Modulation of cerebral somatosensory evoked potentials arising from tibial and sural nerve stimulation during rhythmic active and passive movements of the human lower limb.

The magnitudes of cerebral somatosensory evoked potentials (SEPs), following stimulation of cutaneous or muscle afferents in the upper limb, are reduced during active and passive movements of the fingers. The generalizability of such a movement effect was tested for lower limb events. We measured SEP magnitudes following activation of cutaneous (sural) and mixed (tibial) nerves during the flexion phase of active and passive rhythmic movements of the human lower limb. In eight volunteers, 150 SEPs per condition were recorded from Cz' referenced to Fpz'. Compared to stationary controls, both active and passive movements significantly depressed the early SEP components (P1-N1) [mean values, to 12.8%, 9.9% respectively for tibial nerve and to 29.6%, 25.6% for sural nerve stimulation, p < 0.05]. The attenuation was still observed when only one leg was moved and with stimulation at an earlier point in the flexion phase of movement. Visual fixation did not significantly affect P1-N1 amplitudes, compared to eyes closed. As previously shown, soleus H reflexes with stable M waves were significantly depressed during the movements (p < 0.05). The general construct may be that centripetal flow initiated from somatosensory receptors during limb movement leads to modulation of both spinal and cortical responses following large diameter cutaneous or muscle afferent activation.