Short communicationDynamic force measurements for a high bar using 3D motion capturing
Introduction
Measurement and knowledge of time-dependent forces acting on the athlete during performances on the high bar are of high relevance for better understanding of an athlete's physical requirements and abilities as well as for studying, modeling, and optimizing different exercises. Different studies aimed to model and optimize such gymnastic movements on the horizontal bar (Hiley and Yeadon, 2005, Hiley and Yeadon, 2008), others tried to describe the energetic processes during different exercises (Arampatzis and Brüggemann, 1999, Arampatzis and Brüggemann, 2001), but little information about forces during such high bar performances is available in the literature. Force measurements from bar mounted strain gauges were presented in Arampatzis and Brüggemann (1999). There, two different systems (at different framing rates) have been used to record the strain gauge readings and video data. The new approach presented within this work uses only one single system for all measurements, which eliminates such synchronization related errors as described in Lipfert et al. (2009).
The aim of this study was to deploy a Vicon 3D motion capturing system not only for tracking and analysis of an athlete's performance on the high bar but also for dynamic force measurements of that same exercise. Tests indicated that the spatial resolution of a Vicon system is sufficient to detect the bar displacement with a precision and accuracy needed to use the bar itself as a dynamometer. This approach only needs one system to be set up and further facilitates data evaluation as all data is recorded with the same system and renders subsequent data synchronizations unnecessary. Additionally, the effect of the high bar's inertia can be addressed since forces, displacement vectors, and derivatives thereof are available.
Section snippets
Methods
A FIG conform horizontal bar (Fédération Internationale de Gymnastique—Apparatus Commission, 2006) was set-up and centered within a Vicon V612 motion capturing system (Oxford Metrics Ltd, UK) with 8 M2 near-infrared cameras operating at 100 Hz. The (Cartesian) coordinate origin was set in the center-of-mass of the horizontal bar itself. The calibrated 3D volume covered 3 m in x-direction (axis through bar), 5.5 m in z-direction (height), and 6 m in y-direction (length). For the purpose of bar
Results
The calibration coefficients at the origin for the three studied tensions are listed in Table 1. A continuous trend towards smaller ratios of the calibration coefficients in y- and z-direction, , with increasing stiffness of the tension cables is observed. Unsurprisingly, higher tensions at the cables do have a larger influence on the y as on the z-direction (additional contribution from bar supports). To calculate the displacement for any other position along the bar further information
Discussion
We have developed a dynamic method to use the displacement of a horizontal bar to measure the forces acting on the bar by means of a motion capturing system. The main advantage of this approach is the dual use of the same equipment for both, motion analysis of exercises on the high bar and simultaneous force measurements thereof. Once the initial calibration has been performed the main measurement is easily performed together with the motion capturing. The same data evaluation procedures as for
Conflict of interest statement
None declared.
Acknowledgments
The authors would like to acknowledge the help of Alfred Fürhapter-Rieger and Patrick Rosmann during setup and data acquisition.
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