Postural control in the elderly: An analysis of test-retest and interrater reliability of the COP-COM variable

Objectives: To estimate the test-retest and interrater reliability of the center of pressure-center of mass (COP-COM) variable of postural control in the elderly. Design: The biomechanic variable COP-COM, which represents the distance between the COP and the COM, was determined from 2 AMTI force platforms and 3 OPTOTRAK(T) position sensors. Measurements were taken in quiet position, double leg stance, and eyes open and eyes closed conditions. Setting: Laboratory environment. Participants: Forty-five healthy subjects, 8 patients with diabetes neuropathy, and 7 stroke survivors, all of whom were at least 60 years old. Interventions: Subjects were evaluated on 2 separate occasions within 7 days by the same evaluator to determine test-retest reliability. Interrater reliability was determined the same day. Main Outcome Measures: The biomechanic variable COP-COM, which represents the distance between the COP and the COM in terms of root mean square. The mean of 4 trials of the COP-COM variable for each condition was used for statistical analysis. Intraclass correlation coefficients (ICCs) were used. Results: The COP-COM variable has good reliability for both the test-retest and interrater studies, but its reliability varies according to the direction of the COP-COM. For the test-retest and interrater studies, the ICC ranged from .89 to .93 in the anteroposterior direction and from .74 to .79 in the mediolateral direction. Conclusion: The equivalence of the test-retest and interrater coefficients obtained suggests that the measurement error of the COP-COM variable is mainly linked to the biologic variability of this measure over a short period of time. Using the mean of 4 trials stabilizes the COP-COM variable enough to be potentially used to evaluate clinical change.