Intrasession reliability of the "center of pressure minus center of mass" variable of postural control in the healthy elderly

Objectives: To estimate the intrasession reliability of a single measure of the biomechanical variable "center of pressure minus center of mass" (COP-COM) to determine, first, how many trials must be averaged to obtain a reliable measure of postural stability, and second, the minimal metrically detectable change of the COP-COM. Participants: Community-living, healthy, elderly people over 60 years of age (n = 7). Design: Measurements were made in double leg stance, eyes-open condition. Nine successive trials, with rests between, were carried out for each subject. Data Analysis: Intraclass correlation coefficients (ICCs). Main Outcome Measure: The biomechanical variable COP-COM, which represents the distance between the center of pressure and the center of mass, was measured from two force platforms and three position sensors. Results: The ICCs obtained for one measure of the COP-COM were .79 in the anterior-posterior (AP) direction and .69 in mediolateral (ML) direction. With four trials, the COP-COM variable is reliable at .94 in the AP direction and .90 in the ML direction. Minimal metrically detectable changes were .10mm (AP) and .16mm (ML). Conclusion: Using four repetitions of the COP-COM variable provides a reliable measurement of postural stability.