A simple force platform

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The force platform consists of a sandwhich of steel, Rockwool® and concrete plates about 900 × 700 mm in surface. Four steel rings were bolted to the under side of the steel plate in each corner. Each steel ring was furnished with only one strain gauge, two of which were placed on the outer- respectively on the inner side of each ring. The four strain gauges were connected to a measuring bridge. Before mounting the rings on the steel plate, the sensitivity to pressure of each ring was adjusted in such a way that they were all similar. Because of this the platform responded with a signal which was independent of where a pressure was applied within the surface of the platform. The platform showed a rectilinear response forstatic forces up to 500 kp with a stable zero value. In response todynamic forces the platform showed a resonance frequency of about 50 Hz, with a damping factor of 0.15. Calibration of dynamic forces was carried out by calculation of the forces during a vertical jump compared with what would be expected from the time of flight also registered by the platform-measuring-bridge-ink-writer-set-up. The time of flight was significantly higher (11%) than was expected from the time-force relations before take-off. This was explained partly by the relatively low damping factor in the system, partly by the subjects not extending their knees at landing on the platform.