Topographic differences in task-dependent facilitation of magnetic motor evoked potentials.

OBJECTIVE: To look for differences in task-dependent facilitation of magnetic motor evoked potentials (MEPs) in proximal and distal upper extremity muscles.

BACKGROUND: Postexercise facilitation of MEPs has been demonstrated repeatedly in forearm muscles. Proximal muscles are prominently involved in limb stabilization, a largely isometric activity. In contrast, distal hand muscles specialize in precision movements. Based on these functional differences between proximal and distal muscles, we postulated that there might be topographic differences in task-dependent facilitation of MEPs.

METHODS: We studied the effects of isometric exercise and a precision grip task on MEPs in proximal and distal upper extremity muscles.

RESULTS: Isometric exercise of the target muscle was associated with significant facilitation of MEPs in biceps and extensor carpi radialis muscles, but not in first dorsal interosseous or abductor digiti minimi muscles. In contrast, a precision grip task was associated with significant post-task facilitation of MEPs in first dorsal interosseous, but not in extensor carpi radialis.

CONCLUSIONS: These differences in the facilitation of MEPs depending on the motor task and whether the muscle is proximal or distal may reflect the relative importance of proximal muscles in maintaining posture.