Phasic relationships of the extrinsic muscles of the normal hand.

The objective of this investigation was to improve the interpretation of dynamic electromyogram studies of upper extremity musculature by providing normal reference data. Fine wire electrodes recorded electromyogram data from the extrinsic forearm and hand muscles of 10 normal adults. Simultaneous wrist and finger motions were recorded by goniometers with the electromyogram data. Subjects performed hand opening and closing while actively maintaining selected elbow, forearm, and wrist positions. Phasic muscle activity was identified in each position for individual subjects. Normal adult subjects use gravity and tenodesis effects in addition to active muscle contraction to achieve motor goals. Individual subjects used different muscles to accomplish the same motor task, but each subject was remarkably consistent in muscle assignment. These findings suggest that individuals employ "motor planning strategies" in hand function.