370 



THE HUMAN MOTOR 



showing the expenditure of energy in walking and running. An 

 inspection of these curves will show that beyond 160 to 170 paces 

 per minute it is more economical to run than to walk, and also 

 that the maximum economy in running is attained at a speed of 

 about 220 paces per minute. 



The values shown in fig. 265 are only approximate, but they 

 clearly exhibit the point at which running becomes preferable to 

 walking. In the case of very rapid walking the enforced exten- 

 sion of the legs causes painful sensations. In running, on the 

 other hand, the flexion of the legs renders great speed possible 

 without any such painful sensations. 



283. Jumping. The jump f 1 ) is a muscular movement by 

 which the body is projected upwards and caused to traverse a 

 certain space in the air at a considerable speed. 



In jumping, the body follows the same laws as those which 

 govern the movements of projectiles ( 263). The various phases 

 of the action are as follows : 



There is a first " preparatory " stage in which the trunk is bent 

 forward, the knees are flexed, and the arms are drawn back. 

 Next there is the phase of " impulsion," which is effected by the 

 sharp contraction of the muscles of the calf (triceps), and of the 

 thigh (quadriceps) ; the arms are raised and the body develops 

 a momentum whose amount is M V ( 27) . Next comes the period 

 of " suspension " during which the body is in the air, followed 

 by the " fall." The various phases in the action of jumping are 

 depicted in fig. 266, which is a reproduction of chronophotographs 

 by Richer and Londe. It will be seen that in the act of falling 

 the positions of the muscles are such as to " cushion " the shock 

 of contact with the 



ground. Were this not 

 done the shock of im- 

 pact would probably have 

 an injurious effect on the 

 brain. 



It will be observed 

 from fig. 267 that the 

 pressure of the feet is 

 much less in the former 

 than in the latter case. 

 The unilateral jump 

 (with one leg) is more ad- 

 vantageous than the bi- 

 lateral jump, becauses it 



( l ) Marey (Comptes Rendus Sciences, vol. ciii., 1888) ; du Bois-Reymond, 

 Zur Physiologic des Springens (Arch. f.Anat. u. Physiol. Abt. Suppl., 1905. 



Fio. 263. 



.Pressures of the feet in jumping 

 (1) cushioned fall ; (2) uncushioned fall. 



