Process stationarity and reliability of trunk postural stability.
BACKGROUND
Empirical assessments of torso stability can be estimated from postural variability and nonlinear analyses of seated balance tasks. However, processing methods require sufficient signal duration and test-retest experiments require the assessment must be reliable. Our goal was to characterize the reliability and establish the trial duration for torso stability assessment.


METHODS
Kinetic and kinematic data were recorded while subjects maintained a seated posture on a wobbly seat pan. Stability was evaluated from dynamic variability and nonlinear stability analyses. Process stationarity of the measured signals characterized the minimum necessary trial duration. Intra-class correlations measured within-session and between-session reliability.


RESULTS
Trial duration necessary to achieve process stationarity was 30.2 s. Shorter time to stationarity was observed with measures that included multi-dimensional movement behavior. Summary statistics of movement variability demonstrated moderate intra-session reliability, intra-class correlation=0.64 (range 0.38-0.87). Inter-session reliability for movement variance was moderate, intra-class correlation=0.42 (range 0.22-0.64). Nonlinear stability measures typically performed better than estimates of variability with inter-session reliability as high as intra-class correlation=0.83. Process stationarity and reliability were improved in more difficult balance conditions.


CONCLUSIONS
To adequately capture torso dynamics during the stability assessment the trial duration should be at least 30 s. Moderate to excellent test-retest reliability can be achieved in intra-session analyses, but more repeated measurements are required for inter-session comparisons. Stability diffusion exponents, H(S), and the Lyapunov exponents provide excellent measures for intra-session analyses, while H(S) provides excellent inter-session comparisons of torso stability.
Lee
HyunWook
H
School of Biomedical Engineering and Sciences, Virginia Polytechnic Institute and State University, 219 Norris Hall (0219), Blacksburg, VA 24061, USA. hwlee@vt.edu
Granata
Kevin P
KP
D000005:Abdomen / Q000502:physiology*; D000328:Adult; D001415:Back / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D010808:Physical Examination / Q000379:methods*; D004856:Postural Balance / Q000502:physiology*; D011187:Posture / Q000502:physiology*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D013909:Thorax / Q000502:physiology*