Posterior root–muscle reflexes elicited by transcutaneous stimulation of the human lumbosacral cord

Continuous epidural stimulation of lumbar posterior root afferents can modify the activity of lumbar cord networks and motoneurons, resulting in suppression of spasticity or elicitation of locomotor-like movements in spinal cord-injured people. The aim of the present study was to demonstrate that posterior root afferents can also be depolarized by transcutaneous stimulation with moderate stimulus intensities. In healthy subjects, single stimuli applied through surface electrodes placed over the T11-T12 vertebrae with a mean intensity of 28.6 V elicited simultaneous, bilateral monosynaptic reflexes in quadriceps, hamstrings, tibialis anterior, and triceps surae by depolarization of lumbosacral posterior root fibers. The nature of these posterior root-muscle reflexes was demonstrated by the duration of the refractory period, and by modifying the responses with vibration and active and passive movements. Stimulation over the L4-L5 vertebrae selectively depolarized posterior root fibers or additionally activated anterior root fibers within the cauda equina depending on stimulus intensity. Transcutaneous posterior root stimulation with single pulses allows neurophysiological studies of state- and task-dependent modulations of monosynaptic reflexes at multiple segmental levels. Continuous transcutaneous posterior root stimulation represents a novel, non-invasive, neuromodulative approach for individuals with different neurological disorders.