Medicine & Science in Sports & Exercise:
CLINICAL SCIENCES: Clinical Investigations
Dynamic Postural Stability Deficits in Subjects with Self-Reported Ankle Instability
WIKSTROM, ERIK A.1; TILLMAN, MARK D.1; CHMIELEWSKI, TERESE L.2; CAURAUGH, JAMES H.1; BORSA, PAUL A.1
1Center for Exercise Science; and 2Department of Physical Therapy, University of Florida, Gainesville, FL
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: email@example.com.
Submitted for publication June 2006.
Accepted for publication October 2006.
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 ± 0.09, STABLE = 0.30 ± 0.06). Similar results were seen for the vertical stability index (FAI = 0.73 ± 0.17, STABLE = 0.61 ± 0.13), the normalized dynamic postural stability index (FAI = 0.85 ± 0.17, STABLE = 0.73 ± 0.12), the normalized vertical stability index (FAI = 0.007 ± 0.004, STABLE = 0.005 ± 0.001), and the dynamic postural stability index (FAI = 0.008 ± 0.003, STABLE = 0.006 ± 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.
Lateral 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 proprioception, 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 protocols. 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 stability, has detected differences between stable and FAI patients (1,18-20,23). However, close examination of the original time-to-stabilization formula has revealed calculation flaws (19,26). Specifically, time to stabilization gives three separate measures of dynamic postural stability for the different force directions (medial/lateral, anterior/posterior, and vertical), rather than a common measure among the three force directions. In addition, earlier studies reported that pathological adaptations (increased postural sway) could bias the baseline values and permit unequal group comparisons (19,26).
Since the original time-to-stabilization technique was developed, the calculation flaws have been corrected (19), and a new measure of dynamic postural stability, the dynamic postural stability index (DPSI), has been developed (26) to eliminate the inherent flaws of the original time-to-stabilization measure while maintaining its inherent strengths. Similar to the time-to-stabilization measure, the DPSI indicates how well a subject can dissipate resultant ground reaction forces (GRF) from a jump landing. Moreover, the DPSI is a measure of motor control for the lower extremity and is dependent on proprioceptive feedback as well as reflexive, preprogrammed, and voluntary muscle responses (10). Previous investigations have reported that the DPSI is a more reliable (r = 0.95) and precise (SEM = 0.03) measure than the sequential estimation technique of calculating time to stabilization during a single-leg-hop stabilization maneuver (26). This technique (DPSI) has been used to detect gender differences (25), but it has not been used to investigate injured populations. Given the potential for altered dynamic postural stability with FAI, the purpose of this study was to determine differences in the DPSI in individuals with FAI compared with those with stable ankles. The primary hypothesis was that subjects with FAI would have a higher overall DPSI as well as higher scores in the anterior/posterior and vertical directional components.
This study is a retrospective analysis of data collected during 2 yr in our laboratory. The data were originally collected to study the time-to-stabilization measure (22,23,25). This retrospective design analyzed dynamic postural stability in stable and unstable ankles using the DPSI technique. All previous studies incorporated identical methodology and were conducted by the same investigator.
A total of 108 subjects were randomly selected from the data pool of 157 after being tested for a single-leg-hop stabilization maneuver (54 subjects with stable ankles (STABLE group); 54 subjects with FAI (FAI group)). Group demographics are shown in Table 1. After a written IRB-approved informed consent was read and signed, subjects completed a medical history and ankle-stability questionnaire to determine eligibility. All STABLE subjects were free from lower-extremity injury and head injury for the 3 months before testing and did not suffer from any equilibrium disorders. All FAI subjects were free from acute lower-extremity injury and head injury for 3 months before testing and did not suffer from any equilibrium disorders. In addition, all subjects in the FAI group met the criteria established by Hubbard and Kaminski (9), including sensations of weakness, episodes of giving way during daily activity without any history of fractures to the involved ankle, injury within the past 3 months, and no formal rehabilitation of the affected ankle.
The jump protocol was performed as first described by Ross and Guskiewicz (18). Subjects stood 70 cm from the center of the force plate and jumped with both legs to touch an overhead marker placed at a position equivalent to 50% of each subject's maximum vertical leap before landing on one leg on the force plate. Each subject was instructed to jump with one's head up and hands in a position to touch the designated marker. Subjects were instructed to land on the test leg, stabilize as quickly as possible, and balance for 10 s with hands on one's hips while looking straight ahead. If a subject lost balance and touched the floor with the contralateral limb, the trial was discarded and repeated. Likewise, if a short additional hop occurred on landing, the trial was discarded and repeated. A Bertec triaxial force plate (Bertec Corporation, Columbus, OH) was used to collect the baseline and jump-landing data at a rate of 180 or 200 Hz (22,23,25,26). Sample rate varied because the time-to-stabilization calculation technique used (unbounded third-order polynomial, or sequential estimation) in the original investigations differed (22,23,25,26). In the current investigation, all data were analyzed using the DPSI technique described below. All force plate data underwent an analog-to-digital conversion and were stored on a PC-type computer using the DATAPAC 2000 (Run Technologies, Laguna Hills, CA) analog data-acquisition, processing, and analysis system.
STABLE subjects reported to the research laboratory and were randomly assigned to perform the jump-landing task on either the dominant or nondominant limb for a unilateral assessment, or they were instructed to perform the jump-landing task on each limb in random order for bilateral assessment based on the procedure for the individual study each subject was involved in. In the case of bilateral assessment, each subject was entered into our database only once, and the limb selected was chosen at random by a coin flip. All FAI subjects were tested on their involved limb, and all possessed unilateral FAI as dictated by the ankle-stability questionnaire (9). The dominant limb for each subject was defined as the limb that the subject would use to kick a soccer ball. Subjects had their maximum vertical jumps tested as described by Wikstrom et al. (24). Single-leg baseline values of static stance and body weight were recorded at 180 or 200 Hz on a force plate during 5-s windows (22,23,25,26), depending on the time-to-stabilization calculation technique used (see the Jump Protocol section). Data from three successful jump protocol trials were averaged and further analyzed.
GRF data were reduced as initially described by Wikstrom et al. (26). For the current investigation, the data were exported into a QuickBasic subroutine (version 4.5, Microsoft Corporation, Redmond, WA). The subroutine calculated stability indices in the three principal directions (medial/lateral, anterior/posterior, and vertical) and the DPSI. These indices are created from the standard deviation fluctuations around a zero point, rather than a group mean, and they are divided by the number of samples within the collection time period. The medial/lateral stability index and anterior/posterior stability index assess the fluctuations from zero along the frontal (x) and sagittal (y) axes of the force plate, respectively. The vertical stability index assesses the fluctuation from the subject's body weight equivalent to standardize the vertical GRF along the z-axis of the force plate. This is done to normalize the vertical scores among individuals with different body weights (mass). The DPSI is a composite of the medial/lateral, anterior/posterior, and vertical stability index and is sensitive to changes in all three directions (26). The formulas used to calculate these indices are displayed in Table 2.
A significant difference was noted between groups: the FAI group had greater height and mass than the healthy group, and therefore, the normalized stability indices were reported according to the method described by Wikstrom et al. (25). The indices were normalized by the energy dissipated during the landings performed by each individual. Moreover, three indices were normalized to relevant energy values (horizontal kinetic energy or vertical potential energy). The anterior/posterior stability index is a measure of variance of the GRF values in the anterior/posterior direction and is sensitive to the horizontal velocity and mass of the jumper. Thus, the anterior/posterior stability index was normalized to the horizontal kinetic energy (0.5 × mass × squared horizontal velocity), with horizontal velocity calculated as 0.7 m divided by time from jump to landing in seconds. Alternatively, the vertical stability index was normalized to the potential energy (mass × gravity × jump height) dissipated during the landing, because the vertical stability index is a measure of variance in the vertical GRF.
To ensure that the dataset represented equal variances throughout the 2-yr collection period, Levene's test for the equality of variances was conducted on all dependent variables among STABLE and FAI groups separately. The results indicated unequal variances in several variables, and therefore unequal variance t-tests were conducted in these situations. Separate independent-sample t-tests were deemed appropriate where equal variances were seen and were conducted to analyze the data between the injured limb of the FAI subjects and randomly selected limbs of the STABLE group. Effect sizes were calculated using mmax − mmin/σ, with the following classifications: small < 0.10, medium = 0.11-0.39, large > 0.40 (3).
Conducting seven separate independent t-tests may produce spurious significant effects with an alpha set at the traditional level of 0.05. Therefore, a more conservative Bonferroni adjustment procedure was used to control the experiment-wise error rate (16). The adjustment procedure resulted in a new level of significance (α = 0.007).
The analyses indicate that subjects with FAI demonstrated higher dynamic postural stability scores. Specifically, subjects with FAI showed significantly worse (higher) anterior/posterior and vertical stability index and DPSI scores. In addition, when the data were normalized, the FAI group displayed worse VSI and DPSI scores. However, no significant differences were found for the medial/lateral stability index or for the normalized anterior/posterior stability index, as shown in Table 3.
These results confirm that the group with FAI had worse dynamic postural stability than the STABLE subjects. Specifically, the findings indicate that individuals with FAI produced significantly higher dynamic postural stability scores in the anterior/posterior and vertical plane and also overall DPSI while completing a jump-landing protocol. The current results support the hypothesis that individuals with FAI would have increased (worse) dynamic postural stability scores in the anterior/posterior and vertical directions and worse DPSI scores.
In addition, these findings are consistent with recent results by several investigators (1,18-20,23). Ross et al. (18-20) used the same jump-landing protocol and were able to identify dynamic postural stability deficits in the anterior/posterior and medial/lateral direction using the time-to-stabilization measure. Similarly, anterior/posterior time-to-stabilization deficits between FAI and healthy individuals were detected in other investigations, although no medial/lateral deficits were found (1,23).
Researchers have postulated that individuals with FAI perform differently than those with stable ankles during a single-leg-hop test, because the jump-landing protocol detects the neuromuscular deficits present in FAI subjects (1,2,18). The results of the current investigation support this finding. Perhaps greater preparatory muscle activity contributes to proper movement patterns. McKinley and Pedotti (15) have reported that greater preparatory muscle activity as measured by electromyography would provide a better preprogrammed dynamic defense mechanism, thus minimizing dynamic postural stability scores. Specifically, subjects with greater and earlier cocontraction of lower-leg muscles before landing displayed lower time-to-stabilization scores (15). This muscle cocontraction created greater muscle stiffness in the ankle joint and resulted in faster reactions to the landing surface. In addition, the responsibility for dynamic control of the ankle during functional activities is a consequence of an interaction between central programming and peripheral feedback (13). However, people with FAI have been shown to have deficits in proprioception and deafferentation of the ankle region (12,28).
Ross et al. (18) speculate that the damage caused by lateral ankle sprains and subsequent ankle instability might be responsible for the increased (worse) dynamic postural stability scores. Ross et al. (18) theorize that people with FAI take longer to decelerate their center-of-mass oscillations because they allow their center of mass to approach the limits of stability, causing large external moments that act to destabilize the body (18). However, it is unknown whether the increased dynamic postural stability scores seen in the current investigation or in previous investigations are attributable to ankle-instability symptoms such as the self-reported weakness, or whether they are attributable to the fact that individuals with FAI use a different landing strategy to improve stabilization time, as suggested by Ross et al. (18).
An alternative viable explanation is that ankle instability causes motor control changes, forcing people with FAI to use a nonankle strategy and predisposing them to recurrent injury. Evidence is available to support this argument. For example, individuals with a history of ankle sprains but no symptoms of instability do not have deficits in proprioception (28). In addition, no bilateral differences in center-of-pressure measurements were found between the uninvolved (stable) ankle and the unstable ankle of soccer players. However, both limbs had significantly higher scores compared with those of a healthy reference group, indicating that FAI can possibly change postural control strategies (21). Further, Caulfield and Garrett (2) note that subjects with unilateral FAI land in a more dorsiflexed position, causing increased GRF but, possibly, "splinting" the lateral ankle ligaments compared with those of a healthy reference group. Similarly, Pintsaar et al. (17) have found that individuals suffering from unilateral FAI use a hip strategy to compensate for the lack of support in the ankle during a single-leg balance task compared with an external control group. Although these investigations support the alternative explanation, only prospective studies can confirm whether motor control changes do take place.
The results support our hypothesis that medial/lateral differences would not be seen during the current investigation; we hypothesized that no differences would be seen for two main reasons. First, the medial/lateral stability index was shown previously to have poor test-retest reliability (r = 0.27 and 0.38) (25,26). In addition, the medial/lateral stability index was shown to have a high standard error of the measurement as a percentage of the mean score (26.1%) relative to the anterior/posterior (5%), vertical (4.6%), and DPSI (3.7%) (26). Second, the functional anatomy of both the talocural and subtalar joints allows triplanar (7) motion regardless of jump direction, although the forces generated during the sagittal jump-landing protocol did not seem sufficient to elicit group differences. Previous investigations using identical jump-landing protocols have consistently found anterior/posterior group differences but not medial/lateral group differences (1,18-20,23).
There are limitations to this study. Subjects were randomly selected from our database rather than being matched for gender, height, and weight. Although one could argue that random selection would be advantageous, the selection process generated group differences in height and weight between the FAI and STABLE groups. These differences could have affected the GRF produced during the jump-landing protocol. Therefore, data were normalized to relevant energy values (horizontal kinetic energy or vertical potential energy) (25), and both the standard and normalized results were reported. Further, the data were not filtered to correct for the discrepancy in sampling rates (180 Hz, 200 Hz). Filtering was not conducted for two reasons: 1) previous work revealed that according to a fast Fourier analysis, the raw analog signals of the single-leg-hop stabilization maneuver used in this investigation were below 30 Hz (20), and sampling rates were at least six times greater than the analog signal that was under investigation; and 2) previous work in our laboratory has indicated that manipulating the sampling rate (200, 500, or 1000 Hz) did not affect DPSI scores in healthy individuals (27). Finally, the number of sprains or the self-reported function of our FAI sample were not quantitatively described. Because these data were not described, the ability to compare these results with those of investigations that reported such information is limited, because it is impossible to know how impaired the FAI subjects in the current investigation were.
The results of this investigation and those of previous studies performed in our lab indicate numerous potential applications for the DPSI in future research and clinical settings (25,26). The DPSI has been shown to have excellent reliability, precision, and clinical usefulness in detecting group differences between FAI and stable ankles. The clinical usefulness of the DPSI is similar to the modified time-to-stabilization technique (anterior/posterior = 0.63 and medial/lateral = 0.71) as measured by an effect size (19). These similarities indicate that researchers and clinicians now have another tool available to measure dynamic postural stability in conjunction with a functional task: a single-leg-jump stabilization maneuver. However, further evidence is needed to definitively establish the clinical validity of the DPSI. Specifically, future research should assess the responsiveness of the DPSI to changes in healthy status and whether the change in DPSI score correlates with changes in clinical outcome (i.e., self-reported function).
The current findings indicate that subjects with FAI have significant deficits in dynamic postural stability. Five of seven variables of dynamic postural stability were increased in subjects with FAI compared with subjects from the STABLE group. These variables included the anterior/posterior stability index, vertical stability index, DPSI, and normalized vertical stability index and DPSI. DPSI and normalized DPSI are considered the most important because they correct for the flaws in the original time-to-stabilization technique and because they are sensitive to changes in each of the three principle directions (26).
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