Transcutaneous electrical nerve stimulation enhances locomotor adaptation savings in people with multiple sclerosis

Locomotor adaptation on a split-belt treadmill can improve gait symmetry across various clinical populations, including people with multiple sclerosis (PwMS). As many PwMS experience sensory impairments, mobility interventions relying on sensory prediction errors may be less effective. Transcutaneous electrical nerve stimulation (TENS) has been shown to amplify sensorimotor function in PwMS and healthy controls, but its influence on motor learning remains unexplored. This randomized crossover trial investigated the effects of TENS on locomotor adaptation and cortical activation in PwMS. In total, 28 PwMS and 20 age- and sex-matched healthy controls completed two locomotor adaptation sessions, one with active TENS and one with inactive TENS. Locomotor adaptation was evaluated using step length asymmetry, quantified across four outcome metrics: adaptation magnitude, early change, after-effect and savings. Functional near-infrared spectroscopy recorded cortical activation, and linear mixed-effect models assessed group, visit and TENS condition effects on behavioural and cortical activation outcomes. PwMS exhibited reduced adaptation magnitude compared with healthy controls. TENS did not influence early change (representing adaptation rate) but significantly improved adaptation savings for PwMS who received TENS during their second visit only (initial savings: adj-P = 0.005, d = 1.35; early savings: adj-P = 0.014, d = 1.13). Additionally, both PwMS and healthy controls exhibited decreased cortical activation during locomotor adaptation with TENS, particularly in the dorsal premotor cortex for PwMS (adj-P = 0.019, d = 0.84). These findings indicate that TENS promotes the retention of prior locomotor adaptation, enhancing the efficiency of relearning. Additionally, reduced cortical activation with TENS in both groups indicates reduced cortical reliance during adaptation. Together, these effects suggest that TENS could have broader utility for enhancing motor learning in populations with sensory impairments, potentially leading to amplified retention and automaticity during motor rehabilitation paradigms.