Responses of finger flexor and extensor muscles to transcranial magnetic stimulation during isometric force production tasks

INTRODUCTION: In this study we investigated neural mechanisms of finger force control. METHODS: Ten right-handed subjects performed isometric finger flexion and extension force productions at 10-60% of maximum voluntary contraction (MVC) using 4 fingers of the dominant hand. Transcranial magnetic stimulation (TMS) was applied over the contralateral hand motor area. We measured fluctuation of the background force and TMS responses from finger flexor and extensor muscles. RESULTS: Force fluctuation was greater during finger extension than during finger flexion. Motor evoked potentials (MEPs) increased with force levels in the flexor digitorum superficialis (FDS) during finger flexion and in the extensor digitorum communis (EDC) during finger extension. TMS-induced forces increased up to 40% MVC and then decreased during finger flexion, whereas they decreased continuously through the tested force levels during finger extension. CONCLUSIONS: These results suggest that FDS and EDC are controlled by different neural mechanisms, most likely attributable to their different functional roles in daily activities.