Research suggests that individuals with chronic ankle instability use different movement strategies to maintain postural control than individuals with healthy ankles. These changes may be related to alterations in movement variability associated with ankle instability.
By Lisa Chinn, MS, ATC, and C. Collin Herb, MEd, ATC
Chronic ankle instability (CAI) is a phenomenon that has been reported to occur in 30% to 70% of individuals who suffer an initial lateral ankle sprain.1,2 CAI has been defined as the occurrence of repetitive bouts of lateral ankle instability and feelings of “giving way,” often resulting in numerous subsequent ankle sprains.3
Repeated ankle sprains represent a serious healthcare issue in the U.S. CAI has been connected to degenerative changes,4 reduced physical activity,5,6 activity limitations,7,8 withdrawing from professional activities,9 and, potentially, increased body mass index.10 Although the underlying etiology of CAI is not fully understood, it is believed that an initial ankle sprain can lead to sensorimotor function alterations. One aspect of the sensorimotor system that has received much research attention is postural control.
Postural control research
Postural control has been defined as the ability to maintain stability above a narrow base of support.11,12 Although the general definition of postural control is well understood, the methods used for measuring postural control are diverse. Researchers and clinicians have used single- and double-limb stance, static and dynamic tasks, different constraints on subjects, as well as instrumented and noninstrumented measures. Each method has advantages and disadvantages; however, with all the various methods and measurements of postural control, the published literature is not consistent regarding quantification of altered postural control.
Riemann13 published a 2002 review of literature on CAI and postural control. In his report, Riemann references Freeman and colleagues,11,14 the first researchers to propose a link between CAI and altered postural control. Since the early work of Freeman et al, however, researchers have presented mixed results from studies comparing individuals with CAI and healthy controls.12,15-19 Riemann points out that research has also produced conflicting results with regard to alterations in balance in injured versus noninjured limbs of individuals with CAI.16,20
While Riemann did not perform any statistical analysis, he concluded that the current literature could not establish a clear link between CAI and altered postural control. He believed the lack of universal inclusion and exclusion criteria for CAI and variations in research procedures are the cause of disparities in published results. Studies reported using a wide range of populations, including professional basketball players, amateur soccer players, gymnasts, college students, and others.
Researchers also looked noninstrumented outcomes, such as time to stabilization following a jump, or counting “errors” during single-limb stance of unstable surfaces, as well as instrumented outcomes such as center of pressure force plate measures. The many variations in methods meant no strong conclusions could be drawn.
In 2008, McKeon and Hertel21 performed a systematic review to evaluate whether postural control is affected adversely in CAI. Their paper included only studies in which participants balanced in a modified Romberg position and that presented center of pressure measurements from a force plate. They identified eight articles16,18-20,22-25 that fit their inclusion criteria. After calculating effect sizes, the authors stated that postural control appears to be adversely affected in those with CAI compared with healthy controls, but also noted the results were not conclusive. The effect sizes indicated that postural control was worse in individuals with CAI, however, the effect size point measures were generally weak and several of the confidence intervals were large, crossed zero, or both. Even when limiting the included studies to those with single-limb balance on a force plate, similar to Riemann,13 McKeon and Hertel cited inconsistent criteria for CAI, as well as the variation in participants, methods, and outcome measures, as potential reasons for conflicting results. The included studies also reported varying length in trial times, sampling rates, and participant demographics.
A relatively new force plate measure that can be assessed during static balance is the time-to-boundary (TTB) method of assessing spatiotemporal characteristics.26 Whereas center of pressure measures assess only the location of center of pressure in relation to the base of support, TTB incorporates both the location and the velocity of each center of pressure point. TTB estimates the time it would take for the center of pressure to reach the boundary of the base of support if the center of pressure was to continue on its trajectory at its instantaneous velocity.22 A lower TTB indicates the individual in question has less time to make a postural correction before losing control, or, more simply, a lower TTB means greater postural instability.
Hertel and Olmstead-Kramer22 reported that the five of six TTB measures showed significant differences between persons with CAI and healthy controls, whereas only one of eight traditional center of pressure measures found similar results. Other studies have also found that persons with CAI display lower TTB measures while maintaining single-limb balance.27-29 The differences in TTB measures indicate that, while preserving postural control, CAI patients function closer to their boundary of support or have faster velocities compared with healthy controls.
Researchers have attempted to evaluate force plate measures to determine if there is one specific measurement to distinguish between the ankles of controls and those with CAI.30,31 Ross et al30 recorded 15 different center of pressure force plate measures while participants performed single-limb, eyes-open static balance for 20-second trials. They report that standard deviation of the ground reaction force in the medial-lateral direction had “fair” accuracy for discriminating between unstable and stable ankles. This measure accurately classified 68.2% of the measured ankles.
Knapp et al31 also evaluated force plate measures, including TTB, to determine if a single force plate measure could discriminate between CAI and healthy ankles. In their study, participants performed single-limb balance for 10 seconds, however, they reported both eyes-open and eyes-closed results. Overall, of the 30 different force plate outcomes evaluated, three measures were predictors of CAI status: eyes-closed standard deviation of center of pressure in the medial-lateral direction; eyes-closed percentage of center of pressure range used in the medial-lateral direction; and eyes-closed absolute minimum of TTB in the medial-lateral direction. However, none of the significant measurements had strong likelihood ratios associated with them, indicating that a single force plate measure cannot completely predict CAI status.
Pope et al32 further evaluated postural control by assessing the location, on the plantar aspect of the foot, of the center of pressure and the most unstable (TTB minima) data points. Overall, they found the CAI group positioned their center of pressure more anteriorly and laterally on their base of support compared with healthy controls. Other researchers have reported lateral plantar pressure distributions while walking in CAI,33,34 but this was the first study to evaluate the location of the center of pressure.
Many authors13,21,32,35-38 point out that published studies look only at successful trials of postural control. In order to complete the required task, CAI subjects may utilize different postural control strategies compared with healthy individuals. In healthy volunteers, measurements of postural control in single-leg stance reveal that the head, trunk, and leg function as one unit, with rotations and corrections occurring at the ankle. Horak and Nashner describe this mechanism as an “ankle strategy” where muscular contractions in the leg will reposition the body over the supportive foot to maintain balance.39 In contrast, Riemann13 believes that CAI patients are specifically using a “hip strategy” to facilitate “normal” postural control, leading investigators to conflicting results. McKeon and Hertel21 and Pope et al32 include the potential use of a knee strategy as well as a hip strategy to maintain postural control. Although it is difficult to accurately document and quantify the use of a hip or knee strategy, studies have shown that those with CAI have altered proximal kinematics35,40-43 and neuromuscular stimulation43-47 during a variety of tasks, indicating that CAI has altered movement patterns. Pope at al32 refer to various variability theories as an explanation for these alterations.
The belief that movement variability is essential for the stability and function of the sensorimotor system is a concept that has developed only recently.48 Movement variability has been viewed as both detrimental and beneficial to skilled coordinated movement. The generalized motor program theory identifies variability as error in movement, planning, and execution. This theory states that, with practice, variability will be reduced or potentially eliminated, whereas the uncontrolled manifold hypothesis states that variability is associated with motor redundancy. In addition, generalized motor program theory maintains that a controller allows some elements to have high variability as long as they remain in a selected subspace. In contrast, dynamical systems theory states that inherent movement variability is essential for enabling individuals to maintain postural control under changing conditions.48,49 This theory has been used in neuromuscular dysfunction and Parkinson disease research along with sports medicine injury research.49,50
Variability in systems depends on constraints of the systems, the task being accomplished, degrees of freedom of the system, and the difficulty of the task. Stergiou et al51 augmented this theory by stating that desired variability should be nonlinear. They believe there is an optimal zone of variability, and that a decrease in this optimal zone creates a rigid system while an increase results in a noisy and unstable system.
The human body has many joints and degrees of freedom between these joints, giving the sensorimotor system numerous pathways to accomplish tasks.48-50 This affords the body many ways to accomplish goals and, if one part of the system falters, the body can go to another option to accomplish its goal.52 Spontaneous pattern formation between component parts has been found to emerge through self-organization.50 This process of self-organization depends on constraints that are placed on the body by the task, the environment, and the organism itself.50 Davids et al50 defines constraints as boundaries or features that interact to limit the form of biological systems searching for optimal states of organization. When tasks are required by the body, the ability to accomplish a goal in multiple ways places less stress on specific tissues and allows flexibility to perturbations or constraints. This system flexibility allows the body to disperse stresses among different tissues, providing a longer adaptation time between loading events.
Organized movement patterns are known as coordinative structures and are believed to be the body’s way of organizing and controlling complex movements.50 These coordinative structures allow the human body to exploit the interconnectedness of the system. However, in some parts of the system variability can reflect individuals’ compensatory action. In sports variability can be observed in athletes who are skillful at highly dimensional tasks and exploiting available degrees of freedom.50 Less skilled athletes fix their degrees of freedom and demonstrate more variability that is not functional or necessary.
The variability, or “noise,” produced during postural control studies in the past was not explained, but this variability has now been identified as unavoidable and functional.50 Cognitive scientists have been unable to reveal, even in elite athletes, reproducible invariant movement patterns. Therefore, the goal of treatment in sports medicine should not be to achieve an ideal movement pattern but to encourage patients to overcome the constraints that affect them in order to improve functional capacity and, subsequently, performance.50 Evidence has indicated that variability in joint coordination is an essential component of functional movement, providing necessary flexibility for successful task execution.53
Studies of movement variability during jump-landing tasks suggest that athletes with CAI appear to have less movement variability than healthy athletes.54 Interestingly, athletes with CAI also have less movement variability than “copers,” who have a history of ankle sprain but no lingering instability. Additional research suggests that movement variability may be influenced by whether the ankle instability is functional or mechanical in nature.55 Athletes with functionally unstable ankles demonstrate greater frontal-plane movement variability, while those with mechanically unstable ankles demonstrate greater anterior-posterior ground reaction force variability. The implications of these variability patterns for postural control in patients with CAI, however, are unclear.
Researchers are interested in determining the underlying causes of postural instability in individuals with CAI. Clinicians and athletes, hoverer, ultimately want to know if rehabilitation can change the alterations. McKeon et al28 found that, after four weeks of balance training, force plate measures improved in those with CAI compared with a group with CAI who did not complete the training. Similarly, Sefton et al56 had a group of persons with CAI perform six weeks of balance training and reported that participants improved significantly compared with a group of healthy controls who did not perform any rehabilitation. Further research should be done to determine the most effective rehabilitation protocol in altering postural control to help clinicians treat CAI patients.
Overall, people who suffer ankle sprains do not completely comprehend the potential long-term effects of not fully treating and rehabilitating the injury. Individuals who develop CAI most likely suffer from postural control changes, however, they often believe that little can be done for their pathology and choose to alter their lifestyle to accommodate their instability. Research confirms the potential of rehabilitation protocols for improving postural control in those with CAI. Clinicians should be aware of these rehabilitation protocols, educate patients, and implement balance training rehabilitation protocols to help those with CAI.
Lisa Chinn, MS, ATC, and C. Collin Herb, MEd, ATC, are doctoral students studying chronic ankle instability in the Kinesiology Program at the University of Virginia in Charlottesville.
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