_C_ shows the arms and legs in an intermediate position, _i.e._ when they are neither flexed nor extended. The arms and legs require to be in the position shown at _A_ before they can assume that represented at _B_, and they require to be in the position shown at _B_ before they can assume that represented at _C_. When the arms and legs are successively assuming the positions indicated at _A_, _B_, and _C_, they move in ellipses, as explained.--_Original._] By alternately flexing and extending the limbs, the angles made by their several parts with each other are decreased and increased,--an arrangement which diminishes and augments the degree of resistance experienced by the swimming surfaces, which by this means are made to elude and seize the water by turns. This result is further secured by the limbs being made to move more slowly in flexion than in extension, and by the limbs being made to rotate in the direction of their length in such a manner as to diminish the resistance experienced during the former movement, and increase it during the latter. When the arms are extended, the palms of the hands and the inner surfaces of the arms are directed downwards, and assist in buoying up the anterior portion of the body. The hands are screwed slightly round towards the end of extension, the palms acting in an outward and backward direction (fig. 41, _B_). In this movement the posterior surfaces of the arms take part; the palms and posterior portions of the arms contributing to the propulsion of the body. When the arms are flexed, the flat of the hands is directed downwards (fig. 41, _C_). Towards the end of flexion the hands are slightly depressed, which has the effect of forcing the body upwards, and hence the bobbing or vertical wave-movement observed in the majority of swimmers.[52] [52] The professional swimmer avoids bobbing, and rests the side of his head on the water to diminish its weight and increase speed.