Intrasession reliability of center of pressure measures of postural steadiness in healthy elderly people

Objectives: To estimate the immediate test-retest reliability of a single measure of several center of pressure (COP) variables, to report the number of trials to be averaged to obtain a reliable measurement of postural steadiness, and to determine the minimal metrically detectable change (MMDC). Design: Cross-sectional study. Setting: University biomechanics laboratory. Participants: Seven community-living, healthy elderly people over the age of 60 years (range, 62-73y). Interventions: Not applicable. Main Outcome Measures: The COP was estimated from 2 force platforms and the following measures were calculated: (1) root mean square (RMS), (2) COP range, (3) COP mean velocity, (4) mean power frequency (MPF), (5) median power frequency (MedPF), and (6) sway area. Intraclass correlation coefficients (ICCs) were determined by using 9 successive quiet standing trials. Results: The ICCs obtained for 1 measure of 120 seconds were .58 and .58 for the RMS, .83 and .94 for the COP mean velocity, .52 and .62 for the COP range, .44 and .30 for the MPF, and .34 and .47 for the MedPF in anteroposterior (AP) and mediolateral (ML) directions, respectively. The ICC of the COP sway area obtained for 1 measure was .41. Only 2 trials had to be averaged to obtain an ICC over .90 for the COP mean velocity associated with an MMDC of +/- 1.2mm/s (AP) and +/-0.6mm/s (ML). Conclusions: Mean velocity was the most reliable COP measure and using 2 repetitions allowed for reliable measurement of postural steadiness. For the other COP variables, 3 trials of 120 seconds were needed to obtain an ICC over .80.