Symmetric facilitation between motor cortices during contraction of ipsilateral hand muscles

Using transcranial magnetic stimulation (TMS) over the contralateral motor cortex, motor evoked potentials (MEPs) were recorded from resting abductor pollicis brevis (APB) and first dorsal interosseous (FDI) muscles of eight subjects while they either rested or produced one of six levels of force wi th the APE ipsilateral to the TMS. F-waves were recorded from each APE at r est in response to median nerve stimulation while subjects either rested or produced one of two levels of force with their contralateral APE. Contract ion of the APE ipsilateral to TMS produced facilitation of the MEPs recorde d from resting APE and FDI muscles contralateral to TMS but did not modulat e F-wave amplitude. Negligible asymmetries in MEP facilitation were observe d between dominant and subdominant hands. These results suggest that facili tation arising from isometric contraction of ipsilateral hand muscles occur s primarily at supraspinal levels, and this occurs symmetrically between do minant and subdominant hemispheres.