Biomechanics and Medicine in Swimming XI

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These trials allowed verifying that the force signal produced by the wave was reasonably symmetric (Fig. 3). In normal turning situations, swimmers touch the platform, allowing it to record the swimmer’s pushing action against the wall. However, as already stated, before the swimmer touches the wall, a water wave hits the platform being also recorded. To remove this extraneous force, two assumptions were made: (i) the wave force to time curve being symmetrical, and (ii) that the peak value of the water wave force occurs at the time instant of swimmer’s wall contact. From the force platform data was extracted the time of water wave force start, and from the image-based kinematics the time of swimmer’s first contact was obtained. We assume this to be coincident with half water wave curve. This half force to time curve of the water wave was then mirrored and its values were subtracted from the total force values registered by the platform (Fig. 2) to allow obtaining: (i) the swimmer’s contact force to time curve, and (ii) the water wave force to time curve. Solid line: force registered; dotted line: wave mirroring; vertical line: time of contact Solid line: force registered; dotted line: force after wave removal; vertical line: time of contact Figure 2. Graphical representation of the removal of the water wave of the total force to time curve obtained during a crawl flip turn. With a sample of 17 swimmers, 9 males and 8 females, aged 17.88 ± 3.19 years, height 1.73 ± 0.09, body mass 64.48 ± 11.90 kg, all participants at the Absolute Portuguese National Championships, 154 valid turns were analysed. The force to time curve produced by the water wave was eliminated graphically from the force to time curve produced by the swimmer’s contact. Moreover, the wave force to time curve was also treated to characterize their main characteristics. Descriptive statistics were used for data analysis. RESULTS The force curves generated by the water wave obtained without the swimmer touching the wall resulted in a mean curve (Fig. 3). Figure 3. Mean force curve of the water wave generated by the displacement of the swimmer during the turn without touching the force platform. chaPter2.Biomechanics It is possible to observe that the behavior of the curve is not linear but follows a pattern tending to a symmetrical condition, as observed before by Fujishima (1999). Mean results obtained for the quantitative parameters of the water wave obtained with the turns performed without touching the wall are presented in Table 1, as well as the data regarding eight turns from the same swimmer touching the wall and the 154 turns performed by the total sample of 17 swimmers. Table 1. Mean values (Mean) ± standard deviations (SD) obtained for the variables: maximum and mean force value produced by the water wave, and wave impulse, grouped in normalized and absolute values in 3 situations of wave measurement during the flip turn. WAVE DATA Turning trials 8 turns without touching the wall 8 turns touching the wall 154 turns touching the wall NORMALIZADED VALUES ABSOLUT VALUES Maximal (N/bw) Mean ± SD Mean (N/ bw) Mean ± SD Impulse (N.s/bw) Mean ± SD 0.38 ± 0.77 0.16 ± 0.33 0.01 ± 0.002 0.52 ± 0.12 0.24 ± 0.06 0.01 ± 0.008 0.49 ± 0.24 0.19 ± 0.12 0.04 ± 0.02 Maximal (N) Mean ± SD 320.17 ± 65.59 448.88 ± 106.32 323.17 ± 191.94 Mean (N) Mean ± SD 140.62 ± 28.05 206.97 ± 65.76 128.68 ± 90.44 Impulse (N.s) Mean ± SD 10.5 ± 0.17 15.44 ± 4.07 27.16 ± 19.29 Figures 4 and 5 represent an example of the swimmer’s contact/impulse force to time curve, before (Fig. 4) and after removal (Fig. 5) of the water wave specific curve. In this case, the wave was almost fully incorporated into the swimmers’ contact curve, and the interference of the wave phenomenon into the swimmers’ kinetics is presumably important, making it more difficult to assess the effect of the wave over the impulse curve. Figure 4. Example of a force to time curve produced during a flip turn with the effect of the water wave impact. Figure 5. Example of a force to time curve produced during a flip turn without the effect of the water wave impact. 149
BiomechanicsandmedicineinswimmingXi The comparison of the impulse (time integral of the force curve) with and without the water wave kinetic effect showed values of 314.48 ± 62.75, and 288.57 ± 51.79 N.s, respectively. This difference stresses out a reduction of 10.87 % of the original force to time curve area when the water wave effect is removed. dIscussIon The behavior of the force to time curve recorded at the turning wall due to the water wave produced by a swimmer is variable, and the factors that determine its behavior are not yet fully understood. Supposedly, they are likely related to the aided mass of water displaced with the swimmer, to its cross-sectional area, and to the speed and direction of its displacement. The data collected during the turning experiments where the swimmer did not touch the wall are in agreement with Roesler (2002), but only partially with Lyttle and Mason (1997). In fact these authors found higher maximum values, but using a method in which the wave kinetics may include a not negligible contribution of the swimmers’ contact force (once the instrumentation that allowed the assessment of the initial contact instant was triggered manually). The possibly larger force plate used by these researchers might also influence the appropriateness of this comparison. The kinetic characteristics of the water wave at the wall (mean data of 154 turns performed by 17 swimmers) were similar to those provided by the mean results of eight trials performed without contact at the wall by one swimmer, despite the diversity of individual responses. However, when comparing the results of the same swimmer touching or not the wall, it was observed that the water wave force values without touching the wall tended to be smaller, probably because the swimmer refrained himself in order to avoid touching the wall. Since he is one of the best and stronger swimmers of the sample, his results tended to be higher than those obtained for the global sample. conclusIon The comparison of the force to time curve characteristics of the water wave produced by a swimmer during a flip turn, with and without touching the wall, support satisfactorily the solution proposed to remove the water wave effect upon the swimmers’ kinetics. It can be accepted that the error associated with the wave data elimination through this procedure is not significant, ensuring data integrity of the swimmer’s kinetics at the wall, and allowing greater accuracy for its assessment. reFerences Barbosa, T. M., & Vilas-Boas, J. P. (2005). Study of different concepts of efficiency of human locomotion in water. Portuguese Journal of Sport Sciences, 3(5), 337–347. Blanksby, B., Simpson, J. R., Elliot, B., & McElroy, K. (1998). Biomechanical factors influencing breaststroke turns by age-group swimmers. Journal of Applied Biomechanics, 14(2), 181–189. Blanksby, B., Skender, S., Elliott, B., McElroy, K., & Landers, G. (2004). An analysis of rollover backstroke turns by age-group swimmers. Sports Biomechanics, 3(3), 1-14. Fujishima, M., Sato, Y., & Miyashita, M. (1999). Improvement of wave absorption by a new lane-marker covered with mesh. Paper presented at the International Symposium on Biomechanics and Medicine in Swimming, Jyväskylä, Finland. Lyttle, A. (2000). Hydrodynamics of the human body during the freestyle tumble turn. The University of Western Australia. Lyttle, A., & Mason, B. (1997). A kinematic and kinetic analysis of freestyle and butterfly turns. Journal of Swimming Research, 12, 7–11. Roesler, H. (1997). Development of underwater platform for measuring forces and moments in the three coordinate axes for use in biomechanics. Universidade do Rio Grande do Sul, Porto Alegre. 150 Roesler, H. (2002, June, 2003). Turning force measurement in swimming using underwater force platform. Paper presented at the International Symposium on Biomechanics and Medicine in Swimming, Saint-Etienne, France. AcKnoWledGeMents This work was supported in part by a grant of the FCT - Science and Technology Foundation, Portugal.
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Figure 1. General biomechanical par
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average VO2 measured during resting
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Figure 2. Repeatability of the LTin
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etween experimental groups and cont
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Pahlen Norge AS Pahlen Norge AS Cha
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