Force plate analyses of human jumping
Conflicts in published research raised a series of questions on the precision of the measurements used to differentiate between vertical jumps performed with and without pre-stretch. Procedures outlined by previous researchers (eg. Komi and Bosco, 1978a; Bedi et al., 1987) were repeated and extended. Force plate data were collected for a series of squat, counter movement and rebound jumps. Individual subjects responded differently and no evidence could be found for an optimal rebound dropping height. Modal analysis of the force plate highlighted the need for improving its mounting. A frame was designed to raise the resonant frequency of the plate and static and dynamic calibrations revealed point of force application errors. 16 mm cinefilm was selected in preference to video for the subsequent inverse dynamics analysis of rebound jumping. French physiologist, Marey, observed that people appeared to jump higher following a rebound than a counter movement. A 'Marey' style jumping exercise was used to examine different takeoff and landing strategies. Variations in kinematic data filtering, body segment inertia parameters and quasi-static analysis techniques on the resultant moment moments were investigated. No differences in maximum jump height were found between counter movement and rebound jump takeoffs. This apparent contraction to the findings in previous research was accounted for by variations in the subjects' stretch heights at takeoff. A general proximal to distal sequencing of muscle moment peaking was observed in both takeoff actions, but moments peaked later in rebound takeoffs than when following counter movements. Larger peak moments occurred during landings preceding coming to rest than during the landing phase of the rebound jump. Quasi-statically determined muscle moments about the ankles and knees matched closely with the inverse dynamics values, but joint and overall support moments were consistently over estimated. Conflicts with selected published research findings were shown to arise from a lack of measurement precision. Takeoff velocities were greater following counter movements, but were insufficient to differentiate between jumping techniques. Rebounding was found to increase leg extension. Improvements in automatic measurement procedures combined with an enhanced understanding of musculo-skeletal modelling were seen as a way of improving future knowledge of neuromuscular coordination and power production in jumping.