Inhibition of ipsilateral motor cortex during phasic generation of low force

Objective: To study the effect of different types of unilateral pinch grips on excitability of the ipsilateral motor cortex. Methods: In 9 healthy volunteers, transcranial magnetic stimuli (TMS) were applied over one motor cortex while the subjects performed either phasic or tonic ipsilateral pinch grips with different force levels (range 1-40% max imum voluntary contraction, MVC). Motor evoked potentials (MEP) were record ed from the relaxed contralateral first dorsal interosseous muscle (FDI) an d were compared to MEPs obtained during muscle relaxation of both hands. In additional experiments, transcranial electrical stimuli (TES) were adminis tered and F waves were recorded after electrical stimulation of the ulnar n erve. Results: Phasic pinch grips with low force (1 and 2% MVC) induced a signifi cant decrease of TMS-induced MEP amplitudes. The effect lasted for about 10 0 Ins after reaching the force level and was similar for both right and lef t-handed pinch grips. TES-induced MEPs and F waves remained unchanged. In c ontrast, tonic contractions (20 and 40% MVC) enhanced MEPs in the homologou s FDI. Conclusions: Phasic pinch grips with low force inhibit the motor cortex res ponsible for the contralateral homologous hand muscle. This effect, which i s probably mediated transcallosally, might act at the level of the motor co rtex. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.