Effects of Wearable Continuous Normal-Force Tactile Feedback Applied to the Fingertip on Standing Balance in Healthy Young Adults.

Wearable balance-assistance systems predominantly rely on vibrotactile cues that are well suited for transient event detection but may provide a physiologically mismatched representation for the slow, quasi-static postural sway during quiet standing. To address this gap, this study investigates the effects of wearable continuous normal-force feedback on standing balance. We developed a real-time closed-loop system that maps mediolateral center-of-pressure deviation ( $\Delta $ COP ${}_{x}\text {)}$ to a continuous normal-force cue on fingertip using a dead zone-based piecewise linear algorithm (dead zone: ±3 mm; gain: 0.5; force limited to < 2 N). Twelve healthy young adults completed quiet-standing trials under a $2\times 2$ within-subject design (eyes open/closed $\times $ feedback on/off). Postural sway was quantified from COP ${}_{x}$ trajectories using root mean square (RMS), mean velocity (MV), maximum deviation (MD), and centroid frequency (CF), and analyzed with linear mixed-effects (LME) models. Continuous normal-force feedback significantly reduced sway magnitude (RMS; p < 0.001) and increased control activity (MV: p < 0.001; CF: p < 0.001), whereas no main effect was observed for MD. The absence of significant Vision $\times $ Feedback interactions across all behavioral metrics (p > 0.05) suggests that normal-force feedback provided similar benefits across visual conditions in the present study. These findings support the potential of continuous normal-force feedback as a physiologically grounded sensory augmentation strategy that contributes to postural regulation and may inform the development of future wearable balance-assistance devices. However, the physiological mechanisms underlying this visual-tactile integration, as well as its effectiveness in broader populations, remain to be elucidated.