This study investigated whether force data could be derived accurately using segment inertia data determined by the elliptical zone method (Jensen, 1976), automatic digitizing from high-speed video using a Motion Analysis VP110 system, and for an activity that does not require flexion of the thorax. The criterion fonctions were the force-time records of the jumps recorded at 500 Hz by a Kistler 9281B force platform. A second-order Butterworth digital filter was used to smooth the derived data, with frequency cutoffs being selected on the basis of root mean square error of the smoothed function with respect to the criterion force function. In a second procedure, the criterion function was the directly measured force-time record after filtering with a second-order Butterworth digital filter at 5 Hz to remove the high frequency part of the force signal. The closeness of fit of the derived data to the low frequency part of the criterion force was then assessed. It was concluded that, using the techniques described, the low frequency components of the ground reaction forces of drop jumps could be derived accurately.
Ross H. Sanders and Barry D. Wilson are with the School of Physical Education, University of Otago, Dunedin, New Zealand. Robert K. Jensen is with the School of Human Movement, Laurentian University, Sudbury, Ontario, Canada.