Fig. 2. (A) Schematic representation of the kinematics of the leg (flutter) kick. (B) Vertical displacement of the hip, knee and ankle as a function of time, obtained by video analysis. The kick frequency in A is about half of that shown in B. At variance with walking, where the displacements of these anatomical landmarks are in-phase with each other, when swimming using the leg kick each co-ordinate reaches its minimum/maximum at a phase-shift (t) with respect to the others. The distance between the anatomical landmarks (l_T, the thigh length; l_S, the shank length) divided by the time lag (or phase shift, t, in s) gives the velocity of the bending wave along the body (c=l_T/t_T; c=l_S/t_S).

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