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Power snatch execution asymmetry capturing method
UDC 796.886
PhD L.A. Khasin1 A.L. Drozdov1
1Moscow State Academy of Physical Culture, Malakhovka, Moscow Region
Corresponding author: niit@mgafk.ru
Abstract
Objective of the study was to develop and test benefits of a power snatch execution asymmetry capturing method applicable in the weightlifting elite training systems.
Methods and structure of the study. We used in the study detailed power snatch video captures produced by three high-speed cameras shooting from different angles. Cameras 1 and 2 were fixed 7m from the shaft butt on the right and left sides (respectively) of the weightlifting platform with the shooting axes parallel to the bar shaft. Camera 3 was fixed 10m far from the shaft perpendicular to the shaft center for the full-face shooting. We sampled for the power snatch kinematics analysis an elite female Master of Sport in weightlifting competing in the 59kg weight class.
Results and conclusion. We analyzed an individual asymmetry of the power snatch execution by elite weightlifter L to compute the horizontal/ vertical tilts and detect the tilt/ imbalance startup moment - and find reasons for the power snatch asymmetry. We also found that the vertical tilt/ imbalance starts up and grows after the shaft-hips contact. Despite the contact points and movement times of the right and left plates being even, the horizontal acceleration peaks and forces on the both plates were found to differ. In the analyzed power snatch sequence, the right plate was found to move faster with higher acceleration than the left one to force a vertical tilt on the weight. Horizontal tilt of the weight was found to start in the depreciation (pre-dip-under) phase, with a positive acceleration peak on the left butt. As a result, the vertical speeds of the right and left plates were different to result in the weight control asymmetry. This power snatch execution tilt/ asymmetry is may be explained by the individual anthropometrics-specific speed-strength imbalances plus a compensatory grip asymmetry.
Keywords: weightlifting, power snatch, high-speed video captures, kinematics, dynamics, asymmetry.
Background. Presently the weightlifting research community in its efforts to excel the training and competitive fitness systems gives a special priority to the weight control pacing and timing (spatial-temporal test rates), kinematics, dynamics, tilts and asymmetries/ imbalances in the weightlifting moves including the power snatch sequence, with a special role in the studies played by the high-speed videos to capture every movement detail in a contactless manner with high accuracy.
Objective of the study was to develop and test benefits of a power snatch execution asymmetry capturing method applicable in the weightlifting elite training systems.
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Methods and structure of the study. We used in the study detailed power snatch video captures produced by three high-speed cameras shooting from different angles. Cameras 1 and 2 were fixed 7m from the shaft butt on the right and left sides (respectively) of the weightlifting platform with the shooting axes parallel to the bar shaft. Camera 3 was fixed 10m far from the shaft perpendicular to the shaft center for the full-face shooting.
We sampled for the power snatch kinematics analysis an elite women's weightlifting Master of Sport competing in the 59kg weight class and coached by S.A. Syrtsov. We captured the 60kg power snatch sequence (every phase from startup to squat and
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standup) at 250 fps, with the weight rated at 95% of the personal best. Targets for Cameras 1 and 2 on the shaft butts were marked in digitalized in a coordinate system. Targets for Camera 3 were marked and digitalized at the shaft ends, shaft center and hand centers. Coordinates of every marked target were processed by a digital filter of our own design (State Registration Certificate No. 2017613826) to track the target movement speeds and accelerations. We also computed the vertical and horizontal power snatch force components on the weight as described in the prior study reports [1, 2].
Results and discussion. We obtained the right/ left shaft butt video captures with digitized coordinates of every right/ left shaft butt point in every frame on the sagittal plane, with the frames scaled for the plate centers - knowing that the plate shift effect on the scale never exceeds one percent. We then used the digital filter to compute, within these coordinates, the vertical and horizontal speed and acceleration coordinates of the targets. This means that every frame fixed coordinates, speeds and accelerations of the shaft butts.
A comparative analysis of the right/ left shaft butt vertical coordinate variations with time made it possible to find the right/ left shaft butt travel maximums (peaks) versus the starting point (weight on the platform). In this case the right and left butt travel maximums were 1.062 m and 1.11 m, respectively, with the right/ left shaft butt travel difference of 0.048 m with an average scaling error of 0.007 m - that means the shaft tilted to the right. The right/ left shaft butt difference was maximized at 0.063 m in the final weight fixing/ standup point. The right/ left shaft butt travel speed maximums were estimated at 2.038 m/s and 1.965 m/s on the left and right butts, respectively - that means the left butt being pushed up faster, as additionally verified by a visual examination of the full face frame in the squat/ standup starting position: see Fig. 1.
The vertical right/ left shaft butt coordinates were found to little vary (<0.006 m) till the 0.616s time point since the startup - to grow thereafter. Note that this time point refers to the depreciation (pre-dip-under) phase: see Figure 2. The growth may be due to a changed balance of forces on the right and lift plates plus a grip asymmetry. The athlete's right and left hand distances to the shaft ends were estimated at 0.196m and 0.186m, respectively, with a 0.032m grip shift from the shaft center indicative of the individual
anthropometric strength application imbalances/ asymmetry. The vertical strength maximums on the right and left plates were estimated at 371 N and 438 N, respectively, with this difference explaining the extra acceleration of the right butt.
Figure 1. Squat phase
Figure 2. Pre-dip-under (depreciation) phase
The horizontal right/ left shaft butt coordinates were found to differ much wider. For the horizontal coordinates tracking purposes, we took the either butt coordinates of the weight on the platform as a zero point - and found the horizontal coordinates different by 0.122m in the squat phase. To find causes for the difference, we visualized the horizontal coordinates difference variation with time - see Fig. 3.
Figure 3. RLB horizontal coordinates: difference variation with time
Note that till the 0.98s time point, the right/ left shaft butt horizontal coordinates differ within 0.01 m only, with the difference growing thereafter to peak at
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0.122 m in the final phase, with a tilt of 3.12 degrees. Our video analysis found this time point referring to the moment of the shaft-hips contact (pre-dip-under) point when the horizontal speed vector changes, with the shaft moved off the body: see Fig. 4. This hip contact point on diagrams (see Figure 5) is represented by peaks. It should be mentioned that the horizontal accelerations of the left and right butts are positive and negative at 36.22 m/s2 and 38.22 m/s2, respectively; whilst the butt acceleration times are the same at 0.076s. The right and left butt travel distances for this phase were estimated at 0.036m and 0.029m, respectively. On the whole, the above acceleration and travel profiles are indicative of the shaft-hips contact asymmetry - that results in the shaft tilting thereafter particularly visible at the end of the movement sequence: see Fig. 2.
Figure 4. Shaft-hips contact point prior to the dip-under phase
Figure 5. Horizontal accelerations of the left (positive peak) and right (negative peak) butts of the shaft
As was done in the barbell tilt analysis, we may analyze the horizontal forces in the weight control process
by the right/ left shaft butt travel and acceleration profiles using the mathematical model described in our prior study [2]. The maximal shaft-hips contact forces on the right and left plates were estimated at 780 N and 654 N, respectively - see the shaft-hips contact point prior to the dip-under phase on Figure 4.
Conclusion. We analyzed an individual asymmetry of the power snatch execution by elite weightlifter L to compute the horizontal/ vertical tilts and detect the tilt/ imbalance startup moment - and find reasons for the power snatch asymmetry. We also found that the vertical tilt/ imbalance starts up and grows after the shaft-hips contact. Despite the contact points and movement times of the right and left plates being even, the horizontal acceleration peaks and forces on the both plates were found to differ. In the analyzed power snatch sequence, the right plate was found to move faster with higher acceleration than the left one to force a vertical tilt on the weight. Horizontal tilt of the weight was found to start in the depreciation (pre-dip-under) phase, with a positive acceleration peak on the left butt. As a result, the vertical speeds of the right and left plates were different to result in the weight control asymmetry. This power snatch execution tilt/ asymmetry is may be explained by the individual an-thropometrics-specific speed-strength imbalances plus a compensatory grip asymmetry.
References
1. Khasin L.A., Buryan S.B. Video captures and mathematical modeling to rate horizontal forces in snatch sequence. Teoriya i praktika fiz. kultury. 2019. No. 6. pp. 29-31.
2. Khasin L.A. Biomechanical analysis of technique of highly skilled weightlifters with the application of mathematical modeling and high-speed video recording. Advances in Intelligent Systems and Computing. 2020. 1028 AISC. pp. 96-105.
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