342 THE HUMAN MOTOR 



An acrobat during a leap can, by the use of his arms, turn 

 himself around a transverse axis.p) How- 

 ever, apart from the slight resistance of the 

 air, it is yet true that the height, the speed, 

 and the duration of the leap are governed 

 and connected b the formulae : 



g 



If the angle of inclination of the initial 

 speed is a the time t- - . The body 



g Fir,. 242. 



reaches its maximum height by a parabolic Trajectory of a 

 curve (vide fig. 242). J um P- 



The laws which govern the movement of projectiles are equally 

 applicable in the case of the human body. Thus to obtain the 

 greatest length of jump the direction of the jumper should be at 

 45 with the horizontal. The actual distance covered falls short 

 of the calculated distance owing to the resistance of the air ( 24). 



Gravity acts on each component of the human body. Otto 

 Fischer, by a laborious investigation determined the centres of 

 gravity of the various members. But the action of gravity is 

 balanced or overcome by muscular action. The result is motion, 

 muscular dynamics. This motion is conditioned, as to its form, 

 by the degrees of liberty, and as to its speed by the magnitude of 

 the stresses applied. 



264. Remarks on Muscular Movement. It is of great import- 

 ance carefully to examine the motions both of the upper limbs 

 (the arms, the fore-arms, and the hands) and of the lower limbs 

 (the thighs, the legs and the feet). When one osseus segment is 

 caused by muscular contraction to approach another (for example, 

 when the elbow or the knee are bent) the motion is denominated 

 flexion. Motion in the opposite direction is called extension, 

 A bduction is the movement of a member in a direction away from 

 the body. Adduction is the reverse movement towards the body. 

 When a member turns on a longitudinal axis, the motion is called 

 supination if the movement is outwards, and pronation if the 

 movement is inwards. Thus the forearm is in supination when 

 the back of the hand faces the ground, and in pronation when the 

 palm of the hand is downwards. 



The joints of the shoulder and of the hip have so great a degree 

 of freedom that these members are able to describe circles, or arcs 



(*) E. Kohlrauch (loc. cit., p. 45). This is why good jumpers show 

 highly developed shoulder muscles (Marey, Exposit. Intern. Univ. de. t 1900, 

 Sect. 13, Paris, 1901). 



