Symmetry can be defined as an exact correspondence between opposite halves of a figure or a form, conversely asymmetry is any deviation from this "ideal" structure. In front crawl swimming, Haffner and Cappaert (1999) reported symmetry on 2-D angular values of the upper limb for international swimmers when Cappaert and Van Heest (1999) concluded that symmetry in kinematic stroke parameters represented the most efficient swimming stroke and thus characterised best swimmers. To our knowledge, no studies investigated the symmetry of underwater hand path while previous studies showed the importance of hand in sprint performance (Berger et al., 1995). Therefore, the aim of this study was to examine symmetry of 3-D spatio-temporal stroke parameters in international swimmers and to investigate their relationships with stroke velocity. Methods Eight male international swimmers (1.87 +/- 0.07 m; 79 +/- 6.53 kg; 22.50 +/- 2.29 yrs) performed a 25m swim at maximal velocity under breath-holding conditions. All subjects were medallists or finalists at the European championship (2002). Two digital video cameras (25 Hz) filmed frontal and sagittal views of underwater arm stroke in the middle portion of the test. Right and left fingertips, shoulders and hips were semi-manually digitised frame-by-frame to determine the swimmer displacement, bodyroll (BR) and fingertips trajectories for one complete cycle. Stroke parameters were stroke rate (SR), stroke length (SL) and stroke velocity (SV). According to Maglischo (1986), characteristic points of the fingertip trajectory were studied: minimal and maximal coordinates on the antero-posterior axis (B, F), maximal depth on the vertical axis (D) and maximal outward (O), inward (I), hand entry (Entry) and exit (Exit) on the transversal axis. Symmetry of bodyroll and of fingertip trajectories was evaluated with the absolute symmetry index (ASI) proposed by Karamanidis et al. (2003) in running: ASI = (IXR-XLI) / (0.5 x (XR+XL)) x 100 (%) (where XR is the parameter recorded from the right side and XL the corresponding parameter from the left one). A zero value for ASI reflected perfect symmetry. Acceptable symmetry was considered for an ASI of <10% (Herzog et al., 1989). Mean, standard deviation and coefficient of variation (Cv) were calculated for each parameter. Coefficient of correlation (p<0.05) was calculated between ASI parameters and SV. Results Stroke parameters were 1.53 +/- 0.1 m.s-1 (6.18%) for SV, 2.11 +/- 0.25 m (11.85%) for SL and 43.9 +/- 3.5 strokes.min-1(7.97%) for SR. Spatial trajectories of the fingertip and of the bodyroll were symmetrical while temporal parameters presented asymmetry for all parameters (Table 1). No significant correlations between SV and ASI both for spatial and temporal parameters were obtained. These preliminary results demonstrate that top level swimmers were characterised by spatial symmetry and by temporal asymmetry. Asymmetry could be due either to strength difference between both arms or to the preferred side of breathing. The lower force of one of the arm as observed by Goldfuss and Nelson (1971) in tethered swimming could explain that swimmer spent more time to realise the same spatial trajectory than the stronger one. Yanai and Hay (2000) observed that on the preferred side of breathing, even though the swimmer did not breathed, stroke duration and body roll were more important for this side. Opposite to Cappaert`s and VanHeest`s (1999) results, symmetry or asymmetry did not have any effect on SV. The divergence of findings could result from the studied parameters.
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