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


METHODS
Subjects (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.


RESULTS
Low-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.


CONCLUSIONS
Muscle lengthening contributes to low-level EMG activity in the silent period, through spinal reflex facilitation by muscle spindle afferents.


CONCLUSIONS
The silent period depth is not only dependent on cortical output but also reflex effects evoked by muscle lengthening.
Neuroscience Research Australia, Randwick, NSW 2031, Australia. j.butler@neura.edu.au
Jane E
JE
Butler
Nicolas C
NC
Petersen
Robert D
RD
Herbert
Simon C
SC
Gandevia
Janet L
JL
Taylor
D000328:Adult; D001132:Arm / Q000502:physiology; D004576:Electromyography; D019054:Evoked Potentials, Motor / Q000502:physiology; D006801:Humans; D007537:Isometric Contraction / Q000502:physiology; D009044:Motor Cortex / Q000502:physiology*; D009046:Motor Neurons / Q000502:physiology; D018482:Muscle, Skeletal / Q000033:anatomy & histology / Q000294:innervation* / Q000502:physiology*; D013997:Time Factors; D050781:Transcranial Magnetic Stimulation