The underwater dolphin kick is a cyclic motion where the toes oscillate in a regular fashion with one spatial maximum (up-peak) and one spatial minimum (down-peak) in the vertical direction occurring over the course of one cycle. Underwater dolphin kick can be broken down into two phases; the down kick (DK) and upkick (UK). In a ventral body position, the DK is characterised by hip flexion and knee extension and occurs from the up-peak position and ends in the down-peak position; the UK is characterised by hip extension and knee flexion and occurs from the down-peak position and ends in the up-peak position. Kicking symmetry in the UDK is defined as the ability to produce equivalent propulsion during the DK and UK phases. This is accomplished through similar kinematics between the DK and UK phases. Theoretically, symmetry between DK and UK phases should result in more consistent centre of mass (CM) velocity as there are two equivalently propulsive phases, compared with one propulsive and one resistive phase in an asymmetrical kick cycle. In swimming, vortices represent the transfer of momentum from water to body and vice versa, resulting in body acceleration (Ungerechts, Persyn & Col man 1999). Previous studies have demonstrated that efficient swimmers create a large static vortex at the end of the downward kick and a small vortex at the end of the upward kick, whereas inefficient swimmers create small translating vortices at the end of the downward kick and no vortices at the end of upward kick (Arellano 1999; Arellano et al. 2000). Previous studies have compared the kinematics of human undulatory propulsion to the kinematics of dolphins/cetaceans (Ungerechts 1983; Von Loebbecke, Mittal, Fish & Mark 2009a; Von Loebbecke, Mittal, Fish & Mark 2009b). Ungerechts (1983) compared the kinematics of the butterfly stroke with dolphins and found that the primary difference between the human swimmers and dolphins was that dolphins were able to perform symmetrical DK and UK phases; in contrast, the human swimmers were relatively less effective at the UK phase, and he concluded that only swimmers who were able to hyperextend their knees would be able to perform the UK phase effectively. Von Loebbecke et al. (2009a, 2009b) compared the kinematics of the UDK in humans with odontocete cetaceans, finding that humans were less propulsively efficient and slower than cetaceans over the range of kicking frequencies and kicking amplitudes selected by the human swimmers. The differences between humans and cetaceans were attributed to the disadvantageous anatomy and musculature of humans, such as narrow feet and less-flexible joints, which especially limit the performance of the UK phase. Given their anatomical differences, symmetry between the DK and UK phases is an obvious limitation for human swimmers when compared with cetaceans; however, the relationship between kicking symmetry and performance has not been adequately studied in human swimmers. The purposes of this study were to evaluate the kinematics of DK and UK phases and how symmetry between OK and UK is related to performance. Symmetry in this experiment was evaluated by comparing joint marker paths, joint angles, horizontal displacement of the CM, horizontal velocity of the CM, and vertical toe velocities during the DK and UK phases.
© Copyright 2014 XIIth International Symposium for Biomechanics and Medicine in Swimming. Published by Australian Institute of Sport. All rights reserved.