Phase-dependent modulation of muscle activation evoked by superficial radial nerve stimulation during walking in humans

Researchers have been trying to develop an understanding of the complex neural mechanisms involved in controlling and regulating the coordination of numerous muscles during locomotion. Many sensory inputs received by the central nervous system from cutaneous and muscle receptors during movement that might help adjust to varying demands of the environment during the movement have been examined. It has been established that some nerve fibers involved in the control of locomotion interconnect the cervical and lumbar segments of the spinal cord, and that stimulation of upper limb nerves can alter the activation in lower limb motor pools. This study analyzed specific details of muscle activation responses in eight lower limb muscles evoked by superficial radial nerve stimulation in the right upper limb during several static postures and walking. Eighteen healthy volunteer adult participants with no documented neurological impairment or musculoskeletal injuries (ages 18-35) were recruited for the study. After determining the maximum isometric voluntary contraction (MVC) for each muscle being studied, muscle activation responses to brief nerve stimulation at random intervals during sustained contraction of 30% MVC, six static postures representing the different phases of the step cycle, and continuous walking were analyzed. The target level of ~30% MVC was selected to present a suitable baseline for observing both inhibition and excitation. This study specifically aimed to assess whether differences in reflexive muscle activation in response to electrical stimulation during different static postures were in some ways task-dependent and/or phase-dependent during locomotion. To assess that, muscle activation responses during the six static postures were compared with muscle activation responses during comparable phases of walking on a treadmill. The results from the study indicated the presence and nature of inter-limb reflexes in healthy individuals. Findings from this study may help understand further position-dependency and phase dependency of such reflexes in humans