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.

Reliability of SEMG measurements for trunk muscles during lifting variable loads in healthy subjects.

OBJECTIVE: The reliability of surface electromyography (SEMG) derived parameters is of high importance, but there is distinct lack of studies concerning the reliability during dynamic contractions. Particularly, the similarity index reliability has not been investigated during functional task yet. The purpose of this study was to evaluate the reliability of trunk muscle activities measured by means of surface electromyography during lifting tasks over repeated trials within a day. METHODS: Seven volunteers were assessed twice a day. SEMG signals were recorded bilaterally from seven trunk muscles. Intra class correlation coefficients (ICC) and standard error of similarity index, magnitude and timing were calculated. RESULTS: Investigation of ICC and correlation between trials showed that similarity index (ICC 0.73-0.97, correlation 0.6-0.95) is more reliable and repeatable than muscles magnitude (ICC 0.41-0.69, correlation 0.54-0.65) and it's timing (ICC 0.41-0.69, correlation 0.31-0.74) while timing had the least reliability. CONCLUSION: Similarity index can be mentioned as an appropriate motor control index and evaluation of muscle recruitment pattern should be done by considering similarity index and magnitude.