INDIRECT MEASUREMENT OF FORCES ON THE GYMNASTICS RINGS
Keywords: gymnastics, rings, force, video, strain gauge
AbstractINTRODUCTION: Forces produced during the Men’s Gymnastics Rings Exercise have been used to identify factors associated with excellence, and techniques which increase the risk of injury (Cheetham, 1987; Nissinen, 1995). Load cells positioned, either within the metal framework at the attachment of the ring cable, or connected in series between the ring cable and the frame have been used to record the forces applied by gymnasts. The purpose of this study was to develop an indirect video based method for determining the forces exerted on the rings during a gymnastics exercise. METHODS: Two genlocked video cameras were used to record a series of swinging skills performed by a member of the Great Britain Senior Men’s National Team. A third genlocked video camera recorded a close-up view of the displacement of the frame and of the ‘bungs’ (elastic damping devices from which the rings cables are suspended). A strain-gauged load cell was fitted into one of the rings cables to directly measure the applied forces for evaluation purposes. Prior to the main recording session, static calibration loads were applied to the rings in steps up to 6kN and synchronised video recordings of the frame and ‘bung’ deflections were recorded with the close-up camera. All video data were obtained using a Target high resolution video digitising system. The Direct Linear Transformation was used to reconstruct the 3D coordinates of the gymnast’s joint centres and the orientation of the rings cables from the two main camera views. Force values recorded during a series of swings by the gymnast were obtained indirectly from digitised video data of the frame and bung displacements and from the directly measured strain gauge output. RESULTS: During calibration the displacement of the rings frame was linear, with a displacement of 10.4 mm resulting when the maximum load of just under 6 kN was applied to the rings. Each ‘bung’ comprised a set of concave metal washers which were progressively flattened under loading in a non-linear relationship up to a maximum displacement of 19.4 mm at a submaximal applied load of 4690 N. Two mathematical functions, which accurately reflected the differing behaviours of the linear spring (frame) and the damped spring (bung), were fitted to the static calibration data using a simulated annealing procedure. The root mean squared differences between the measured and predicted force values for a backward longswing, forward longswing and basic swing were 45 N, 63 N and 37 N. CONCLUSION: The indirect method, based on video digitisation of the deflections in the rings frame and ‘bungs’, provided data which matched the overall profile of the force-time histories in the rings cables, and estimated cable tension to an accuracy of approximately 2 percent of the overall force range. REFERENCES: Cheetham, P. J. et al., (1987). Sports Psyche Editions, 99-106. Nissinen, M. A., (1995), Biomechanics XV, 680-681.
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