Phasic modulation of somatosensory potentials during passive movement

Tibial nerve somatosensory evoked potential (SEP) amplitude modulates to passive stretch of leg extensors with movement, paralleling spinal reflex modulation. We therefore hypothesized that SEP amplitude is phasically attenuated during flexion in passive pedalling. SEPs and soleus H reflexes were evoked at four phase positions when the leg was static and passively moved. Initial SEPs were attenuated at full flexion compared with extension for both conditions (p < 0.05). SEPs during movement were significantly lower than those in the static condition (p < 0.05). There were no significant movement or phase effects on subsequent SEP components. H reflex modulation resembled that for initial SEPs. We conclude that movement-induced amplitude modulation of initial SEPs arises, partly, from phasic discharge of extensor muscle spindles.