of the body could be 

 followed in space, they would be seen to constitute a 

 series of vertical oscillations in accordance with the 

 movements of the feet. At the same time during each 

 oscillation there is a certain movement of translation, 

 sometimes at an increased and sometimes at a decreased 

 velocity.* 



Another way in which energy is expended is when 

 movements are alternately communicated to the legs, 

 movements which gravity might account for if they 

 were, as the brothers Weber believed, comparable to 

 oscillations of a pendulum. In practice, however, these 

 movements usually require the help of the muscles. 



The measurements (Fig. 106) of different movements 



* We ignore as unimportant the movements of the centre of gravity 

 which occur outside the vertical piano of progression. 



