162 MOVEMENT 



Relative Amount of Work done in executing Various 

 Paces. — If the values of the different factors constituting 



extent of 100 grammetres. But let us go a little further. The ball 

 in consequence will be flattened against the ground, and rebouud to 

 a certain height, 0*00 metre for instance. When the ball has reached 

 this height, of the energy which 1ms accrued from the effects of 

 gravity, only 40 grammetres have been expended, beeause on letting 

 it fall down again we shall find that there are GO grammetres of 

 energy left. These 60 grammetres, then, were recovered by the elastic 

 force of the ball, which had stored them up. 



Is there anything analogous, when at the end of a movement the 

 antagonistic muscles tend to slop it ; and will these muscles contribute 

 anything towards the succeeding movement in the opposite direction ? 

 The following facts lead us to be ieve that there is such a restitution. 



When one exerts one's self to jump and reach an object above 

 one's head, if the first attempt is unsuccessful, fcometimes the second 

 succeeds. 



Chronophotography shows that the second jump is always higher 

 than the first. What is it that occurs in such successive acts? In 

 the first jump, the total effort of the extensor muscles of the thighs 

 and legs projects the body a certain height. In descending, these 

 same muscles are contracted in order to break the fall, i.e. in order 

 to counteract the energy generated by the body. Then these muscles 

 are again contracted to project the body into the air a second time. 



Now, since the height of the second jump is greater than the first, 

 it must be admitted that the elastic force of the muscles which are 

 contracted to break the fall, is added to the muscular action consciously 

 brought into play for the second jump. 



Now, is this elastic force of rebound due to a physical property of 

 the muscles, or is it due to an additional expenditure of energy? 

 Weber demonstrated that a muscle when in action acquired, by some 

 intimate change within its fibres, a greater elastic force, and that it 

 was this force which produced movement. The same thing happens, 

 then, in a living tissue as in a steam-engine, in which the elastic force 

 of a gas is converted into work. 



Now, from the physiological point of view, in the second jump theie 

 was no obvious liberation of stored-up energy, but such liberation as 

 there was must have been the result of intrinsic action accompanying 

 all muscular contractions. If the body attained to a greater height in 

 the second jump it was because the muscular energy was greater. We 

 said, however, that in the first jump we exercised our muscles to the 

 utmost extent. That is true, but it u:ay not be by the most energetic, 

 but by the most prolonged effort that we attained to so great a height 

 in the second jump. It will be remembered that, in the experiments 

 with the dynaniograph, the "area of impetus," or the amount of 

 movement communicated to the body, was proportional to the square 

 root of the height jumped; and that the height of the jump did not 

 depend on the mere height of the dynamographic curve, because the 

 latter only expressed the degree of effort at one particular moment; 

 the duration of the effort had also to be taken into consideration. For, 

 as we mentioned, often the highest jumps corresponded to curves of 

 the lowest amplitude. 



