Intra-session reliability of traditional and nonlinear time-series posturographic measures in a semi-tandem stance: A reference to age

The purpose of this study was to evaluate the intra-session reliability of traditional and nonlinear time-series center of pressure (CoP) based measures of postural sway during upright semi-tandem quiet stance in young vs. older subjects. Seventy-four healthy volunteers (40 young individuals, mean age: 27 ± 6 yrs; 34 older individuals, mean age: 79 ± 8 yrs) performed three 30-s trials of semi-tandem quiet stance on a force plate with their eyes open. Traditional posturography analysis and 4 nonlinear time-series analyses (sway density plots, diffusion plots, recurrence quantification, sample entropy analyses) were applied to the recorded CoP time series, based on which 13 traditional and 32 nonlinear time-series CoP-based measures were calculated. Significant trial-to-trial improvements (particularly between the first and second trial) were observed in the calculated CoP measures in only 2 and 4 cases in the young and older individuals respectively. Regarding the traditional CoP measures, the highest intraclass correlation coefficients (ICCaverage, i.e. ICC2,k) (0.75-0.96) and the lowest coefficients of variation (CV) (8.2-27.3%) in both age groups were observed in the mean frequency of CoP local extrema, the mean amplitude of CoP excursion, and the time-normalized CoP area, although for some of the measures we recommend an additional familiarization trial due to systematic bias. Regarding the structural mechanics CoP measures, the highest ICCa (0.70-0.94) and the lowest CVs (1.2-8.4%) in both age groups were observed in those derived from recurrence quantification and sample entropy analyses, respectively. These CoP measures could be recommended for further use in basic and clinical studies, particularly those on age-related changes in postural control during the semi-tandem quiet stance.