Origin of the low-level EMG during the silent period following transcranial magnetic stimulation

ObjectiveThe cortical silent period refers to a period of near silence in the electromyogram (EMG) after transcranial magnetic stimulation (TMS) of the motor cortex during contraction. However, low-level EMG of unknown origin is often present. We hypothesised that it arises through spinal reflexes. Sudden lengthening of the muscle as force drops during the silent period could excite muscle spindles and facilitate motoneurones.MethodsSubjects (n = 8) performed maximal isometric, shortening and lengthening contractions of the elbow flexors during which TMS (90–100% output) was delivered over the motor cortex. The rate of flexion during shortening contractions reduced muscle lengthening caused by muscle relaxation. Surface EMG was recorded from biceps brachii and brachioradialis, and the low-level EMG during silent periods produced by TMS was measured.ResultsLow-level EMG activity was reduced on average by 68% in biceps and 63% in brachioradialis in the shortening contraction compared to all other contraction conditions (p < 0.001). Levels of pre-stimulus EMG were similar between conditions.ConclusionsMuscle lengthening contributes to low-level EMG activity in the silent period, through spinal reflex facilitation by muscle spindle afferents.SignificanceThe silent period depth is not only dependent on cortical output but also reflex effects evoked by muscle lengthening.