FIG. 28.--Preparing to run, from a design by Flaxman. Adapted. In the original of this figure the right arm is depending and placed on the right thigh.]
In order to enable the right leg to swing forward, it is evident that it must be flexed, and that the left leg must be extended, and the trunk raised. The raising of the trunk causes it to assume a more vertical position, and this prevents the swinging leg from going too far forwards; the swinging leg tending to oscillate in a slightly backward direction as the trunk is elevated. The body is more inclined forwards in running than in walking, and there is a period when both legs are off the ground, no such period occurring in walking. “In quick walking, the propelling leg acts more obliquely on the trunk, which is more inclined, and forced forwards more rapidly than in slow walking. The time when both legs are on the ground diminishes as the velocity increases, and it vanishes altogether when the velocity is at a maximum. In quick running the length of step rapidly increases, whilst the duration slowly diminishes; but in slow running the length diminishes rapidly, whilst the time remains nearly the same. The time of a step in quick running, compared to that in quick walking, is nearly as two to three, whilst the length of the steps are as two to one; consequently a person can run in a given time three times as fast as he can walk. In running, the object is to acquire a greater velocity in progression than can be attained in walking. In order to accomplish this, instead of the body being supported on each leg alternately, the action is divided into two periods, during one of which the body is supported on one leg, and during the other it is not supported at all.
“The velocity in running is usually at the rate of about ten miles an hour, but there are many persons who, for a limited period, can exceed this velocity.”[40]
[40] Cyc. of Anat. and Phy., article “Motion.”
If we direct our attention to the water, we encounter a medium less dense than the earth, and considerably more dense than the air. As this element, in virtue of its fluidity, yields readily to external pressure, it follows that a certain relation exists between it and the shape, size, and weight of the animal progressing along or through it. Those animals make the greatest headway which are of the same specific gravity, or are a little heavier, and furnished _with extensive surfaces_, which, by a dexterous tilting or twisting (for the one implies the other), or by a sudden contraction and expansion, they apply wholly or in part to obtain the maximum of resistance in the one direction, and the minimum of displacement in the other. The change of shape, and the peculiar movements of the swimming surfaces, are rendered necessary by the fact, first pointed out by Sir Isaac Newton, that bodies or animals moving in water and likewise in air experience a sensible resistance, which is greater or less in proportion to the density and tenacity of the fluid and the figure, superficies, and velocity of the animal.