Consistency and robustness of virtual time-to-contact in assessing postural sway in people with multiple sclerosis

Traditional sway metrics (path length, sway area) show high inter- and intra-subject variability and ignore the Center-of-Mass (CoM)/Center-of-Pressure (CoP) relationship with the base-of-support (BoS), limiting their ability to fully capture balance control mechanisms in people with multiple sclerosis (PwMS).

Does virtual time-to-contact (VTC) provide a more consistent assessment of quiet-stance balance than traditional sway metrics in PwMS and healthy controls?

119 PwMS and 48 matched healthy controls completed three 30 s standing trials under eyes-open, eyes-closed, and foam-surface conditions. CoM (pelvic markers) and BoS (foot markers) were used to compute VTC, path length, and sway area. Mixed ANOVA method examined task and group effects. Consistency was quantified with coefficient of variation (CV; intra-subject variability across trials) and Robust MAD/IQR (inter-subject variability). Bonferroni-corrected paired tests compared metrics.

Increasing task difficulty impaired balance across all metrics (p < 0.001). Controls exhibited higher VTC and lower path length and sway area than PwMS (p < 0.001). Across all task conditions and in both groups, VTC displayed the lowest CV compared to path length and sway area (p < 0.001), underscoring its superior trial-to-trial consistency. Similarly, VTC showed the smallest inter-subject variability, with both Robust MAD and IQR significantly lower than those of traditional sway metrics (p < 0.001).

By integrating the critical CoM-BoS relationship, VTC yields substantially more consistent quiet-stance measurements than traditional sway metrics in PwMS and controls. Its higher repeatability across trials and lower between-subject spread enhance sensitivity for detecting clinical change and evaluating interventions.