We examined whether repetitive electrical stimulation to discrete foot sole regions that is phase-locked to the step cycle modulates activity patterns of ankle muscles and induces neuronal adaptation during human walking. Non-noxious repetitive foot sole stimulation (STIM; 67 pulses @333 Hz) was given to the medial forefoot (f-M) or heel (HL) regions at (1) the stance-to-swing transition, (2) swing-to-stance transition, or (3) mid-stance, during every step cycle for 10 min. Stance, but not swing, durations were prolonged f-M STIM delivered at stance-to-swing transition, and these changes remained for up to 20-30 min after the intervention. Electromyographic (EMG) burst durations and amplitudes in the ankle extensors were also prolonged and persisted for 20 min after the intervention. Interestingly, STIM to HL was ineffective at inducing modulation, suggesting stimulation location-specific adaptation. In contrast, STIM to HL (but not f-M), at the swing-to-stance phase transition, shortened the step cycle by premature termination of swing. Furthermore, the onset of EMG bursts in the ankle extensors appeared earlier than in the control condition. STIM delivered during the mid-stance phase was ineffective at modulating the step cycle, highlighting phase-dependent adaptation. These effects were absent when STIM was applied while mimicking static postures for each walking phase during standing. Our findings suggest that the combination of walking-related neuronal activity with repetitive sensory inputs from the foot can generate short-term adaptation that is phase-dependent and localized to the site of STIM.
Local GABA Concentration Predicts Perceptual Improvements After Repetitive Sensory Stimulation in Humans
