Biomechanics and Medicine in Swimming XI

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Influences of the Back Plate on Competitive Swimming Starting Motion in Particular Projection Skill nomura, t. 1 , takeda, t. 2 , takagi, h. 2 1 Kyoto Institute of Technology, Kyoto, Japan 2 University of Tsukuba, Tsukuba, Japan The purpose of this study was to identify the influences of the back plate to competitive swimming starting motion in particular projection skill. Ten male college elite competitive swimmers performed the track start from the conventional platform and from the platform with the back plate. The influences of the back plate to competitive swimming starting motion in particular projection skill were identified as follows: (1) At the set position, the centre of mass displaced to a anterior position (-0.205±0.054 m); (2) The rear leg knee joint angle of the set position was about 90 degree (84.3±11.3 degree); (3) Just before take off, the body was accelerated into the horizontal direction (horizontal 8.76±0.87 m/s 2 , vertical 0.16±1.13 m/s 2 ); (4) At take off, the projection angle could be kept near horizontal (-8.2±5.2 degree); (5) There were no significant differences in flight phase. Key words: Back plate block, competitive swimming start, Projection skill IntroductIon A faster start has characteristics that move the centre of mass fast in the forward direction while the feet are in contact with the starting block, maximize the force exerted through the feet in the backward direction, and maximize the force exerted through the hands against the starting block in the forward and upward direction (Guimarães & Hay, 1985). Various investigations have been conducted on styles of set position as grab, track, handle, and so on (Bkanksby et al., 2002; Issurin & Verbitsky, 2003; Takeda & Nomura, 2006; Galbraith et al., 2008; Welcher et al., 2008). They found that there was no significant difference of start time among the grab start, the track start, and the handle start. Furthermore, the 94% of variance in the start time was explained by the water time that was the duration of the period between from first water contact to 9 m (Guimarães & Hay, 1985). It was considered that the swimming start consisted of plural skills, as projection, immersion and underwater actions. The underwater speed depends on the horizontal velocity at the entry. Furthermore, the horizontal velocity at entry results from the horizontal velocity at takeoff. Therefore, an important factor is the starting motion on the block to start fast. On the other hand, few studies have been considering the conditions of the starting block (Pereira et al., 2003). The starting platform with an adjustable back plate (posterior foot support) setting is approved by the international swimming governing body (FINA) in the facility rule 2.7. But there is no scientific evidence of the influences of the platform with the back plate on the performance of competitive swimming start. Therefore, the purpose of this study was to identify the influences of the back plate on competitive swimming starting motion in particular projection skill. Methods Ten male college elite competitive swimmers were used as subjects in this study (Height = 177.5 ± 5.3 cm, Weight = 74.6 ± 8.4 kg, Age = 21.1 ± 1.3 yrs). The characteristics of subjects are showed in Table 1. Subjects performed the track start in two conditions. One trial started from the conventional platform (CON). The other trial started from the platform with the back plate (BKP). Table 1. Characteristics of subjects. No. Height Mass Age Specialty Distance chaPter2.Biomechanics Personal Best (cm) (kg) (yrs) (m) (min’sec) 0 188.0 86 23 Free style 50 23.33 1 176.5 69 21 Butterfly 100 52.40 2 175.0 73 22 Breast stroke 100 1’01.51 3 175.8 81 21 Individual Medley 400 4’22.30 4 179.0 81 21 Free style 100 50.09 5 174.0 67 22 Free style 100 50.78 6 171.0 65 21 Butterfly 100 53.77 7 181.0 81 19 Breast stroke 100 1’01.78 8 172.0 62 19 Free style 200 3’59.38 9 183.0 81 22 Breaststroke 100 1’01.71 The starting block specifications were as follows; Height from water surface was 0.75m. The conventional platform was 0.5m width × 0.7m length, with a 10º slope. As a back plate a pedal for a track and field’s starting equipment was used. It was set 0.44m from the front edge of the platform, and with a 35º slope from the platform. A digital video camera equipment (Fps = 60Hz, Shutter speed = 250Hz) was connected to a starting and timing system and used to record the performance of each trial from a sagittal view. Seventeen calibration points were set over a 2.1m width × 2.5m height frame placed at the start area on the swimmer’s plane of movement. The digital video was analyzed by digitizing 13 points of the body: left finger tip, left wrist, left elbow, left shoulder, left hip, left ear, vertex, left knee, left ankle, left toe, right knee, right ankle, right toe. It was assumed that arm and torso were symmetrical. Kinematic variables were calculated using 2D-DLT method with a self-made motion analysis software (Note Player). The accuracy of coordinates was evaluated by the difference of the actual value and the estimated value (horizontal = 0.034m, vertical = 0.013m). Butterworth low-pass filter with a cut-off frequency of 6 Hz was used to remove noise from all raw data. The objective in performing a swimming start while the feet are in contact with the starting block is to move the centre of mass (CM) as fast as possible in the forward direction. For the purpose of this study, the projection motion was divided into set position, acceleration phase, take-off and flight phase. The set position was a still position previous to the starting signal. The acceleration phase was from the starting signal until take-off. The take-off is the instant on time when the forward leg leaved from the block. The flight phase was defined from the take-off until the first contact with the water surface. Kinematic variables for the projection motion consisted of 15 items, as shown in Table 2 and Figure 1. The x-axis was defined to be the horizontal axis in the main plane of movement. The y-axis was defined to be the vertical one. results At the starting position, the coordinate of CM in BKP was (-0.205±0.054 m, 1.367±0.029 m). It was significantly forwarded and at a higher position than in CON (-0.253±0.054 m, 1.355±0.031 m). The knee joint angles of front and rear leg in BKP were respectively 140.1±5.7º, and 84.3±11.3º. The knees were extended significantly narrower than in CON (145.5±8.0º and 97.1±11.4º). The ankle joint angles of front and rear leg in BKP were, respectively, 140.6±8.4º, and 104.1±8.4º. The front ankle was dorsal-flexed significantly narrower than that in CON (147.1±10.5º). On the other hand, the rear ankle in 135
BiomechanicsandmedicineinswimmingXi Table 2. Kinematic variables for projective motion. Phase Symbol Unit Explanation Set position Acceleration Take-off Flight 136 Px m Py m Horizontal position of CM from the front edge of the platform Vertical position of CM from the water surface θ_knee deg Angle of knee joint at the set position θ_ankle deg Angle of ankle joint at the set position θ_before deg Mean pre-projection angle on velocity vector of CM during 0.3 second just before to the take-off Ax_before m/s 2 Mean horizontal acceleration of CM during 0.3 second just before the take-off Ay_before m/s 2 Mean vertical acceleration of CM during 0.3 second just before the take-off T_take-off sec θ_take-off deg θ_body deg Time duration from starting signal to take-off Angle of the velocity vector of CM at the take-off Angle of CM connected with the front edge of platform and the horizontal line at the take-off V_take-off m/s Velocity of CM at the take-off Vx_take-off m/s Horizontal velocity of CM at the take-off Vy_take-off m/s Vertical velocity of CM the take-off D_flight m Distance of flight (horizontal distance from the wall to fingertip contact with water surface) Vx_flight m/s Mean horizontal velocity during flight CM Ax_before ƒ Æ_before CM Vx_take-off Ay_before ƒ Æ_take-off Vy_take-off V_take-off ƒ Æ_body CM Vx_flight dVx/dt CM dVy/dt Ax_before (Vx_before) ƒ Æ_before CM Vx_take-off Ay_before (Vy_before) Px, Py ƒ Æ_knee (V_before) ƒ Æ_take-off Vy_take-off V_take-off ƒ Æ_ankle ƒ Æ_body CM Vx_flight dVx/dt CM dVy/dt Ax_before (Vx_before) ƒ Æ_before Ay_before (Vy_before) (V_before) dVx/dt dVy/dt (Vx_before) (Vy_before) Px, Py ƒ Æ_knee (V_before) ƒ Æ_ankle Back plate 35 deg Platform 10 deg Height 0.75m (0,0) D_flight T_take-off Acceleration Flight Set position Take-off Entry Figure 1. Diagrammatic representation of kinematic variables. BKP was plantar-flexed and positioned significantly wider than one in CON (76.4±7.2º). During the last 0.3 second before to the take-off in the acceleration phase, the mean pre-projection angle of CM in BKP was -6.7±4.4º. It was significantly nearer to the horizontal than that in CON (-8.2±4.3º). Mean horizontal acceleration in BKP was 8.76±0.87 m/s 2 . It was significantly larger than that in CON (7.96±0.79 m/s 2 ). On the other hand, mean vertical acceleration in BKP was 0.16±1.13 m/s 2 . It was near to the zero than that in CON (-0.58±0.79 m/s 2 ). At take-off, projection angle of CM in BKP was -8.2±5.2º. It was significantly more close to the horizontal and larger than that in CON (-10.5±4.9º). Verti- cal velocity of CM at the take-off in BKP was -0.65±0.45 m/s. It was near to the zero than that in CON (-0.81±0.45 m/s). In other items for the take-off (T_take-off, θ_body, V_take-off and Vx_take-off ), significant differences were not observed between starting conditions. During flight phase, non-significant differences were also perceived. The main results of this study are shown in Table 3. Table 3. Kinematic variables compared between conventional and in back plate starting conditions. Phase Symbol Unit Set position Accele- ration Take- off Flight CON BKP Significant Mean S.D Mean S.D Difference Px m -0.253 ± 0.054 -0.205 ± 0.054 *** Py m 1.355 ± 0.031 1.367 ± 0.029 ** θ_knee (front) deg 145.5 ± 8.0 140.1 ± 5.7 ** θ_knee (rear) deg 97.1 ± 11.4 84.3 ± 11.3 *** θ_ankle (front) deg 147.1 ± 10.5 140.6 ± 8.4 ** θ_ankle (rear) deg 76.4 ± 7.2 104.1 ± 8.4 *** θ_before deg -8.2 ± 4.3 -6.7 ± 4.4 * Ax_before m/s 2 7.96 ± 0.79 8.76 ± 0.87 * Ay_before m/s 2 -0.58 ± 0.79 0.16 ± 1.13 ** T_take-off sec 0.784 ± 0.033 0.764 ± 0.046 N.S. θ_take-off deg -10.5 ± 4.9 -8.2 ± 5.2 * θ_body deg 20.5 ± 5.4 20.9 ± 5.9 N.S. V_take-off m/s 4.47 ± 0.30 4.41 ± 0.32 N.S. Vx_take-off m/s 4.38 ± 0.22 4.34 ± 0.26 N.S. Vy_take-off m/s -0.81 ± 0.45 -0.65 ± 0.45 * D_flight m 3.00 ± 0.19 2.99 ± 0.18 N.S. Vx_flight m/s 4.48 ± 0.16 4.46 ± 0.17 N.S. * p < 0.05, ** p < 0.01, *** p < 0.001 dIscussIon At the set position, the CM at the back plate condition is displaced to comparatively anterior position regarding the CON. It was in agreement with study of squatting-to-standing movement that heel elevation primarily influenced postural adjustment as anterior displacement of the hip (Sriwarno et al., 2008). In the back plate condition, the rear knee joint angle obtained a value close to the 90º. The knee angle of the posterior leg was recommended 90º by a manufacturer of starting blocks with back plate that approved by FINA. However, isometric force-angle relationship of knee extension reported that higher force is produced at 105º to 120º than in other joint angle conditions (Lindahl et al., 1969). Moreover, the relationship isometric knee-hip extension force and the percentage of leg length that was criterion of the knee flexion had investigated. The force exhibited a peak when the foot position was at 80-90% of leg length (Yamauchi et al., 2007). If this ratio was angle converted, it became about 106º to 128º. Therefore, at the set position in the back plate condition, the rear knee joint should be extended a little more. As a result, CM moved ahead slightly. It seemed that the θ_before in BKP approached horizontally was a preferable effect with back plate, because moving CM fast to the forward direction was one important factor for a faster start (Guimarães & Hay, 1985). It was supported by Ax_before in BKP which was grater than one in CON and Ay_before in BKP was approximating to become zero. The force distribution on the starting block had reported that a higher force at last stage of the start movement, and force gradually became lower until take-off (Krüger et al., 2003). It was considered
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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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• Without restriction; • Blinde
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etween experimental groups and cont
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Pahlen Norge AS Pahlen Norge AS Cha
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