Biomechanics and Medicine in Swimming XI

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front-crawl stroke. This mechanical arm has a restrict movement, since the medio-lateral direction was not considered as there was little or no hand motion in this direction due to the mechanical constraints. Therefore, the purposes of this study, which involved synchronized swimmers and swimmers in a real situations, were (i) to verify the agreement between the attack angles calculated using different combinations of vectors, described in the literature and proposed by this study, to define the plane of the hand and (ii) to verify the variation in vector length of the methods found to be agreement in order to establish which method is most recommended when estimating the attack angle during sculling motion performed in a stationary vertical position (head above the water’s surface). Methods The sample consisted of 16 participants (10 female synchronized swimmers and 6 female swimmers; age 13.7±2.3 years; height 1.56±0.11 m; weight 51.7±4.2 kg). In order to participate in the study, swimmers were required to have at least 6 months of exercise including sculling actions during their training program. All the participants or their guardians, in those cases where the participants were not legally capable, signed informed consent forms. The Ethics Committee of the university where the study was undertaken approved this study. Recording took place in an indoor 25 m swimming pool. Two digital video cameras ( JVC GR-DVL 9800) were positioned behind two glass windows in the side of the pool beneath the water level. The distance between these windows was 11.2 m. The sampling frequency of the video cameras was 50 fields-per-second. The Digital Video for Windows (Dvideow) software was used to track the markers. Each camera was connected to a computer, which was linked to an intranet in order to start the recording at same time for both cameras. The spatial resolution of the video system used was 1024 x 768 pixels. A cube (0.80 m x 0.80 m x 0.80 m) was used as a control object with 12 control object points. The distance between the control object and a point equidistant between the windows was 6.7 m. Landmarks were placed on the distal end of the third finger (1), metacarpophalangeal joints of the second (2) and fifth fingers (3), on the centre of the wrist joint (4) and the elbow (5) of the right limb, thus, the movement was considered symmetrical. After this, each participant performed a warm-up with sculling actions and familiarization with the experimental conditions with the right side of the body directed toward the equidistant point between the windows, in the position to be used during recording. All tasks were performed at the calibrated volume. Each participant was asked to perform 20 seconds of sculling actions, maintaining a stationary vertical position with the head above the water surface and with the water at chin level in order to maintain all landmarks submerged during recording. This length of time was chosen in order to allow the participants sufficient time to stabilize the movement, while maintenance of the chosen position was the only criterion for standardization of sculling movement performance. Based on a qualitative criterion of stability, three cycles of sculling motion were chosen for digitalization. Most of the studies found in the literature analyzed only one cycle of sculling motion. By contrast, in the present study, it was decided to analyze three consecutive cycles of sculling, chosen when the movement seemed to be stable. An experienced digitizer, using Dvideow software, manually digitized these. By analyzing three cycles, the influence of any random error that may occur during the digitalizing procedure was minimized. Three-dimensional coordinates were obtained using a direct linear transformation method. The accuracy of the measurements was calculated and was equal to 0.004 m. The coefficient of variation was 0.5% when the distance between two rigidly linked markers at the known distance was reconstructed. All three-dimensional coordinates were smoothed using a seventh order low-pass Butterworth digital filter with cut-offs around 4 and 5 Hz, according to Residual Analysis (Winter, 2005). chaPter2.Biomechanics The calculations of the attack angle were made using the Matlab software (version 7.1). Figures 1 and 2 show the vectors that were used to reconstruct the hand orientation and Table 1 shows the different combinations of these vectors described by Schleihauf (1979), Berger et al. (1995), Lauder et al. (2001) (Lauder 1 – 5) and the new combination (NC) proposed by this study. Figure 1. Vectors on hand that can be used to reconstruct the hand orientation. Figure 2. Vectors on hand and forearm that may be used to reconstruct the hand orientation. The angle of attack was defined as the angle between the plane of the hand and the velocity vector of the hand (Payton and Bartlett, 1995). The plane of the hand was defined by the cross-product of vectors 1 and 2, which define a vector perpendicular (VP) to the plane of the hand. The velocity vector (v) was calculated using the raw coordinate data from the midpoint between the distal end of the second and fifth finger. The attack angle was calculated as 90° minus the angle between v and VP. Once the angles were calculated, (1) the time of each cycle of sculling motion was normalized (from 1 to 100%), (2) each participant was represented by the average of the three cycles and (3) the average cycle of all participants was calculated and used to verify the agreement between the attack angles calculated using different combinations of vectors described in the literature and proposed by this study. The degree of agreement between the attack angles calculated using the different combinations of vectors used to define the plane of hand was established based on graphical techniques from Bland and Altman (1986), in which the data were presented graphically by plotting the difference between each method versus their average obtained by step 3. Thus one hundred points were plotted for each normalized time. The mean differences (bias) and standard deviation (SD) of the differences between the values obtained with the methods, expressed in degree, were calculated. The limits of agreement were set at bias ± 2SD. All statistical procedures were performed using Matlab software. Thus, the analysis of the results comprised of two steps: (1) the agreement between the attack angles calculated was verified using different combinations of vectors, which were used to define the plane of the hand; (2) the means and standard deviations were calculated of these vector lengths in the methods that were found to agree. Therefore, from these values, the variation in vector length of the methods that were found to agree was estimated. Table 1. Different combinations of vectors for the reconstruction of the plane of the hand (see Figures 1 and 2 for location). Combinations Vector 1 Vector 2 Schleihauf VE VC Berger et al. VB VE Lauder 1 VD VC Lauder 2 VD VI Lauder 3 VH VD Lauder 4 VD VG Lauder 5 VF VD NC VH VI 87
BiomechanicsandmedicineinswimmingXi results According to figures 3 to 5, the combination of vectors for the reconstruction of the plane of the hand of Schleihauf is in agreement with the combination proposed in Lauder 1. The two methods are in agreement with the new combination (NC) proposed in this study. The variation in vector length of the methods found to be in agreement is presented in Table 2. Table 2. Variation (%) in vector length of the methods that were in agreement. Combinations Vector 1 Vector 2 Schleihauf 12 17.9 Lauder 1 7.2 17.9 NC 7.8 8.4 dIscussIon The results of this study show that the attack angles calculated from Schleihauf, Lauder 1 and NC methods are in agreement, that is, the three methods may be used interchangeably in order to calculate the attack angle. However, when the variation in vector length of these methods is observed there are greater variations between the Schleihauf and Lauder 1 methods. Lauder at al. (2001) also found the Schleihauf and Lauder 1 methods to be very similar and the mean percentage error in vector length reconstruction was smaller in Lauder 1 than Schleihauf. Furthermore, they concluded that the method proposed by Berger at al. (1995) differed from both Schleihauf and Lauder 1. This finding is in accordance with the findings of this study, since the method from Berger et al. (1995) was found not to be in agreement with the Schleihauf and Lauder 1 methods. The plane of the hand is found by calculating the cross product between two vectors, which represent the plane of the hand. These two vectors are calculated from three points, as in the Berger et al. (1995) and NC methods. Nevertheless, Schleihauf and Lauder 1 use four points to define the plane of the hand, although the points do not necessary lie in a single plane. Although Lauder at al. (2001) suggested that, since the method from Berger et al. (1995) requires fewer points for reconstruction, it could offer a clear advantage in real life, when some of the points may be obscured by water turbulence. Our results suggest it is better to use Schleihauf, Lauder 1 or NC when analyzing the sculling motion. Moreover, the vectors used in the method from Berger et al. (1995) appear to be more sensitive to slight changes in the hand shape than those used in Schleihauf, Lauder 1 and NC. Figure 3. Agreement between Schleihauf and Lauder 1 methods for reconstructing the attack angle. 88 Figure 4. Agreement between Schleihauf and NC methods for reconstructing the attack angle. Figure 5. Agreement between Lauder 1 and NC methods for reconstructing the attack angle. Using three points may present an advantage, and the NC method is better than the Schleihauf and Lauder 1 methods. Furthermore, Lauder 1 and NC presented a smaller variation in their vector lengths than the methods from Schleihauf (1979) and Berger et al. (1995) because there is a greater distance between the points, which might be beneficial in reducing error. In addition, the vectors used in the NC method presented a smaller variation in their lengths than other two. Thus, it is suggested that the NC method should be used in order to calculate the attack angle during analysis of sculling motion. conclusIon The attack angles calculated from Schleihauf, Lauder 1 and NC (the new method proposed by this study) methods were found to be in agreement. However, there was less variation in the length of the vectors used in the NC method than in those of the other two methods. Therefore, given that the results of the present study were obtained in a real situation as opposed to a model, we suggest using the NC method to calculate the attack angle in the analysis of the sculling motion. reFerences Berger, M., Groot, G. & Hollander, P. (1995). Hydrodynamic drag and lift forces on human hand/arm models. J Biomech, 28, 125-33. Bland, J. M. & Altman, D. G. (1986). Statistical methods for assessing agreement between two methods of clinical measurement. Lancet, 327, 307-10. Lauder, M. A., Dabnichki, P. & Bartlett, R. M. (2001). Improved accuracy and reliability of sweepback angle, pitch angle and hand velocity calculations in swimming. J Biomech, 34, 31-9. Payton, C. J. & Bartlett, R. M. (1995). Estimating propulsive forces in swimming from three-dimensional kinematic data. J Sports Sci, 13, 447-54. Schleihauf, R. (1979). A Hydrodynamic Analysis of Swimming Propulsion. Swimming III (pp. 70-109). Edmonton, Canada: University of Alberta. Winter, D. (2005). Biomechanics and Motor Control of Human Movement. New Jersey: John Wiley & Sons, Inc.
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Pahlen Norge AS Pahlen Norge AS Cha
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