Autonomic responses to repeated locomotor perturbations in older adults: dissociation between phasic habituation and sustained tonic arousal.

BACKGROUND: Balance loss during walking is an event that can activate the sympathetic nervous system (SNS). Sympathetic responses to unexpected perturbations specifically during walking and habituation with repeated exposure remain underexplored in older adults. OBJECTIVE: To investigate SNS responses to repeated balance perturbations during walking and determine whether these responses habituate in older adults. METHODS: Eighteen right-leg dominant older adults completed six slip perturbations in the same direction (rightward) during the initial contact phase of gait while walking on a treadmill, followed by a seventh perturbation in the opposite direction (leftward). SNS activity was monitored using electrodermal activity (EDA). Phasic responses were quantified as normalized skin conductance responses (%SCR), and tonic activity as normalized skin conductance levels (%SCL), both measured before and after each perturbation. RESULTS: Each perturbation elicited a significant phasic increase in %SCR. The first perturbation produced nearly double the response of subsequent trials, followed by gradual attenuation across trials 2-6, indicating habituation. The seventh perturbation in the opposite direction elicited a renewed significant %SCR increase, reflecting heightened SNS activation for unexpected events. Tonic %SCL increased after the first perturbation and remained elevated across subsequent trials, demonstrating sustained sympathetic arousal despite phasic habituation. CONCLUSIONS: Balance loss during walking triggers a phasic sympathetic nervous system (SNS) response that habituates with repeated similar perturbations. Changing perturbation direction restores the phasic response. Tonic activity increases after the first perturbation and remains elevated, reflecting sustained vigilance. These findings suggest older adults show an initial stress response but rapidly habituate and may benefit from perturbation-based balance training.