Characterizing within-subject variability in quantified measures of balance control: A cohort study.

Background: To longitudinally assess individuals using quantified measures, we must characterize within-subject variability (WSV) of the measures.

Research Question: What is the natural within-subject variability (WSV) that can be expected in postural control over 3+ days?

Methods: Thirteen individuals without orthopedic or neurologic impairment (mean(SD) = 55 (9) years; 76 (18) kg; 11 females/2 males) were recruited from a community workplace and consented to participate. Participants stood quietly with eyes closed (QEC) on a force platform (5 x 1 min x 6 days) in two stances: comfortable and narrow. We recorded center of pressure (COP) and calculated COP-based balance parameters. To analyze variance components, we applied a linear mixed model for repeated measures, calculating within-subject standard deviation (SDws) from the pooled variance not attributable to between-subject variability. To estimate WSV, we scaled SDws by a confidence interval (CI) factor (e.g. WSV at the 95%CI = WSV = SDws * 1.96) and report WSV for a range of conditions previously reported in the literature and the following measures previously found sensitive to or predictive of health: (primary) WSV of root-mean square amplitude of medial-lateral COP during QEC (RMSml); (secondary) WSV of COP ellipse area (COPa); (secondary) WSV of mean medial-lateral COP velocity (COPvml) during QEC.

Results: WSV was estimated at RMSml = 0.8 mm, COPa = 99mm, and COPvml = 1.1 mm/s among healthy, middle-aged participants standing comfortably for one recommended data duration (4 × 30 s trials). A look up table provides values for alternate protocols that have been suggested in the literature and might prove relevant for clinical translation.

Significance: This work advances longitudinal assessment of individuals using quantified measures of postural control. Results enable practitioners/researchers to assess an individual's progress, maintenance, or decline relative to WSV at a defined CI level.