830 SPECIAL MUSCULAR MECHANISMS. 



would fall backward were it not for the action of the muscles passing from 

 the thighs to the pelvis, assisted by the anterior ligaments of the hip-joints. 

 The line of gravity of the combined head, trunk and thighs falls, moreover, 

 a little behind the knee-joints, so that some, though little, muscular exertion 

 is required to prevent the knees from being bent. Lastly, the line of gravity 

 of the whole body passes in front of the line drawn between the two ankle- 

 joints, the centre of gravity of the whole body being placed at the end of 

 the sacrum ; hence some exertion of the muscles of the calves is required to 

 prevent the body falling forward. 



773. In walking, there is in each step a moment at which the body 

 rests vertically on the foot of one, say the right, leg, while the other, the left 

 leg, is inclined obliquely behind with the heel raised and the toe resting on 

 the ground. The left leg, slightly flexed to avoid contact with the ground, 

 is then swung forward like a pendulum, the length of the swing or step being 

 determined by the length of the leg; and the left toe 1 is brought to the 

 ground. On this left toe as a fulcrum, the body is moved forward, the centre 

 of gravity of the body describing a curve the convexity of which is upward 

 and the left leg necessarily becoming straight and rigid. As the body moves 

 forward, a point will be reached similar to that with which we supposed the 

 step to be started, the body resting vertically on the left foot, and the right 

 leg being directed behind in an oblique position. The movement on the left 

 foot, however, carries the body beyond this point, and in doing so swings the 

 right leg forward until it is the length of a step in advance of its previous 

 position, and its toe in turn forms a fulcrum on which the body, and with it 

 the left leg, is again swung forward. Hence in successive steps the centre 

 of gravity, and with it the top of the head, describes^ series of consecutive 

 curves, with their convexities upward, very similar to the line of flight of 

 many birds. 



Since in standing on both feet the line of gravity falls between the two 

 feet, a lateral displacement of the centre of gravity is necessary in order to 

 balance the body on one foot. Hence in walking the centre of gravity de- 

 scribes not only a series of vertical, but also a series of horizontal curves, 

 inasmuch as at each step the line of gravity is made to fall alternately on 

 each standing foot. While the left leg is swinging, the line of gravity falls 

 within the area of the right foot, and the centre of gravity is on the right 

 side of the pelvis. As the left foot becomes the standing foot, the centre of 

 gravity is shifted to the left side of the pelvis. The actual curve described 

 by the centre of gravity is, therefore, a somewhat complicated one, being 

 composed of vertical and horizontal factors. The natural step is the one 

 which is determined by the length of the swinging leg, since this acts as a 

 pendulum ; and hence the step of a long-legged person is naturally longer 

 than that of a person with short legs. The length of the step, however, may 

 be diminished or increased by a direct muscular effort, as when a line of 

 soldiers keep step in spite of their having legs of different lengths. Such a 

 mode of marching must obviously be fatiguing, inasmuch as it involves an 

 unnecessary expenditure of energy. 



In slow walking there is an appreciable time, during which, while one 

 foot is already in position to serve as a fulcrum, the other, swinging, foot has 

 not yet left the ground. In fast walking this period is so much reduced that 

 one foot leaves the ground the moment the other touches it ; hence there is 

 practically no period during which both feet are on the ground together. 



When the body is swung forward on the one foot acting as a fulcrum 

 with such energy that this foot leaves the ground before the other, swinging, 



1 This indicates perhaps what should he done rather than the actual practice ; most 

 people put the heel to the ground first, the contact with the toe coming later. 



