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.