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.

In motion analysis marker sets or protocols are mostly developed for gait analysis and it has been shown that the marker set used affects the results of gait analysis. These marker sets are also used for the analysis of high dynamic sports movements. Single-leg landings are a common tool to investigate functional knee stability and further to predict injury risks where frontal plane motion and loading seem to play an important role. Until now, it is unknown how the marker sets affect the motion analysis results of such high dynamic movements. Therefore, the aim of the study was to compare six different marker sets. Three-dimensional motion and force data of single-leg landings in 12 healthy subjects were collected. Six different marker sets consisting of up to 26 markers and two clusters were simultaneously attached to the subjects’ lower limb and pelvis. The results show that particularly, the knee joint angles in the frontal and transverse plane showed the greatest differences between marker sets with in part contrary joint angle directions and great differences in angle magnitude. In addition, the amount of joint load was dependent on the marker set used for analysis. These results show that one must be careful when interpreting and comparing data of the frontal or transverse plane during high dynamic movements.

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The authors simulated the vertical movements of a jumper and the force time courses by means of a 4-degrees-of-freedom model consisting of 4 masses, springs, and dampers. Of the motions simulated, only that of the mass imitating the trunk corresponded to the measured data. The best fit to the measured force curves were obtained in the simulation in which time-dependent model parameters were used. From the results, the authors concluded that at the beginning of the landing, a jumper behaves like a 2-mass model in which the leg segments (thighs, shanks, and feet) effectively combine into 1 mass. After approximately 60 ms, the connections between the leg segments become more compliant and the jumper behaves like a 4-mass model with a soft coupling between the leg segments. The process is equivalent to an increase of the degrees of freedom of the movements. At the end of the ground contact phase during hopping, the jumper has to contract the muscles in order to reach the envisaged jump height. In the model, that contraction could not be satisfactorily simulated.

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