Modification of reflex responses to lumbar posterior root stimulation by motor tasks in healthy subjects.

Dynamic task-dependent regulation of reflexes controlled by the central nervous system plays an integral part in neurocontrol of locomotion. Such modifications of sensory-motor transmission can be studied by conditioning a test reflex with specific motor tasks. To elicit short-latency test reflexes, we applied a novel transcutaneous spinal cord stimulation technique that depolarizes large-diameter posterior root afferents. These responses, termed posterior root-muscle (PRM) reflexes, are equivalent to the monosynaptic Hoffmann (H)-reflex but can be evoked in several muscles simultaneously. We elicited PRM reflexes in quadriceps, hamstrings, tibialis anterior, and triceps surae in subjects with intact nervous system. During three different conditioning-test paradigms in a standing position, that is, volitional unilateral single- and multi-joint lower limb movements and leaning backward/forward, we recorded characteristic movement-induced modulations of PRM reflexes in the thigh and leg muscle groups. We could thus demonstrate that monosynaptic PRM reflexes in functional extensor and flexor muscles of the thigh and leg can be elicited in upright standing subjects and can be modulated during the execution of postural maneuvers. The significance is that transcutaneous posterior root stimulation allows extending H-reflex studies of a single muscle to the assessment of synaptic transmission of two-neuron reflex arcs at multiple segmental levels simultaneously.