Time-dependency of test-retest reliability and measurement error of center-of-pressure synchronization and symmetry during quiet standing within specific frequencies in early sub-acute stroke.

Background: People with unilateral sensorimotor impairments after a stroke exhibit poor between-limb synchronization and asymmetries in balance control by relying on their less-affected side to maintain stability during standing. Therefore, there is a growing consensus to include center-of-pressure metrics as between-limb synchronization and dynamic control asymmetry when investigating balance improvements poststroke. However, the test-retest reliability of these metrics remains under-investigated, hindering uptake of these assessments in future stroke rehabilitation and recovery studies.

Research Question: First, are three immediate test-repetitions necessary for obtaining reliable synchronization and asymmetry scores in individuals with sub-acute stroke, or can fewer repetitions suffice? Second, does timing of assessments at 3-, 5-, 8-, and 12-weeks poststroke affect these estimates' test-retest reliability and measurement error?

Methods: Thirty stroke survivors with moderate-to-severe motor impairments were tested at 3-, 5-, 8-, and 12-weeks poststroke. At each timepoint, they completed three 40-second quiet standing trials on a dual force plate which measured center-of-pressure signals separately on each side. We calculated between-limb synchronization (i.e., cross-correlation) and dynamic control asymmetry (i.e., symmetry index) using the original center-of-pressure signal, and after decomposition into high and low (cut-off 0.4 Hz) frequency bands. Intraclass correlation coefficients (ICC) and Bland-Altman plots were used to assess test-retest reliability and measurement error. A cut-off was used to determine acceptable reliability (ICC>0.75).

Results: Between-limb synchronization and dynamic control asymmetry showed good-to-excellent reliability (ICCs=0.80-0.97) across three repetitions at all timepoints. Reducing to two repetitions yielded acceptable reliability (ICC=0.77-0.95) for dynamic control asymmetry and high-frequency measures, yet insufficient reliability for between-limb synchronization. At the 3-weeks timepoint, ICCs were generally lower, with more error, compared to later timepoints.

Conclusion: Novel performance metrics quantifying synchronization and asymmetry of balance control can be reliably obtained during the early sub-acute phase if three test-repetitions are administered per assessment.