Test-retest reliability of linear and nonlinear measures of postural stability during visual deprivation in healthy subjects.

[Purpose] The purpose of this study is to evaluate the reliability of linear and nonlinear measures of the center of pressure (COP) during visual deprivation. [Subjects and Methods] Fifteen young adults participated in this study. COP signals were recorded in two conditions: eyes open and eyes closed. Three trials were performed in each condition with a rest period of approximately 1 min. The intraclass correlation coefficients (ICCs) and standard error of measurement (SEM) were calculated. [Results] The investigation of ICC and SEM between trials showed that the Lyapunov exponent (ICC: 0.76-0.96, SEM: 0.03) and total mean velocity (ICC: 0.71-0.95, SEM: 0.05) were more reliable and repeatable than range and area (95% confidence ellipse), while area had the least reliability (ICC: 0.49-0.77, SEM: 0.56). [Conclusion] The Lyapunov exponent can be considered an appropriate postural control index, and the evaluation of postural stability should be done by considering linear and nonlinear tools.