Biomechanics and Medicine in Swimming XI

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cm, 73.1 ± 6.4 kg and 20 ± 1.2 years, respectively. Before the experiment began, each swimmer received instruction on the no-step start and two types of step starts over a two-weeks; this was achieved by conducting daily training sessions. In each trial of a relay start, swimmers were permitted to swing their arms and to perform counter-movement. In the trial, we select three relay starts: a no-step start (NS), singlestep start (SS) and double-step start (DS) (Figure 1). For each start technique, each swimmer performed six trials with maximum effort; the trials were performed in random order. Each swimmer executed the relay start trial by timing it with the goal touch of the previous front crawl swimmer and then sprinted 15 m by front crawl swimming. No-step start Single-step start start Double-step start start Figure 1. Three types of relay starts performed in this study. Two high-speed cameras (FAST-CAM PCI, Photron Inc., JAPAN) filmed the trials at 250 frames per second. Camera 1 was positioned such that its optical axes were perpendicular to the plane of motion. Camera 2 was positioned 2.5 m above the floor of the side pool deck and filmed the scene at the instant at which the incoming swimmer touched his finishing point and the outgoing swimmer take-off from the starting block. The relay time was defined as the time elapsed between the touch and take-off and was calculated by counting the frames in the video image captured by Camera 2. Ground reaction forces during the relay starts were sampled at 1000 Hz by a waterproof force plate (5253B11, Kistler JAPAN Inc.) that was modified into the starting block. The force data were smoothed using a low-pass Butterworth digital filter with a low pass cutoff frequency of 40 Hz. The velocity at take-off (take-off velocity) was calculated from the data on the ground reaction force by performing time integration until the time of take-off. The take-off angle was defined as the angle between the resultant take-off-velocity vector and horizontal line (upward direction: positive; downward direction: negative). The horizontal ground reaction forces were used to distinguish the forces generated by steps or the body leaning forward and the legs driving out. The force generated by legs driving forward was defined as the force after the time (t 1 ) when the horizontal ground reaction force reached over 30% of body weight. The force before t 1 was obtained was defined as the force by step or body leaning forward (Figure 2). The horizontal reaction force generated by the legs driving was integrated with respect to time, and this value was used to calculate the velocity generated by the legs driving. A one-way repeated measure ANOVA was performed for the start type; this was followed by Bonferroni multiple comparison in each variable. The statistical significance was set at P < 0.05. Ground reaction force [N] Vertical + chaPter2.Biomechanics horizontal + 1400 1200 1000 800 Horizontal component 600 Force by 400 200 0 30% of subject’s body weght legs driving -200 t1 -1.4 -1.3 -1.2 -1.1 -1 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 Time [sec] 0 Figure 2. The definition of the horizontal ground reaction force generated by taking steps and leaning forward and the force generated by the legs driving. results Table 1 lists the mean horizontal take-off velocity, take-off angle and relay time in each trial. The relay times were increased significantly in order of no-step start, single-step start and double-step start. The horizontal velocity generated by legs driving in the no-step start was significantly greater than that in the double-step start. There was no significant difference in the horizontal take-off velocity and take-off angle. Table 2 lists the mean values of the standard deviation in the six trials for each condition. Table 1. Mean values of horizontal take-off velocity, horizontal velocity by legs driving, take-off angle and relay time in each trial. Horizontal take-off velocity Horizontal velocity by legs driving Take-off angle Relay time (m/s) (m/s) (°) (s) No-Step 4.09 ± 0.34 3.50 ± 0.34 -11.92 ± 6.15 0.018 ± 0.029 Single-Step 4.09 ± 0.31 3.28 ± 0.24 -13.18 ± 5.27 0.052 ± 0.044 * Double-Step 4.13 ± 0.32 3.22 ± 0.25 * -13.70 ± 4.80 0.087 ± 0.060 * † *: P < 0.05 in comparison with no-step start trials. †: P < 0.05 in comparison with single-step start trials. Table 2. Mean standard deviations of horizontal take-off velocity, horizontal velocity generated by legs driving, take-off angle and relay time in each trial. Horizontal take-off velocity Horizontal velocity by legs driving Take-off angle Relay time (m/s) (m/s) (°) (s) No-Step 0.15 ± 0.08 0.10 ± 0.05 2.17 ± 1.58 0.054 ± 0.023 Single-Step 0.25 ± 0.09 0.15 ± 0.05 2.11 ± 0.59 0.075 ± 0.031 Double-Step 0.23 ± 0.09 0.13 ± 0.03 2.25 ± 1.08 0.077 ± 0.035 Figure 3 shows the changes in the ground reaction force until take-off, as a typical example for Sub. A. Similar patterns were observed when the three trials involving the other subjects were compared. For all subjects, there were a total of eight trials for each step start. dIscussIon McLean et al. (2000) reported that the horizontal take-off velocity after the double-step start was significantly greater than that after the no-step start. Our hypothesis was similar to the results of McLean’s study. However, there was no significant difference among the horizontal take-off velocities achieved by the three relay start techniques (see Table 1). The advantage of step starts is that horizontal forces are generated until the legs driving (See Figure 3). No-step starts were superior to step starts in terms of the horizontal force generated by the legs driving. The horizontal velocity generated by legs driving in the no-step start was significantly greater than that in the double-step start. In the step start, the swimmers had greater horizontal velocities before the legs started driving. It was difficult for the swimmer to generate a large horizontal force during the leg drive in the step starts. In addition, the time required for the legs to drive decreased because the swimmer was already moving forward with a greater velocity before the legs began driving. 171
BiomechanicsandmedicineinswimmingXi The mean values of the standard deviations for each trial were small. Participants in the present study adopted an effective technique in relay starts. Although there were significant differences in the relay times, these differences were not very large. They may be related to the time elapsed during the step start until take-off, because the step starts involve a greater number of movements associated with stepping to the edge, adjusting the foot placement on the edge, and jumping into the pool. In eight trials among all the trials for all starts, the foot placement was incorrect. This indicates that the degree of difficulty of the step start is higher than that of the no-step start. Therefore, the no-step start is better for achieving consistent and superior performance in an relay event within actual competition.. However, a step start may help achieve greater horizontal take-off velocities. Swimmers should apply the technique aimed at generating a greater horizontal force within a shorter duration of leg driving It is also advantageous to achieve a certain amount of horizontal velocity by stepping, improve the accuracy while placing both feet and to achieve grip by using both feet correctly on the edge of starting block. Force [N] 172 NS SS DS 1400 1200 1000 800 Horizontal component 600 No-Step No-Step 400 Single-Step 200 0 -200 1400 Double-Step 1200 1000 800 600 400 200 0 -200 Vertical component -1.4 -1.2 -1 -0.8 -0.6 -0.4 -0.2 0 Time [sec] Figure 3. Changes in ground reaction force until take-off, as a typical example for Sub. A. For the no-step start, the horizontal take-off velocity was 4.92 m/s and the take-off angle was -10.8°. For the single-step start, the horizontal take-off velocity was 4.89 m/s and the take-off angle was -12.0°. For the double-step start, the horizontal take-off velocity was 4.92 m/s and the take-off angle was -15.5°. conclusIon The take-off velocity generated by the step start was not higher than that achieved by the no-step start. In step starts, the time required to change swimmers in a relay was significantly longer than that in no-step starts and there were eight failed trials (SS, DS). Therefore, the no-step start is better than the step start since it helps achieve consistent and superior relay start performance. reFFerences Gambrel DW, Blanke D, Thigpin K, Mellion M. (1991). A Biomehanical comparison of two relay starts in swimming. The Journal of Swimming Research, 7(2): 5-10. Issurin VB, Verbitsky O. (2003). Track Start vs grab Start; Evidence from the Sydney Olympic Games. Proceedings of the IXth World Symposium on Biomechanics and medicine in swimming. University of Saint-Etienne: 213-218. McLean SP, Holthe MJ, Vint PF, Beckett KD, Hinrichs RN. (2000). Addition of an approach to a swimming Relay Start. Journal of Applied Biomechanics, 16: 342-355. Roffer BJ (1972). The grab start is faster. Swimming technique, 8: 101- 102. Takeda T, Nomura T. (2006). What are the differences between grab and track start? In: Vilas-Boas JP, Alves F. & Marques A. (Eds.), Biomechanics and medicine in swimming X. Portuguese Journal of Sport Sciences, Porto, 192-105.
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Pahlen Norge AS Pahlen Norge AS Cha
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