Intrasubject variability of selected force-platform parameters in the quantification of postural control.

Platform stabilometry is increasingly applied to monitor or re-educate standing balance in clinical rehabilitation. Consequently, insight is needed into the validity, reliability, and sensitivity of different force-platform parameters. This study focuses on the intrasubject variability as the major source of variance (unreliability) in the study of human motor skills. The intrasubject variability of several, commonly applied force-platform parameters was determined across ten repeated tests of quiet two-legged standing in healthy subjects to identify the most consistent and stable parameters in the quantification of postural control. The variability of the root mean square (RMS) amplitude, peak-to-peak amplitude, mean frequency, and RMS velocity of the fore-aft and lateral components of the center-of-pressure fluctuations was investigated under varying (visual and cognitive) task conditions. The results indicate that all selected parameters show considerable intrasubject variability irrespective of the task context. Nonetheless, both the RMS amplitude and RMS velocity in either direction of sway do not demonstrate a significant trend across repeated tests. Among the selected parameters, the RMS velocity in the fore-aft direction shows the greatest intrasubject consistency, as well as a high sensitivity to, for example, visual deprivation. These findings support the reliability and validity of this parameter in the clinical quantification of postural control.