Copyright @ 2007 by the American College of Sports Medicine. Unauthorized reproduction of this article is prohibited. Clinical Investigations Dynamic Postural Stability Deficits in Subjects with Self-Reported Ankle Instability ERIK A. WIKSTROM 1 , MARK D. TILLMAN 1 , TERESE L. CHMIELEWSKI 2 , JAMES H. CAURAUGH 1 , and PAUL A. BORSA 1 1 Center for Exercise Science; and 2 Department of Physical Therapy, University of Florida, Gainesville, FL ABSTRACT WIKSTROM, E. A., M. D. TILLMAN, T. L. CHMIELEWSKI, J. H. CAURAUGH, and P. A. BORSA. Dynamic Postural Stability Deficits in Subjects with Self-Reported Ankle Instability. Med. Sci. Sports Exerc., Vol. 39, No. 3, pp. 397–402, 2007. Purpose: A limited understanding of how functional ankle instability (FAI) affects dynamic postural stability exists because of a lack of reliable and valid measures. Therefore, the purpose of this investigation was to determine whether a new reliable index for dynamic postural stability could differentiate between those with stable ankles and those with FAI. Methods: Data were collected on 108 subjects (54 subjects with stable ankles (STABLE group); 54 subjects with functionally unstable ankles (FAI group)). Subjects performed a single-leg-hop stabilization maneuver in which they stood 70 cm from the center of a force plate, jumped off both legs, touched a designated marker placed at a height equivalent to 50% of their maximum vertical leap, and landed on a single leg. The dynamic postural stability index and directional stability indices (medial/lateral, anterior/posterior, and vertical) were calculated. The raw and normalized (to energy dissipated) indices were compared between groups. Results: Significant differences were noted for the anterior/posterior stability index (FAI = 0.36 T 0.09, STABLE = 0.30 T 0.06). Similar results were seen for the vertical stability index (FAI = 0.73 T 0.17, STABLE = 0.61 T 0.13), the normalized dynamic postural stability index (FAI = 0.85 T 0.17, STABLE = 0.73 T 0.12), the normalized vertical stability index (FAI = 0.007 T 0.004, STABLE = 0.005 T 0.001), and the dynamic postural stability index (FAI = 0.008 T 0.003, STABLE = 0.006 T 0.001). Conclusions: These results indicate that the dynamic postural stability index is a sensitive measure of dynamic postural stability and is capable of detecting differences between individuals with stable ankles and individuals with functionally unstable ankles. Key Words: JUMP LANDINGS, DYNAMIC POSTURAL STABILITY INDEX, KINETICS, POSTURAL CONTROL L ateral ankle sprains are a common injury in sports, with an incredible incidence of about 25,000 daily in the United States (11) and a recurrence rate greater than 70% (5). Frequently, lateral ankle sprains cause ligamentous laxity, altered proprioception, altered muscular function, and complaints of the ankle giving way (7). A combination of these neuromuscular characteristics operationally defines mechanical and functional ankle instability (FAI) (4). Mechanical instability refers to structural changes, most notably ligamentous disruption (i.e., joint laxity). FAI is defined as impaired propriocep- tion, strength, and postural and neuromuscular control with or without ligamentous laxity (4). Approximately 40–75% of individuals develop ankle instability after a lateral ankle sprain (5). The high incidence of ankle instability after ankle sprains is alarming because instability is a known risk factor for secondary osteoarthritis (8,14). There is a critical need to quantify dynamic postural stability after an ankle sprain to allow clinicians and researchers to investigate the effects of FAI during dynamic tasks. A better understanding of residual deficits after an ankle sprain could potentially allow a reduction in the incidence of FAI through improved treatment proto- cols. Dynamic postural stability has been defined as maintaining balance while transitioning from a dynamic to a static state (6). Time to stabilization, the only previous method of quantifiably assessing dynamic postural stabil- ity, has detected differences between stable and FAI patients (1,18–20,23). However, close examination of the original time-to-stabilization formula has revealed calcu- lation flaws (19,26). Specifically, time to stabilization CLINICAL SCIENCES Address for correspondence: Erik A. Wikstrom, M.S., A.T.C., University of Florida, Department of Applied Physiology and Kinesiology, PO Box 118205 Gainesville, FL 32611-8205; E-mail: ewikstrom@hhp.ufl.edu. Submitted for publication June 2006. Accepted for publication October 2006. 0195-9131/07/3903-0397/0 MEDICINE & SCIENCE IN SPORTS & EXERCISE Ò Copyright Ó 2007 by the American College of Sports Medicine DOI: 10.1249/mss0b013e31802d3460 397