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Volume 42 Number 4, July/August 2005
Pages 547 — 556

Abstract - Postural stability index is a more valid measure of stability than equilibrium score

Hans Chaudhry, PhD;1-2 Thomas Findley, MD, PhD;1-2 Karen S. Quigley, PhD;1,3 Zhiming Ji, PhD;4
Miriam Maney, MS;1 Tiffany Sims, MS;1-2 Bruce Bukiet, PhD;5* Richard Foulds, PhD2

1War-Related Illness and Injury Study Center, Department of Veterans Affairs Medical Center, East Orange, NJ; 2Department of Biomedical Engineering, New Jersey Institute of Technology, Newark, NJ: 3Department of Psychiatry, University of Medicine and Dentistry of New Jersey, New Jersey Medical School, Newark, NJ; Departments of 4Mechanical Engineering and 5Mathematical Sciences and New Jersey Institute of Technology, Newark, NJ
Abstract — Researchers, therapists, and physicians often use equilibrium score (ES) from the Sensory Organization Test, a key test in the NeuroCom EquiTest System (a dynamic posturography system) to assess stability. ES reflects the overall coordination of the visual, proprioceptive, and vestibular systems for maintaining standing posture. In our earlier article, we proposed a new measure of anterior-posterior (A-P) postural stability called the Postural Stability Index (PSI), which accounts for more biomechanical aspects than ES. This article showed that PSI provides a clinically important adjunct to ES. In the present article, we show that PSI can provide an acceptable index even if a person falls during the trial, whereas ES assigns a zero score for any fall. We also show that PSI decreases as ankle stiffness increases, which is intuitive, while ES exhibits the opposite behavior. Ankle stiffness is generally recognized as an indicator of postural stability. These results suggest that PSI is a more valid measure of A-P stability than ES.
Key words: ankle stiffness, balance, center of pressure, dynamic posturography, equilibrium score, mathematical model, postural stability, sensory organization test, stabilizing torque, sway angle, sway-referenced motion, two-link model.

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