This chapter addresses the role of rhythm in butterfly swimming. The underlying rhythms of the vertical undulations of the body parts and the motion of a 'body wave' have been quantified using Fourier analysis. In combination with advanced three dimensional analysis of kinematics, kinetics, and energetics, the analysis of rhythms and body wave motion provide insights into how skilled butterfly swimmers are able to re-use energy in the process of rotating and raising the trunk to swim economically. Three main conclusions can be drawn from the evidence and rationale provided. First skilled butterfly swimming is characterised by wave-like undulations of shoulders, hips, knees, and ankles with the vertical undulations consisting almost entirely of a waveform corresponding to the stroke frequency (H1) and a waveform of twice that frequency (H2). Second, the undulations are coordinated to yield travelling 'body waves' to optimise performance from both hydrodynamic and energy transmission and re-use perspectives. Third. the phase relationship between the one-beat H1 frequency and two-beat H2 is important to performance.
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