Vestibular contributions adapt to the balance strategies adopted during incline and decline walking

Vestibular contributions during walking are not uniform, but vary with gait phase, muscle group, and balance demands. Most previous experiments have focused on level-ground walking, with little known about how vestibular contributions change with terrain-imposed balance demands, such as slope. Our purpose was to quantify vestibular contributions during sloped walking. Participants walked on an instrumented treadmill at 1.0 m·s⁻¹ across five slopes (level, ±3°, ±6°), with and without stochastic electrical vestibular stimulation (EVS). To assess stability and gait adaptation, we measured step rate, length, width, and their variability from force plates. To probe vestibular contributions, we assessed time-frequency coherence between EVS and both whole-body responses, measured via mediolateral ground reaction forces, and lower-limb muscle responses, measured using electromyography. We found that whole-body coherence increased during incline but, unexpectedly, decreased during decline. Coherence for more proximal muscles, acting at the hip, mirrored whole body responses—increasing during incline and decreasing during decline, particularly during mid-to-late swing. More distal muscles acting at the ankle showed lower coherence across all slopes. This may reflect a balance strategy during incline walking that upregulates the vestibular system to guide limb trajectory throughout swing, rather than rely on active ankle control during stance, and a strategy during decline walking that downregulates the vestibular system and reduces balance demands through gait adaptations, such as shortened steps. Our findings highlight that vestibular contributions may be modulated in unexpected ways, reflecting not only the balance demands created by the terrain, but also the adaptive strategies adopted by the person.