The suppression of the long-latency stretch reflex in the human tibialis anterior muscle by transcranial magnetic stimulation

The purpose of this study was to investigate the transcortical nature of the long-latency stretch reflex (M3) in the human tibialis anterior muscle. This was achieved by applying a single pulse of subthreshold (90% motor threshold) transcortical magnetic stimulation (subTMS) at the site of the motor cortex. Such a stimulus is able to activate intracortical inhibitory circuits and thereby depress motor cortical output. We hypothesized that it would also suppress a transcortical reflex loop. The stretch reflex was elicited using a pedal attached to an electric motor. SubTMS was applied at several intervals prior to M3. Recordings were repeated 20-40 times. The reflex components were quantified using 20-ms windows in the averaged rectified electromyogram (EMG). SubTMS evoked significantly larger depression of M3 than of the background EMG in the same time frame when applied 55-85 ms prior to M3 (P<0.05, n=10). Furthermore, the effect on M3 was significantly larger than the effect on the spinal M2 (P<0.01, n=7). Our results provide evidence that the long-latency stretch reflex in the tibialis anterior muscle is at least partly transcortical.