Wave characteristics of butterfly swimming

In this study it was hypothesized that elite butterfly performance is characterized by wave motions with particular frequency, amplitude, and phase characteristics. Particular emphasis was accorded the question of whether 'waves' travel along the body during the butterfly stroke. Selected body landmarks and the center of mass (CM) of eight elite males and eight elite female swimmers were quantified. Fourier analysis was conducted to determine the frequency, amplitude, and phase characteristics of the vertical undulations of the vertex of the head, shoulders, hips, knees, and ankles. The differences in phase between these landmarks for the first (H1) and second (H2) Fourier frequencies were investigated to establish whether waves travelled along the body in a caudal direction. The absolute average velocity of H1 wave travel from vertex to ankle was found to be a mean of 0.34 ms-1 faster than the forward velocity of the CM for the male swimmers and 0.17 m s-1 faster for the female swimmers. There was a very strong relationship (p < 0.01) between velocity of H1 wave travel and CM velocity. There was no evidence to suggest that elite swimmers timed their actions to minimise vertical CM displacement to reduce mechanical work. In fact, the phase relationships among adjacent segments suggested that energy gained by raising the CM was transmitted caudally and contributed to a propulsive 'whip-like' action.