Effects of contraction history on control and stability in explosive actions

In this model study, the effect of contraction history in skeletal muscle on joint movement and stability was investigated. A joint system was constructed with two identical (antagonistic) muscles. The muscles were modelled either according to the Hill tradition or as a modified Hill system including history-dependent properties. The joint system underwent movements with full activity of both muscles, mimicking explosive actions with high stability demands. Movements starting away from a balanced mid-position, as well as perturbation experiments, were simulated. The comparison between the Hill and modified Hill systems showed that contraction history improved stability (stiffness under perturbation) and, under certain conditions, caused a shift in the final joint position, which depended on the task characteristics (starting position and perturbations characteristics). This result indicates that modulations of muscle activity, required to move a joint to a particular end-position, do not only depend on the end-position but also on the preceding movements. This finding does not agree with the equilibrium-point hypothesis and is discussed accordingly.