Abstract
Maintaining locomotor stability across changing environments requires the integration of numerous sensory sources, with vestibular information playing a prominent role. Previous research using electrical vestibular stimulation (EVS) has found that vestibular responses are altered by stability requirements. For instance, during locomotion, vestibular contribution is greatest during early stance, the least stable phase of the locomotor cycle. It has also been hypothesized that vestibular contribution may be driven by head kinematic variability, with greater variability requiring greater vestibular contribution. Hence, walking terrain may increase locomotor vestibular balance responses through increased stability demands and/or head variability. We tested this hypothesis by having fifteen participants walk on a treadmill across three terrains: level, 6° incline , and 6° decline. Stability was quantified through mediolateral step variability from the instrumented treadmill, and head variability was quantified through residual variance using an inertial measurement unit. We assessed vestibular contribution by estimating time-frequency coherence, between input stochastic EVS (0-25Hz; 0-5mA) and mediolateral forces. Higher coherence indicates greater reliance on vestibular influence for balance-correction. Step variability did not change with terrain. Linear head variability decreased with incline walking, while angular head variability increased with decline walking. Coherence increased during incline walking and decreased during decline walking. These findings suggest that step variability and head kinematic variability do not predict changes in vestibular-evoked balance responses on sloped terrain. One explanation for the observed coherence changes is that individuals rely more on passive dynamics during decline walking and more on active vestibular feedback during incline walking.
