Intermuscular co-ordination during fast contact control leg tasks in man.

From previous inverse dynamic analyses of human leg extensions, it was hypothesized that the underlying processes for the activation of mono- and biarticular muscles are different; the mono-articular muscles being activated when they shortened, whereas the biarticular muscles appeared responsible for the control of the external force direction. In the present study, experiments were performed on a dynamometer which was especially developed to test this hypothesis. Subjects had to exert different prescribed force vectors on a moving force-plate during leg extension, which they had intensively practised prior to the actual experiments. Of each trial, position, force and EMG activity were recorded. Net joint torques were calculated by the method of inverse dynamics and related to the EMG-patterns of the mono- and biarticular upper leg muscles to reveal whether the previously observed different roles in contact tasks might constitute a general principle in motor control. The results showed that although the action of the biarticular m. rectus femoris and hamstrings muscles was consistent in controlling the direction of the external force, the actions of the mono-articular muscles did not agree with their hypothesized role as simple work generators. The generalizability of a different control for mono- and biarticular muscles could thus not be confirmed for these tasks. They might rather reflect one out of more available strategies the CNS can use to control different contact control tasks.