Force-dependent changes in movement-related cortical potentials.

The purpose of this study was to compare movement-related cortical potentials (MRCPs) associated with different levels of isometric contractions by elbow flexors. Eight healthy, right-handed male subjects participated in this study and performed different levels (10 and 50% of maximal voluntary contraction) of isometric contractions by the right elbow flexors. Electroencephalogram (EEG) signals were recorded from Fz, C3, Cz and C4 of the international 10 20 system. Motor potential (MP) amplitudes (from -200 to approximately -50 ms before force onset) for C3 associated with both force generations was significantly greater (P < 0.01) than those for C4, indicating that contralateral predominance of MRCP was observed in the right arm flexion. In Fz, the potentials of negative slope (NS') (from -600 to approximately -200 ms) and MPs for 50% MVC were significantly greater than those of 10% MVC. In Cz, the MP associated with 50% MVC revealed a significantly greater (P < 0.05) value than that with 10% MVC. In C3 and C4, the MP associated with 50% MVC tended to be greater than that with 10% MVC, but no statistically significant differences were found. These force-dependent changes in MRCPs imply increased activation of neural circuits involved in motor preparation and initiation. It is therefore suggested that the larger potentials from Fz and Cz for 50% MVC compared with 10% MVC reflect a greater activation of supplementary motor area for the preparation of the larger force generation.