Simulation of the complex countermovement jumping by means of a simple four-degrees-of-freedom model.

By means of a four-degrees-of-freedom model the vertical movements of an athlete and the time course of the ground reaction force were simulated during a countermovement jump on a concrete and a wooden surface. The model masses were connected to each other and to the surface by springs and dampers. At first the stiffness of the springs decreased in order to initiate the countermovement. Afterwards the stiffness increased like the muscle activity so that the flexion of the model 'legs' were decelerated before the extension starts. The best result was attained when the stiffness of the spring between the model masses 'thighs' and 'trunk' increased before the other three springs. Compared with the muscle activity this means that for a successful jump the upper body segments have to be accelerated before the segments near to the ground are accelerated. The model 'athlete' was connected to a model of the surface. It could be shown that the jump on a concrete surface results in a better jump height than the jump on an elastic wooden surface if the muscle activation is not adapted to the surface properties.