Motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation of the motor cortex are facilitated by contraction of the target muscle. This facilitation of MEPs is an important consideration when employing brain stimulation to indentify the integrity of corticospinal pathways and for elucidating mechanisms of motor control in human subjects. The present investigation was undertaken to determine the degree of facilitation of MEPs in forearm musculature associated with ipsilateral isometric wrist flexion contractions of varying percentage of maximum voluntary contraction (MVC). Of interest were the questions of whether the antagonist muscle (forearm extensor) MEP is inhibited by contraction of an ipsilateral agonist muscle (forearm fexor).
During the unilateral wrist flexion contractions MEPs were evoked in wrist flexor and extensor muscles. Increasing intensity of the force of contraction resulted in a linear increase in amplitude of the MEP in the ipisilateral flexor muscle, as has been already shown previously (Hess et al. 1986) . The mean latency of response also decreased from 16.5 ms at 0% MVC to 14.3 ms at 40% MVC. There was a modest but consistent increase in amplitude of the MEP of the extensor muscle except 40% MVC with increasing flexion force. Close inspection of the background EMG of the extensor muscle revealed low level but monotonically increasing amplitude of the interference pattern associated with incresed flexion force levels. These results establish that MEPs may be simultaneously evoked in both flexor and extensor muscles of the forearm. The presence of increasing levels of background contraction in the flexor muscle dose not lead to a progressive reciprocal inhibition of the extensor MEP. Instead, the amplitude of the extensor muscle MEP is unaltered; or it is increased if there exists a cocontraction of the extensor muscle.