Reliability of COP-based static balance parameters in participants with knee osteoarthritis (KOA) and low back pain (LBP) in Northwest Iran

To evaluate the Within-session test–retest reliability of center of pressure (COP)–based static balance parameters across age and gender subgroups in individuals with knee osteoarthritis (KOA) and low back pain (LBP). Within-session test–retest reliability study nested within a cross-sectional observational dataset. Participants completed three repeated 10-second standing trials during a single testing session. Reliability was assessed using ICC, SEM, MDC, and CV. Foot Anthropometry Research Center, Northwest Iran. A total of 669 patients (76 with KOA and 593 with LBP) stratified by age and gender. Relative reliability was assessed using the intraclass correlation coefficient (ICC [3,1]) with 95% confidence intervals. Absolute reliability was evaluated using the standard error of measurement (SEM), minimal detectable change (MDC), and coefficient of variation (CV). Velocity-based COP parameters (MVAP and MVML) generally demonstrated the highest reliability across age and gender subgroups (ICC range: 0.61–0.97). In contrast, displacement-, sway area–, and fractal-based parameters showed greater variability, with reliability estimates varying across demographic strata and several secondary measures exhibiting lower stability in specific subgroups. Velocity-based COP parameters demonstrated the most consistent reliability across age and gender subgroups in individuals with knee osteoarthritis and low back pain. In contrast, secondary COP metrics showed greater variability and should be interpreted with consideration of demographic context. These findings highlight the relative stability of velocity-based measures for within-session balance assessment, while further studies are needed to evaluate their validity, responsiveness, and clinical utility.