Beyond the Bottom of the Foot: Topographic Organization of the Foot Dorsum in Walking.

Introduction: Sensory feedback from the foot dorsum during walking has only been studied globally by whole nerve stimulation. Stimulating the main nerve innervating the dorsal surface produces a functional stumble corrective response that is phase-dependently modulated. We speculated that effects evoked by activation of discrete skin regions on the foot dorsum would be topographically organized, as with the foot sole. Methods: Non-noxious electrical stimulation was delivered to five discrete locations on the dorsal surface of the foot during treadmill walking. Muscle activity from muscles acting at the ankle, knee, hip and shoulder were recorded along with ankle, knee and hip kinematics and kinetic information from forces under the foot. All data were sorted based on stimulus occurrence in twelve step-cycle phases, before being averaged together within a phase for subsequent analysis. Results: Results reveal dynamic changes in reflex amplitudes and kinematics that are site-specific and phase-dependent. Most responses from discrete sites on the foot dorsum were seen in the swing phase suggesting function to conform foot trajectory to maintain stability of the moving limb. In general, responses from lateral stimulation differed from medial stimulation and effects were largest from stimulation at the distal end of the foot at the metatarsals. That is, in anatomical locations where actual impact with an object in the environment is most likely during swing. Responses to stimulation extend to include muscles at the hip and shoulder. Conclusions: We reveal afferent feedback from specific cutaneous locations on the foot dorsum influences stance and swing phase corrective responses. This emphasizes the critical importance of feedback from the entire foot surface in locomotor control and has application for rehabilitation following neurological injury and in footwear development. (C) 2017 American College of Sports Medicine

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