Reliability of Traditional and Fractal Dimension Measures of Quiet Stance Center of Pressure in Young, Healthy People

Doyle TL, Newton RU, Burnett AF. Reliability of traditional and fractal dimension measures of quiet stance center of pressure in young, healthy people.ObjectivesTo assess reliability of traditional and fractal dimension measures of quiet stance center of pressure (COP).DesignCross-sectional study.SettingUniversity laboratory.ParticipantsThirty young healthy men (n=20) and women (n=10) (mean age, 23y).InterventionsNot applicable.Main Outcome MeasuresCOP 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.ResultsRange of sway (AP) was the most reliable traditional variable (intraclass correlation coefficient model 2,1 [ICC2,1] range −.28 to .72.). Peak sway velocity (AP) had poorest reliability (ICC2,1 range, .05–.29). Only 1 of the traditional variables had excellent reliability; total excursion area (firm, eyes closed) (ICC2,1=.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%.ConclusionsFractal 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.