Phasic modulation of corticomotor excitability during passive movement of the upper limb: effects of movement frequency and muscle specificity

Modulations in the excitability of spinal reflex pathways during passive rh ythmic movements of the lower limb have been demonstrated by a number of pr evious studies [4]. Less emphasis has been placed on the role of supraspina l pathways during passive movement, and on tasks involving the upper limb. In the present study, transcranial magnetic stimulation (TMS) was delivered to subjects while undergoing passive flexion-extension movements of the co ntralateral wrist. Motor evoked potentials (MEPs) of flexor carpi radialis (FCR) and abductor pollicus brevis (APB) muscles were recorded. Stimuli wer e delivered in eight phases of the movement cycle during three different fr equencies of movement. Evidence of marked modulations in pathway excitabili ty was found in the MEP amplitudes of the FCR muscle, with responses inhibi ted and facilitated from static values in the extension and flexion phases, respectively. The results indicated that at higher frequencies of movement there was greater modulation in pathway excitability. Paired-pulse TMS (su b-threshold conditioning) at short interstimulus intervals revealed modulat ions in the extent of inhibition in MEP amplitude at high movement frequenc ies. In the APE muscle, there was some evidence of phasic modulations of re sponse amplitude, although the effects were less marked than those observed in FCR. It is speculated that these modulatory effects are mediated via Ia afferent pathways and arise as a consequence of the induced forearm muscle shortening and lengthening. Although the level at which this input influen ces the corticomotoneuronal pathway is difficult to discern, a contribution from cortical regions is suggested. (C) 2001 Published by Elsevier Science B.V.