THE CONTRIBUTION OF MUSCLE PROPERTIES IN THE CONTROL OF EXPLOSIVE MOVEMENTS

Explosive movements such as throwing, kicking, and jumping are charact erized by high velocity and short movement time. Due to the fact that latencies of neural feedback loops are long in comparison to movement times, correction of deviations cannot be achieved on the basis of neu ral feedback. In other words, the control signals must be largely prep rogrammed. Furthermore, in many explosive movements the skeletal syste m is mechanically analogous to an inverted pendulum; in such a system, disturbances tend to be amplified as time proceeds. It is difficult t o understand how an inverted-pendulum-like system can be controlled on the basis of some form of open loop control (albeit during a finite p eriod of time only). To investigate if actuator properties, specifical ly the force-length-velocity relationship of muscle, reduce the contro l problem associated with explosive movement tasks such as human verti cal jumping, a direct dynamics modeling and simulation approach was ad opted. In order to identify the role of muscle properties, two types o f open loop control signals were applied: STIM(t), representing the st imulation of muscles, and MOM(t), representing net joint moments. In c ase of STIM control, muscle properties influence the joint moments exe rted on the skeleton; in case of MOM control, these moments are direct ly prescribed. By applying perturbations and comparing the deviations from a reference movement for both types of control, the reduction of the effect of disturbances due to muscle properties was calculated. It was found that the system is very sensitive to perturbations in case of MOM control; the sensitivity to perturbations is markedly less in c ase of STIM control. It was concluded that muscle properties constitut e a peripheral feedback system that has the advantage of zero time del ay. This feedback system reduces the effect of perturbations during hu man vertical jumping to such a degree that when perturbations are not too large, the task may be performed successfully without any adaptati on of the muscle stimulation pattern.