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Balance Measures for Discriminating between Functionally Unstable and Stable Ankles


Medicine & Science in Sports & Exercise: February 2009 - Volume 41 - Issue 2 - p 399-407
doi: 10.1249/MSS.0b013e3181872d89
Applied Sciences

Purpose: To identify force plate measures that discriminate between ankles with functional instability and stable ankles and to determine the most accurate force plate measure for enabling this distinction.

Methods: Twenty-two subjects (177 ± 10 cm, 77 ± 16 kg, 21 ± 2 yr) without a history of ankle injury and 22 subjects (177 ± 10 cm, 77 ± 16 kg, 20 ± 2 yr) with functional ankle instability (FAI) performed a single-leg static balance test and a single-leg jump-landing dynamic balance test. Static force plate measures analyzed in both anterior/posterior (A/P) and medial/lateral (M/L) directions included the following: ground reaction force (GRF) SD; center-of-pressure (COP) SD; mean, maximum, and total COP excursion; and mean and maximum COP velocity. COP area was also analyzed for static balance. A/P and M/L time to stabilization quantified dynamic balance. Greater values of force plate measures indicated impaired balance. A stepwise discriminant function analysis examined group differences, group classification, and accuracy of force plate measures for discriminating between ankle groups.

Results: The FAI group had greater values than the stable ankle group for A/P GRF SD (P = 0.027), M/L GRF SD (P = 0.006), M/L COP SD (P = 0.046), A/P mean COP velocity (P = 0.015), M/L mean COP velocity (P = 0.016), A/P maximum COP velocity (P = 0.037), M/L mean COP excursion (P = 0.014), M/L total COP excursion (P = 0.016), A/P time to stabilization (P = 0.011), and M/L time to stabilization (P = 0.040). M/L GRF SD and A/P time to stabilization had the greatest accuracy scores of 0.73 and 0.72, respectively.

Conclusion: Although 10 measures identified group differences, M/L GRF SD and A/P time to stabilization were the most accurate in discriminating between ankle groups. These results provide evidence for choosing these GRF measures for evaluating static and dynamic balance deficits associated with FAI.

1Virginia Commonwealth University, Richmond, VA; and 2University of North Carolina, Chapel Hill, NC

Address for correspondence: Scott E. Ross, Ph.D., ATC, Department of Health and Human Performance, Virginia Commonwealth University, PO Box 842020, 1015 W. Main St., Richmond, VA 23284-2020; E-mail:

Submitted for publication January 2008.

Accepted for publication July 2008.

©2009The American College of Sports Medicine