Inhibitory Effect of Subthreshold TMS on the Long-latency Response in the Flexor Carpi Radialis

Transcranial magnetic stimulation (TMS) can modulate corticospinal excitability during stretch-evoked long-latency responses (LLR). It has been previously established that suprathreshold TMS intensities can partially inhibit the cortical contribution to LLRs relying on a cortical silent period occurring after TMS. However, it is unknown whether the TMS-induced inhibition of stretch-evoked LLRs that relies on the cortical silent period can also be achieved via subthreshold stimulation. In this study, twelve healthy participants performed a protocol combining surface electromyography (EMG), robot-evoked wrist perturbations, and single pulse TMS applied to the motor cortex to study the effect of TMS intensity on the LLR amplitude in the flexor carpi radialis. We tested two TMS intensities of subthreshold (90%) and suprathreshold (130%) of the active motor threshold applied such that the motor evoked potential (MEP) peak would arrive 50 ms prior to perturbation onset. In suprathreshold TMS trials, TMS significantly reduced the cortical contribution to a LLR when applied prior to perturbation onset. When comparing the effects measured in the presence and absence of robot-applied muscle stretch, we observed that only suprathreshold conditions achieved inhibition of LLR, while subthreshold conditions did not result in any inhibition. Overall, our findings establish a clear distinction between the effect of subthreshold and suprathreshold TMS on the LLR inhibition via the cortical component of the silent period.