Biomechanics and Medicine in Swimming XI

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In order to assess the significance of the effect of angle of attack on the various drag forces experienced by the mannequin, Pearson correlation coefficients were determined between angle of attack and the respective drag force components at all tested depths and velocities. These correlation matrices are presented in Table 1. Table 1 Correlation Matrices of angle of attack to; total drag (A) and wave drag (B) force at all depths and velocities. Correlations with significance of P
BiomechanicsandmedicineinswimmingXi Graphic Removal of Water Wave Impact in the Pool Wall during the Flip Turn Pereira, s.M. 1,2 , Gonçalves, P. 2 , Fernandes, r.J. 2 , Machado, l. 2 , roesler, h. 1 , Vilas-Boas, J.P. 2 1 University of the State of Santa Catarina, Florianópolis, Brazil 2 University of Porto, Faculty of Sport, Cifi2d, Porto, Portugal The aim of this study was to develop a graphical removal technique of the water wave that precedes the contact of the swimmer with the wall during the front-crawl flip turn, as well as to characterize and quantify it. To do this the researchers used an underwater force platform and two digital video cameras. In a pilot approach, a swimmer performed 8 turns without touching the wall. Following the pilot study, 17 swimmers of both genders, included the swimmer who participated in pilot approach, performed 154 complete flip turns. The water wave was removed graphically through a routine programmed in MatLab that was based on the symmetry of the wave, and the instant in time that the wave made contact with the wall. The average values for the maximum force of the wave were similar in both trials as well as to those previously published. Furthermore, the removal technique provided satisfactory results. Key words: dynamometry, swimming, flip turn, wave measurement IntroductIon To move a body through the aquatic environment, swimmers transmit a volume of water that is explained by hydrodynamic drag. The volume of water depends, amongst other factors upon the swimmer’s body volume and the speed of travel (Barbosa & Vilas-Boas, 2005). When the swimmer approaches the wall in the turn, some of the water volume displaced, hits the wall before the swimmer’s feet make contact. This anticipated force record makes it difficult to assess the real contact force of the swimmer. It is difficult to calculate the force generated by the wave and to remove it from the total force curve (Blanksby et al., 1998; Lyttle, 2000; Roesler, 2002). Blanksby et al. (1998) were the first authors to recognize the presence of the bow water wave in the kinetic analysis of the turn in breaststroke swimmers of different ages. These researchers tried to eliminate the wave’s effects through signal processing. However, as the frequency of the wave was similar to the frequency of the swimmer’s force signal, this was not possible to totally eliminate the bow wave without losing a considerable amount of useful data. Lyttle and Mason (1997) attempted to explain the slope of the wave shape during the kinetic analysis of turns in front crawl and butterfly swimmers. The data at the instant in time that the swimmer touched the wall (instrumented with a 3D force platform) was recorded manually. The force records from the wave reached a maximum value of 500 N, and the duration of the wave was not mentioned. The authors concluded that the magnitude of this force was proportional to the size of the force platform since the force recorded by the wave should have been equal to the pressure generated, multiplied by the surface area of the force platform. Roesler (2002) conducted a study using two force platforms, placed side by side on the wall of the pool, where the swimmer’s contact zone was only on one of the platforms. Recorded force peak values were around 371 N with a wave duration of around 0.14 s. The second platform, even without the swimmer’s contact, detected a load seven times smaller than the force produced by the wave on the first platform. This decrease was attributed to the distance that the wave travelled to reach the second platform. The aim of the present study was to develop a graphical removal technique of the bow wave that precedes the contact of the swimmer with the wall during the front-crawl flip turn, as well as characterizing and quantifying it. 148 Methods In order to characterize and quantify the force time curve produced by the wave before the turn, an extensometric underwater force platform was used, developed in accordance with Roesler (1997). Dimensions and characteristics were: 500 x 500 x 70 mm, load and sensitivity of 4000 and 2 N (respectively), natural frequency of 400 Hz, gain = 600. The force platform allowed data acquisition of force only in the perpendicular component to the surface of the platform. The platform mean measurement error was calculated after calibration and was given from the weighting values obtained from the platform and the real weight of dead-weights previously measure in a precision scale. The mean difference between the values acquired on the platform and the dead weights was less than 1%. The acquired signals were converted by an analog to digital (A / D) converter, with a 16 bit resolution (BIOPAC Systems, Inc.) and with input voltage of ± 10 volts. The acquisition rate was of 1000 Hz, powered by a 12 volts source, which allowed the subsequent import of the signal to a PC. The platform was set in an upright position on the opposite wall of the starting blocks, in a metallic support that also included a stainless steel frame for improved comfort and safety of the swimmer. As the platform was 0.7 m apart from the turning wall, the black lines at the bottom of the pool were modified to fit the new setting, remaining at the official distance to allow normal turning performances. Data from the force platform were acquired through the program Acqknowledge ® (BIOPAC System Inc.). Signal processing was programmed into a MatLab base and consisted of: calibration, filtering (Butterworth 4th order, with a cutoff frequency of 100 Hz) and DC offset removal. Normalization was done digitally by the program sharing files and turned over by the force of the weight of the swimmer. Two video cameras were used to monitor the water and the swimmer movements, particularly the time of the initial contact of the swimmer at the force platform. A digital type HC42E Sony camera was installed in a waterproof case (Sony SPK-HCB) and attached to the bottom of the pool perpendicular to the swimmers direction of movement (frontal to sagittal intermediate plan). A surveillance video camera (type B7W submergible camera - AC 230V) was fixed at the bottom of the pool, facing upward perpendicular to the swimming direction (frontal plan), in order to monitor the movement of the wave during the turn. When the swimmer started swimming towards to the turning wall, a trigger was activated to the A/D converting system, which connected a underwater light visible by the video cameras. This process allowed synchronizing the video setup and the force records. A male swimmer of 22 years old, 1.82 m in height and 84.9 kg body weight, finalist of sprint races at the Portuguese National Championships, performed 8 flip turns. He started from the center of the pool, 12.5 m apart from the turning wall, with maximum speed, but without touching the platform, as shown in Figure 1. Figure 1. Sequence of images of the wave generated by the movement of the swimmer performing the turn without touching the force platform.
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Figure 2. Repeatability of the LTin
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Pahlen Norge AS Pahlen Norge AS Cha
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