Functional modulation of shoulder girdle stability.

We have previously reported that electrical stimulation of the ulnar nerve at group I strength evokes reflex excitation of the trapezius muscle in normal human subjects. In the present study this reflex has been evoked while subjects were performing a number of different tasks. When subjects performed a task involving a high degree of manual dexterity, the reflex was larger. On the other hand, when subjects performed a task that can be achieved with virtually no movement of the hand or wrist musculature, the reflex does not appear to show any task dependency. These results support the view that in delicate tasks of the hand, such compensatory reflexes are more important than in basic mechanisms such as lifting an object. The functional consequences of these findings and their significance in relation to the possibility that these task-related reflex gain changes produce intention tremor is discussed.