Spinal Reflex Modulation by a Sensory Neuroprosthesis During Gait in an Individual With Lower Limb Loss: Case Report

Individuals with lower limb loss have elevated fall risk partly due to missing sensory feedback from the missing limb. Plantar sensation is crucial for sensorimotor control of balance and contributes to regulation of spinal excitability necessary for stable gait. A sensory neuroprosthesis developed by our team restores plantar sensation perceived as originating from the missing foot by electrically activating nerves in the residual limb. This study investigated how the sensory neuroprosthesis modulates neural circuitry involved in the sensorimotor control of locomotion. One individual with transtibial limb loss who received the implanted sensory neuroprosthesis system walked on a treadmill while the medial gastrocnemius H-reflex was evoked at multiple gait cycle phases with sensory neuroprosthesis inactive or active. Even with sensory neuroprosthesis inactive, the H-reflex exhibited phase-dependent modulation across the gait cycle, increasing toward late stance and being suppressed in swing (P < 0.001), consistent with observations in able-bodied individuals. Sensory neuroprosthesis activation facilitated the H-reflex across all gait phases (P < 0.001), aligning it more appropriately with the biomechanical demands during gait. This suggests that sensory feedback from the sensory neuroprosthesis is integrated into sensorimotor control of gait and is processed through spinal reflex pathways in a context-appropriate manner, analogous to intact plantar mechanoreceptor inputs.