Fig. 5. In some types of aquatic locomotion, bending waves can be observed moving along the body in a caudal direction (e.g. the ondulatory movements of slender fish). Similar waves were observed in this study in subjects swimming using the leg kick (see also Fig. 2). The velocity of the wave travelling along the body (wave speed: c, m s^-1) increases linearly with the kick frequency (K_F, Hz) with fins (LF, open circles) and without fins (L, closed circles). The relationship between c and K_F is well described by c=2.33K_F (r²=0.911, N=20), the slope of which is the wavelength (, m).