Reliability of traditional and fractal dimension measures of quiet stance center of pressure in young, healthy people

Objectives: To assess reliability of traditional and fractal dimension measures of quiet stance center of pressure (COP). Design: Cross-sectional study. Setting: University laboratory. Participants: Thirty young healthy men (n=20) and women (n=10) (mean age, 23y). Interventions: Not applicable. Main Outcome Measures: COP was recorded for 3 trials across 4 conditions: eyes open and eyes closed standing on firm and foam surfaces. Traditional COP variables - peak sway velocity and range of sway, both in the anteroposterior (AP) and mediolateral (ML) directions, and total excursion area, and fractal dimension of the COP in the AP and ML directions - were calculated. Reliability statistics were calculated. Results: Range of sway (AP) was the most reliable traditional variable (intraclass correlation coefficient model 2,1 [ICC_2,1] range -.28 to .72.). Peak sway velocity (AP) had poorest reliability (ICC _2,1 range, .05-.29). Only 1 of the traditional variables had excellent reliability; total excursion area (firm, eyes closed) (ICC _2,1=.95). All bar 1 fractal dimension measures had excellent ICCs. Relative technical error of measurement ranged from 4% to 7% for the fractal dimension measures. Coefficients of variation were also very good, ranging from 1.8% to 6.7%. Conclusions: Fractal dimension measures were more reliable than traditional measures of COP. Although traditional measures are used extensively to assess COP, their reliability is questionable. Fractal dimension measures show promise to reliably quantify COP and warrant further investigation.