We investigated the dominancy of excitability changes in motor cortex or spinal levels and temporal tuning mechanisms of muscle relaxation. We delivered transcranial magnetic stimulation and electric (H-reflex) stimulations relative to the application of the response signal under the reaction time task. The results showed that a significant change was found in only motor evoked potential during the prerelaxation phase, 30 ms after the response signal presented for muscle relaxation, but similar changes were not found in H-reflex. We suggested that this phenomenon, may be, reflects on excitation of corticospinal neurons, which consequently play an important role for muscle relaxation.
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