Simulation of the influence of sports surfaces on vertical ground reaction forces during landing.

In many biomechanical analyses, the vertical ground reaction force (GRF) is measured by force plates. However, if force plates are fixed on elastic surfaces, the force signals have low-frequency oscillations superimposed. The question arises, as to whether this oscillation results from the response of the athlete to the surface properties or from the fixation of the force plate on the elastic surface. For the simulation of the vertical GRF, a mechanical model was developed that combines three submodels representing the surface, the athlete and the force plate. The simulations were carried out for landings on concrete and wooden elastic surfaces, without and with the force plate, respectively. Comparison of the two surfaces showed that, on the elastic surface, the passive peak of the vertical GRF was lower and was reached later than on the concrete surface. Thus a lower force rate was possible during the landing on the elastic surface (concrete: 186 body weight per second; wooden: 164 body weight per second), which can reduce the risk of damaging the joint cartilage. The simulations also showed that the time course of the GRF was changed by a rippling effect when the force plate was fixed on the elastic surface. The rippling was not the result of a change in the athlete's movements, because the parameters of the athlete submodel were not changed. The rippling induced by the force plate hinders the analysis of the GRF time course involving the real peak force and the force rate.