CERTAIN MOVEMENTS IN MAN 163 



the total energy expended on taking a step are com- 

 pared, it is found that they are not equally (Fig. 108) 



It is necessary, then, to know whether the impetus occasioned by 

 the mubdes has not lasted longer in the second than in the first 

 jump. 



In all contracting muscles, the elastic force starts at zero, and 

 attains its maximum in a certain time. This results from the way in 

 which the contraction is produced, namely, by a gradual summation 

 of a series of contractions. Nww, if we leave a crouching position to 

 jump for the first time, our extensor muscles gradually contract, and 

 the-e muscles will only produce their maximum effect at a more or 

 less advauced phase of extension of ti.e limbs. 



In the second jump, on the contrary, when in breaking the fall we 

 again assume a crouching position, our extensor muscles i.ave already 

 reached their maximum contracting force; and it is this maximum 

 force which will continue to operate in raising us up again until we 

 have left the ground. The body mass will have received the full effect 

 of the muscular contraction during a longer period, and consequently 

 will have received a greater "quantity" of movement. 



A familiar example will help to explain the difference which exists, 

 as far as intensity of result is concerned, between a force gradually 

 developed throughout the duration "f a movement, and another which, 

 acts with all its intensity during the whole period of a movement. 

 When we wish to transmit an impetus to an object by extending a 

 finger we y:ive it a fillip. That is to say, by holding the last phalanx 

 of tie middle finger with the thumb, and strongly contracting the 

 extensors of the linger, we let it go like a spring at the moment the 

 maximum degree of extension is reached. The obj'eet is thus shot 

 away with great force. The impetus would have been very muck 

 weaker if the middle finger had previously only been bent, and we 

 had then suddenly extended it. In all alternating movements, the 

 muscles work to better advantage than in simple movements. The 

 brand^hing of a weapon before striking has no other significance. 



Now, since the work done by a muscle is the same, whether it is 

 merely work of resistance or work represented as motion, we believe 

 that, if we want to estimate the work done in walking, we are justified 

 in doubling the v.due of e'jch of the component factors of the w r ork 

 expended in maintaining the oscillations of the body and limbs. If 

 we said that the estimate thus obtained probably represented the 

 maximum, it would be because the vital energy transmitted to the 

 body in the horizontal d.rection, and that transmitted to the legs at 

 each oscillation, are not totally expended on the movements, part 

 being lost in the form of shocks imparted to the ground. Yital 

 energy is perhaps stored up in real elastic structures of the body, such 

 as tendons. 



Veterinary experts have made a special study of the energy lost by 

 the hoofs striking the ground when a horse is travelling at a rapid 

 pace. They maintain that the flexor of the solitary toe which consti- 

 tutes the foot of a horse, is made to a great extent of elastic tissue. 

 It possesses in consequence a physical property by means of which a 

 more or less important part of the vital energy lost in falling ou the 

 feet is to some extent returned in the form of euergy. 



