Human Cutaneous Reflexes Evoked with Simultaneous Multiple Nerve Stimulation during Rhythmic Locomotor-Like Arm and Leg Cycling in Stroke Subjects

The neural coupling between arms and legs during rhythmic activity has previously been investigated in healthy volunteers by cutaneous stimulation during concurrent arm and leg cycling. In this study, the coupling between arms and legs was investigated in 15 chronic stroke survivors with hemiplegia. Cutaneous reflexes were evoked by electrical stimulation of nerves innervating the foot of the most affected leg. Participants performed rhythmic cycling with the arms and legs during four trials with different stimulus configurations. Non-painful stimulations were delivered at the ankle level to the superficial peroneal, sural, tibial, and by stimulation of all three nerves simultaneously. EMG was recorded from four ipsilateral muscles (biceps femoris (BF), vastus lateralis (VL), medial gastrocnemius (MG), and tibialis anterior (TA) in the more affected leg. Phase-dependency was tested by recording reflexes at four different equally spaced phases of the locomotor cycle. Large excitatory VL reflexes were seen in the relaxation phase of the stimulated leg. For BF, an inhibitory reflex was observed in the power phase while an excitatory reflex was observed in the relaxation phase of the cycle. In the TA muscle, significantly larger reflexes were observed when all three nerves were stimulated simultaneously compared to stimulation of the three nerves individually (ANOVA P<0.05, post-hoc P<0.05). No differences were found between reflexes evoked by the three nerves in any of the muscles. No ANOVA interaction between phases and nerves was observed in any of the ANOVA analysis. Central non-linear summation of afferent non-nociceptive input occurs within the spinal reflex circuits after stroke. Sensory integration in reflex pathways could be exploited in gait rehabilitation after stroke by supporting ankle dorsi-flexion and thereby facilitating the relearning of gait in the subacute phase.