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J Athl Train. 2006; 41(3): 245–250.
PMCID: PMC1569562

Dynamic Postural Stability in Subjects With Braced, Functionally Unstable Ankles

Abstract

Context: Research concerning prophylactic ankle stabilizers (PASs) has focused on healthy subjects, and the results cannot be generalized to the functional ankle instability (FAI) population, a population that has an increased risk of reinjury and is likely to wear PASs.

Objective: To determine whether PASs improve dynamic postural stability in FAI subjects as compared with a control (no-brace) condition.

Design: A crossover design was used to determine the effects of PASs on dynamic postural stability and vertical ground reaction forces.

Setting: Biomechanics laboratory.

Patients or Other Participants: Twenty-eight subjects with unilateral FAI, 13 men (age = 21.5 ± 1.2 years, height = 181.5 ± 10.5 cm, mass = 77.6 ± 17.2 kg) and 15 women (age = 20.5 ± 1.1 years, height = 169.4 ± 8.2 cm, mass = 67.9 ± 8.8 kg).

Intervention(s): A jump protocol required subjects to perform a 2-legged jump to a height equivalent to 50% of their maximum vertical leap and land on a single leg.

Main Outcome Measure(s): The dynamic postural stability index, the directional components (medial-lateral, anterior-posterior, and vertical), and vertical ground reaction force after a jump landing.

Results: Compared with the control condition, only the vertical component score was reduced (improved) with the application of a soft or semirigid PAS (P < .01).

Conclusions: Soft and semirigid PASs did not improve dynamic postural stability as measured by the Dynamic Postural Stability Index. However, PASs may help with the attenuation of vertical forces.

Keywords: prophylactic stabilizers, ankle instability, jump landing, ground reaction force

About 23 000 lateral ankle sprains occur each day in the United States. 1 However, the true incidence may be underestimated, as 56.8% of individuals do not seek medical treatment after suffering a lateral ankle sprain. 2 Previous research has also indicated that lateral ankle sprains are not isolated incidents; 40% to 75% of individuals who sprain their lateral ankle ligaments will develop chronic ankle instability. 3 This condition is thought to be caused by a continuum of pathologic conditions that can be described as mechanical or functional insufficiencies. 4 Tropp et al 5 defined mechanical instability or ligamentous laxity as a positive anterior drawer sign. Most recently, functional ankle instability (FAI) was defined by Hertel 4 as the occurrence of recurrent ankle instability and sensation of joint instability because of proprioceptive or neuromuscular deficits. Deficits in proprioception and neuromuscular control indicate a decreased afferent and efferent efficacy in dynamic joint stability. 6, 7 These deficits have major effects in athletics, as shown by the 73% of national, competitive, and recreational athletes who experience recurrent ankle sprains. 3

As a result of the high incidence and recurrence rates of ankle sprains, many clinicians recommend the use of prophylactic ankle stabilizers (PASs) to prevent injury and reinjury to the lateral ligament complex of the ankle. Two common PASs used in athletes today are the soft and semirigid styles, which improve proprioception, 8, 9 add mechanical support, and reduce the incidence of ankle injuries. 10, 11 Previous researchers have focused on proprioception 8, 9 and range of motion 12 during static conditions as well as on the ability of PASs to control motion, 13 attenuate forces, 14 and improve dynamic postural stability 15 during dynamic testing conditions. The limited studies performed to date have focused on healthy subjects, and the results cannot be generalized to the FAI population, a population that has an increased risk of reinjury and is likely to wear PASs. Thus, assessing the dynamic postural stability of FAI subjects during a jump landing could provide useful information regarding lower extremity function and the potential effects of PASs during functional tasks.

The Dynamic Postural Stability Index (DPSI) is a relatively new measure of dynamic postural stability that determines how well balance is maintained as the subject transitions from a dynamic to a static state. 16 The DPSI is a functional measurement of neuromuscular control because it is calculated during a single-leg hop-stabilization maneuver. It is also more informative than other measures (eg, center of pressure scores) because jump landings are commonly reported as a mechanism for lower extremity injury. 2 As many as 45% of all ankle injuries occurred during jump landings; 50% of these injuries were the result of landing with half of the foot on the playing surface and the other half on another player's foot. 2

During the landing phase of a jump landing, the lower extremity musculature is responsible for decelerating and stabilizing the body's center of mass by producing extensor moments to resist the collapse of the lower extremity and attenuate the vertical ground reaction force (vGRF). 17 Subjects with FAI take longer to stabilize their ankles and their center of mass oscillations after perturbations and jump landings, respectively. 18, 19 These impairments could be caused by ligamentous laxity or proprioceptive deficits, 18, 19 2 components of ankle instability that PASs have been shown to improve in healthy subjects. 8, 9 Therefore, PASs would be expected to improve the DPSI scores of FAI subjects. Thus, the purpose of our investigation was to examine the effects of PASs on vGRF and the DPSI and its directional components in a sample of young adults with FAI.

METHODS

Subjects

Twenty-eight subjects meeting the FAI criteria, 20, 21 including 13 men (age = 21.5 ± 1.2 years, height = 181.5 ± 10.5 cm, mass = 77.6 ± 17.2 kg) and 15 women (age = 20.5 ± 1.1 years, height = 169.4 ± 8.2 cm, mass = 67.9 ± 8.8 kg), volunteered to participate in this study. Subjects were eligible if they reported no history of disorders that could affect equilibrium or neuromuscular control and no lower extremity or head injury during the past 3 months. The criteria for FAI require that subjects have perceived sensations of weakness and episodes of giving way during daily activity. Sensations of weakness were operationally defined as instances when the ankle in question did not feel strong enough to successfully complete a high-level functional task (eg, cutting, jumping). Subjects were not excluded based on the number of previous ankle sprains but were determined to be free from mechanical instability, or “excessive ligamentous laxity,” through bilateral anterior drawer and talar tilt tests. Excessive ligamentous laxity was defined as greater than normal laxity palpated during the evaluation. A certified athletic trainer (M.A.A.) with 3 years of experience performed these tests. Additional exclusionary criteria included a history of fractures to the ankle, musculoskeletal injury within the past 3 months, and formal rehabilitation of the affected ankle. Once deemed eligible and before participating, subjects read and signed the informed consent approved by the university's institutional review board, which also approved the study.

Instrumentation

The McDavid Ultra Ankle 188 brace (McDavid Sports Medical Products, Woodridge, IL) and the Mueller Lace-Up ankle brace (Mueller Sports Medicine Inc, Prairie du Sac, WI) were tested in this study (Figure 1). According to the manufacturer, the McDavid Ultra Ankle 188 brace is a semirigid brace that supports both the lateral and medial sides of the ankle and is secured with a hook-and-loop strap. Similarly, the manufacturer describes the Mueller Lace-Up brace as a soft brace made of canvas material that provides support for ankle inversion, eversion, and rotation motions in the transverse plane. A certified athletic trainer fit all PASs according to the manufacturer's specifications. The DPSI and vGRF were measured at a frequency of 180 Hz using a Bertec triaxial forceplate (model 4060-10; Bertec Corp, Columbus, OH). 19 The forceplate uses a series of force transducers to record the ground reaction force and its point of application over time.

Figure 1
The McDavid Ultra ankle brace (left) and the Mueller Lace-Up ankle brace (right)

Procedures

All subjects arrived at the research laboratory for a single test session. After we confirmed each subject's eligibility, his or her maximum vertical leap was measured using a Vertec vertical jump tester (Sports Imports, Columbus, OH). To begin the measurement, each subject was instructed to stand next to the Vertec vertical jump tester, reach up, and touch the highest vane (marked in 1.27-cm increments) possible while maintaining a double-leg stance on the toes. We recorded the height reached as the subject's standing reach height. We then instructed the subject to perform a maximum jump using a countermovement jump technique and touch the highest vane possible. Maximum vertical jump height was determined as the difference between the maximum height reached during the countermovement jump and the standing reach height. 16, 22, 23 Next, the subject performed a 5-second single-limb static stance to determine body mass. The subject was then instructed in the jump protocol and was allowed as many practice trials as needed to confidently complete the jump-landing protocol (mean number of trials = 6.2 ± 1.7). To begin the jump-landing protocol, each subject stood 70 cm from the center of the forceplate and was instructed to jump with both legs to a height at which he or she could touch the 50% height marker with a single arm and land in the center of the forceplate (Figure 2A and B). The subject was instructed to land in the center of the forceplate on only the involved leg, stabilize as quickly as possible, and balance while standing erect and looking straight ahead with the hands on the hips for 3 seconds. 16 For all trials, subjects wore athletic shoes and kept their eyes open. In addition, no limitations were placed on arm or hand positioning during the flight phase of the jumping task. A trial was discarded and repeated if the subject lost balance (mean number of trials discarded = 4.1 ± 1.2 per subject). Loss of balance was operationally defined as the contralateral limb touching the floor or the subject hopping after landing. Three successful jump trials for each testing condition (control, soft, and semirigid) were then conducted after a 2-minute rest period. The order of presentation of the test conditions was randomized for the first subject and then counterbalanced using a Latin square design.

Figure 2
Jump protocol. A, Starting position. B, Ending position

Data Reduction

The DPSI scores were assessed for each subject after a single-leg landing from a jump height of 50% of his or her maximum vertical jump. 16, 19, 22–24 After sampling, the forceplate data underwent an analog-to-digital conversion and were stored on a personal computer using DATAPAC software (version 2000; Run Technologies, Laguna Hills, CA). All data were reduced using a QuickBasic subroutine (version 4.5; Microsoft Corp, Redmond, WA), which calculated the DPSI and its directional components:

equation image

We modified these formulas from those previously reported by Wikstrom et al 16 by altering the order of operations within the formulas. Specifically, the original formula used the square root of the number of samples as the denominator. The modified formulas use the number of samples as the denominator. This modification was made to allow us to calculate the average magnitude of the ground reaction force vector around zero points in the frontal (medial-lateral), sagittal (anterior-posterior) and vertical planes of the force plate. 16 Despite the modification, the DPSI is still a composite of the 3 directional indices. The vertical plane assesses the average magnitude difference from the subject's body weight to standardize the vGRF and normalize the vertical scores among individuals with different body weights. Because the DPSI is a composite score, it is sensitive to changes in all 3 directions. The average values from the 3 successful trials for each condition were then used for the analysis.

We also used the raw data signals to calculate the vGRF, which were expressed as the magnitude of the peak force (N) divided by the subject's body weight. The subject's body weight was determined from the 5-second static single-leg stance. The average of 3 successful trials for each condition was used for further analysis. 16, 22, 23

Statistical Analyses

Separate repeated-measures, within-factors analyses of variance were used for each of the dependent variables (MLSI, APSI, VSI, DPSI, and vGRF) across the brace conditions (control, soft, semirigid). A traditional alpha level of .05 was the criterion for rejection of the hypothesis of no difference for all statistical tests. In addition, the Tukey post hoc test was conducted when appropriate. We used SPSS (version 14.0; SPSS Inc, Chicago, IL) for the statistical analysis.

RESULTS

The VSI values differed across PAS conditions (F 2,54 = 39.24, P < .01), and the Tukey post hoc test revealed that the soft (.0278 ± .006) and semirigid (.0280 ± .006) PAS conditions significantly improved (produced smaller) VSI scores in comparison with the no-brace condition (.0311 ± .008) (Table). However, no differences were seen between the soft and semirigid braced conditions for the VSI. Unlike the VSI, no significant differences were detected for the DPSI (F 2,54 = 0.69, P = .50), APSI (F 2,54 = 0.39, P = .67), MLSI (F 2,54 = 1.09, P = .34), or vGRF (F 2,54 = 1.97, P = .15) among the bracing conditions.

Table thumbnail
Values for the Dynamic Postural Stability Index, Directional Components, and Vertical Ground Reaction Force by Brace Condition

DISCUSSION

We examined the effects of PASs while subjects landed from a jump, a common mechanism of injury. Few authors have examined PASs and peak vGRF, and none have examined PASs and the DPSI. A sagittal jump-landing task was chosen for this investigation for 2 reasons. First, we found no previous study indicating that a particular direction of jump landing was the primary cause of lateral ankle ligament injuries. Furthermore, the functional anatomy of both the talocrural and subtalar joints allows for triplanar motion 4 and, therefore, stresses the lateral ligament complex (especially the anterior talofibular ligament), regardless of the jump direction used. Second, this jump protocol had been used by previous investigators who have examined FAI and dynamic postural stability. 16, 19, 22–24

The current belief is that healthy and FAI subjects react differently to test conditions because of different anatomical or functional preconditions. 6 This phenomenon has been observed in several investigations of ankle stability. For example, Hiller et al 18 found that subjects with FAI took longer to stabilize ankle oscillations after a perturbation and failed more trials than healthy controls. In addition, several authors 19, 24 found that FAI subjects have longer (worse) time to stabilization scores than healthy controls. These neuromuscular control deficits are thought to be caused by proprioceptive deficits, which adversely affect reflex stabilization. 19 Clinically, PASs are believed to help athletes with FAI compensate for poor neuromuscular control through afferent feedback mechanisms and increased mechanical stability.

Previous researchers have shown that PASs generally improve measurements of the sensorimotor system, including active plantar-flexion position sense 9 and ankle joint kinesthesia. 25 Additionally, Feuerbach et al 8 reported that semirigid PASs result in decreased postural sway during stance in healthy subjects. Based on a follow-up investigation, they concluded that their results were because of an increase in afferent information from wearing PASs. 26 Similarly, PASs have been shown to add mechanical stability to the ankle joint. 27 However, the underlying mechanism by which braces provide this stability is still unclear. Cordova et al 28 examined PASs during a controlled lateral shuffling movement designed to produce a dynamic inversion loading of the ankle and mimic a common mechanism of injury. The results indicated that PASs might not act as a force bypass when the ankle joint is being dynamically loaded in the frontal plane. Other investigators have shown that the role of PASs is more likely to restrict motion during the free-fall phase of inversion (before the lateral ligaments are loaded with body weight). 13, 15 This suggestion is based on the findings that 10 different PASs all allowed 6° to 10° of inversion once loading began and that the free-fall angle accounted for 99% of the variance in the data. 13

Based on this evidence, it would be expected that the PASs used in this investigation during a jump-landing protocol should have improved proprioception (joint position sense and kinesthesia) via afferent mechanisms 26 and dynamic postural stability via enhanced mechanical stability. Our results, however, do not support this hypothesis, as dynamic postural stability was not improved during a jump protocol (single-leg hop stabilization test) under either the soft or semirigid PAS conditions over the control (no-brace) condition. However, the VSI was significantly reduced in the braced conditions compared with the control condition, indicating that PASs may help to control the magnitude of the vertical force vector. It is important to note, though, that this theory is completely speculative, as no significant change in vGRF was seen. Presumably individuals instinctively protect a joint after injury, and over time, this protective technique alters their landing patterns to avoid future injuries. Caulfield and Garrett 29 indicated that subjects with FAI have significantly more dorsiflexion before and after a single-leg landing. They theorized that the ankle ligaments were maximally protected against injury while in dorsiflexion. These pathologic restrictions prevent the ankle joint from being in an optimal position to attenuate the forces generated while landing. Perhaps the PASs worn by our subjects were able to partially correct for these restrictions.

For example, one possibility is that PASs increased proprioceptive awareness about the ankle, a finding shown in previous investigations. 9, 25, 26 Additionally, the PASs could have increased the motor neuron excitability of the peroneal muscles. 30 These increases could improve the efficacy of the ankle musculature in decelerating the body's center of mass and attenuating the vGRF. 17

Another explanation for the reduced magnitude of the vertical force vector during the jump-landing task is the restriction PASs place on dorsiflexion. Cordova et al 27 indicated that dorsiflexion and plantar flexion were limited by PASs but to a lesser degree than inversion and eversion. Additionally, McCaw and Cerullo 31 noted that PASs limited dorsiflexion and dorsiflexion angular velocity during drop landings. These restrictions may have prevented subjects from maximally dorsiflexing their ankles and thus placing the foot and ankle in a better position to attenuate ground reaction forces. A combination of better ankle positioning and heightened muscular efficacy about the ankle because of PASs during the jump-landing task may have been the reasons for the significant difference between the braced and control conditions.

Future authors should continue to study the effects of PASs during various functional tasks in both healthy and FAI subjects in order to address some of the limitations of our investigation. We reported only kinetic data, which provide but a small piece of the proverbial puzzle. Kinematic and electromyographic analyses are needed to confirm or refute the possible explanations we have presented, such as ankle positioning and heightened muscular efficiency. In addition, our analyses indicate low observed power, small effect sizes, and lack of significant differences. These results suggest that our sample size was small and that a larger sample size is needed.

CLINICAL RELEVANCE

Impaired static postural sway has been associated with an increased risk of lateral ankle sprains. 32 Although we measured dynamic postural stability, the DPSI is calculated while performing a more functional and sport-specific task. In a cost-benefit analysis review, Olmsted et al 33 indicated that PASs can reduce the number of lateral ankle sprains in individuals with and without a history of lateral ankle sprains. In addition, the authors found that to prevent 1 lateral ankle sprain in athletes with a history of sprains, a clinician would need to tape 26 ankles and brace 18 ankles. 33 Similarly, PASs are more cost-effective (3.05 times less expensive) over the course of a single competitive season than prophylactic ankle taping. 33 Therefore, clinicians should use and encourage the use of PASs because of the increased proprioceptive benefits, mechanical stability, and cost-effectiveness and decreased risk of injury and reinjury.

CONCLUSIONS

The PASs used in this study did not improve the dynamic postural stability of FAI subjects as tested during a sagittal jump-landing task. However, PASs may be able to improve the attenuation of vertical forces during a jump-landing procedure.

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