Suppression of Electromyogram of Hand Muscle Elicited by Transcranial Magnetic Stimulation Over the Primary Motor Cortex

Cortical motor neuronal activation by transcranial magnetic stimulation (TMS) over the primary motor cortex area (M1) produces efferent signals that pass through the cortico-spinal tracts. As a result, motor evoked potentials (MEPs) are observed at muscles innervated by the stimulated motor neurons. It is well known that TMS can cause a silent period in the voluntary electromyogram (EMG). In the present study, TMS was applied to the M1 during thumb tapping. The influence of TMS on voluntary muscle contraction was discussed. Thumb trajectory was observed with a three-dimensional tracking device, and EMG of the abductor pollicis brevis (APB) muscle was also measured. The authors confirmed a change in thumb velocity in the downward direction during voluntary movement when TMS was applied, indicating that tissue excitation elicited by TMS causes loss of control of peripheral muscle motor control during voluntary movement.