[52] The professional swimmer avoids bobbing, and rests the side of his head on the water to diminish its weight and increase speed.
During flexion the posterior surfaces of the arms act powerfully as propellers, from the fact of their striking the water obliquely in a backward direction. I avoid the terms _back_ and _forward_ strokes, because the arms and hands, so long as they move, support and propel. There is no period either in extension or flexion in which they are not effective. When the legs are pushed away from the body, or extended (a movement which is effected rapidly and with great energy, as shown at fig. 41, _B_), the soles of the feet, the anterior surfaces of the legs, and the posterior surfaces of the thighs, are directed outwards and backwards. This enables them to seize the water with great avidity, and to propel the body forward. The efficiency of the legs and feet as propelling organs during extension is increased by their becoming more or less straight, and by their being moved with greater rapidity than in flexion; there being a general back-thrust of the limbs as a whole, and a particular back-thrust of their several parts.[53] In this movement the inner surfaces of the legs and thighs act as sustaining organs and assist in floating the posterior part of the body. The slightly inclined position of the body in the water, and the forward motion acquired in swimming, contribute to this result. When the legs and feet are drawn towards the body or flexed, as seen at fig. 41, _C_, _A_, their movements are slowed, an arrangement which reduces the degree of friction experienced by the several parts of the limbs when they are, as it were, being drawn off the water preparatory to a second extension.
[53] The greater power possessed by the limbs during extension, and more especially towards the end of extension, is well illustrated by the kick of the horse; the hind feet dealing a terrible blow when they have reached their maximum distance from the body. Ostlers are well aware of this fact, and in grooming a horse keep always very close to his hind quarters, so that if he does throw up they are forced back but not injured.
There are several grave objections to the ordinary or old method of swimming just described. _1st_, The body is laid prone on the water, which exposes a large resisting surface (fig. 41, _A_, _B_, _C_, p. 82). _2d_, The arms and legs are spread out on either side of the trunk, so that they are applied very indirectly as propelling organs (fig. 41, _B_, _C_). _3d_, The most effective part of the stroke of the arms and legs corresponds to something like a quarter of an ellipse, the remaining three quarters being dedicated to getting the arms and legs into position. This arrangement wastes power and greatly increases friction; the attitudes assumed by the body at _B_ and _C_ of fig. 41 being the worst possible for getting through the water. _4th_, The arms and legs are drawn towards the trunk the one instant (fig. 41, _A_), and pushed away from it the next (fig. 41, _B_). This gives rise to dead points, there being a period when neither of the extremities are moving. The body is consequently impelled by a series of jerks, the swimming mass getting up and losing momentum between the strokes.