Rotational kinematics of pelvis and upper trunk at butterfly stroke: Can fins affect the dynamics of the system?

The purpose of the present study was to investigate the rotational kinematics pattern of the upper trunk and the pelvis and the complexity-variability of their movement, during the sprint butterfly stroke between male and female swimmers with long fins (18-26 cm) and without fins. Two pairs of 3D accelerometers and gyroscopes were used to measure segments׳ flexion-extension angles. There were no gender differences apart from the record. The amplitude values at the pelvis were significantly larger when swimming without fins while at the C7 they were significantly larger when swimming with them. Autocorrelation coefficients of angles׳ time histories were higher when swimming with fins for both segments. The power spectrums revealed a dominant frequency representing the stroke period which grew significantly stronger for the fin condition. Correlation Dimension verified a lower dimensionality for the fin condition for the C7 segment movement pattern. Overall fin use seems to offer “strength” to the swimmer's movement pattern. This does not imply better movement coordination, but rather a trend toward a more stable attractor.