WORK AGAINST GRAVITY AND WORK DUE TO VELOCITY CHANGES IN RUNNING

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Abstract
From measurements made on moving picture films of sprinters, figures are obtained from which some of the work done in running can be calculated. The movements of the center of gravity within the body due to changes in the positions of the limbs are determined. Both arms and legs tend to raise the center of gravity within the body at the end of the period when the foot is in contact with the ground, the total average rise being 2.9 cm. Simultaneously the body itself rises an average distance of 3.1 cm., the total rise of the center of gravity in relation to the ground being therefore 6 cm. This rise occurs while the foot is on the ground. The corresponding rate of energy expediture is 0.215 H.P. Backward and forward movements of the limbs are largely compensatory on the 2 sides of the body. The velocity of the head is greater than that of the pelvis while both feet are in the air but the reverse is true while either foot is in contact with the ground. The changes in velocity of the pelvis are not so great as they appear to be from direct measurement of the film because there is a compensatory movement of the center of gravity within the body which is shown to move backwards when the apparent forward velocity of the pelvis is at a maximum. An apparatus is described by which the horizontal pressure of the foot on the ground is measured during sprinting. During each period of contact there is first a forward pressure and then a backward one. This constitutes a direct method of measuring energy loss due to fluctuations in the forward velocity. The forward pressure on the ground represents for the whole run a loss of 0.34 H.P. and the backward pressure an expenditure of 0.5 H.P. The difference of 0.16 H.P. is due to wind resistances. The loss of velocity due to contact of the foot with the ground is 13 cm./sec. as measured on the special apparatus (running platform) described and it is 12-24 cm./sec. when calculated from the vector velocity of the center of gravity of the runner and the angle of contact with the ground.

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