THE INFLUENCE OF SWIMMING VELOCITY ON THE KINEMATIC CHARACTERISTICS OF BACKSTROKE SWIMMING
Keywords: swimming, kinematics, stroke cycle, velocity, backstroke, performance
AbstractUnder standard workout conditions, swimmers spend most of their time swimming at rates significantly lower than racing speed. Specificity of training is not just a metabolic problem, but also a technical and a kinesiological one. The purpose of this study was to determine to what extent swimming the backstroke at different speeds implies a change in the stroke pattern and in the kinematics of the propulsive movements. We also studied the technical characteristics associated with performance in short distance efforts. A third aim of this study was to compare the intra-cycle velocity variation of the center of gravity of the body to the intra-cycle velocity variation of the hip point. Seven well-trained male swimmers participated in this study (age: 14.71±0.76 years, height: 1.74±0.39 m, body mass: 63.14±5.52 kg, %FAT: 15.40±1.81). Each subject performed 3 x 50 m backstroke repeats with 15 min of rest at a velocity corresponding to 90% (v90), 95% (v95) and 100% (vmax) of best performance in the 50 m backstroke. The swimmers were filmed on the sagittal plane, underwater and above the water, with synchronized cameras (60 Hz). Images were mixed and then digitized for further analysis using the APAS system. Peak mean horizontal velocities of the hand occurred during the upsweep at v90 and v95 and during the finish/exit at vmax. Total stroke duration decreased from v90 to v100 due to the shorter duration of the downsweep, the final downsweep and the finish/exit, but the relative duration of the phases did not show any significant changes. Increasing velocity caused an increase in the distance between point of entry into the water and point of exit of the hand from the water, but mean horizontal pulling length decreased. Swimming velocity at sub-maximal paces correlated well to mean body velocity at the upsweep, but at vmax, the highest association was to body velocity at the final downsweep. Maximal velocity in the 50 m backstroke was inversely correlated to the range of intra-cycle velocity variations of the body center of gravity. This parameter showed a poor individual correlation to the hip velocity variation (r=0.58 ± 0.18). The correlation between the coefficient of variation of the hip and the center of gravity intra-cycle velocities had, on the contrary, a high significance. Movement of temporal and spatial structures seem to vary little with velocity changes in swimmers who have attained a good stabilization of motor execution. In fast swimming, nevertheless, swimmers apparently achieve a greater anterioposterior stabilization of the hand, which may indicate more pronounced lift oriented sculling actions, and performance becomes more dependent on the final portion of the underwater path. The variation of the mean velocity of the hip cannot be used for quantification of the changes of body velocity from phase to phase, but a coefficient of variation (SD . mean-1 . 100) of intra-cycle hip velocity seems to be an adequate indicator of the intra-cycle velocity variation of the body center of gravity.
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