ACTIVATION PATTERNS OF MONO-AND BIARTICULAR ARM MUSCLES AS A FUNCTIONOF FORCE AND MOVEMENT DIRECTION OF THE WRIST IN HUMANS

1. In order to explain the task-dependent activation of muscles, we ha ve investigated the hypothesis that mono-and bi-articular muscles have a different functional role in the control of multijoint movements. A ccording to this hypothesis, bi-articular muscles are activated in a w ay to control the direction of external force. The mono-articular musc les are thought to be activated to contribute to joint torque mainly d uring shortening movements. 2. To investigate this hypothesis, surface electromyographic (EMG) recordings were obtained from several mono-an d bi-articular arm muscles during voluntary slow movements of the wris t in a horizontal plane against an external force. The direction of fo rce produced at the wrist and the direction of movement of the wrist w ere varied independently. 3. The results revealed distinct differences between the activation patterns of mono-and biarticular muscles. The activation of the bi-articular muscles was not affected by movement di rection, but appeared to vary exclusively with the direction of force. 4. The mono-articular muscles showed significantly more EMG activity for movements in a specific direction, which equalled the movement dir ection corresponding to the largest shortening velocity of the muscle. The EMG activity decreased gradually for movements in other direction s. This direction-dependent activation appeared to be independent of t he direction of the external force.