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Lower Limb Interjoint Postural Coordination One Year after First-Time Lateral Ankle Sprain

DOHERTY, CAILBHE1; BLEAKLEY, CHRIS2; HERTEL, JAY3; CAULFIELD, BRIAN1; RYAN, JOHN4; SWEENEY, KEVIN5; PATTERSON, MATTHEW R.5; DELAHUNT, EAMONN1,6

Medicine & Science in Sports & Exercise: November 2015 - Volume 47 - Issue 11 - p 2398–2405
doi: 10.1249/MSS.0000000000000673
APPLIED SCIENCES
Free

Introduction Longitudinal analyses of participants with a history of lateral ankle sprain are lacking. This investigation combined measures of lower limb interjoint coordination and stabilometry to evaluate static unipedal stance with the eyes open (condition 1) and closed (condition 2) in a group of participants with chronic ankle instability (CAI) compared to lateral ankle sprain “copers” (both recruited 12 months after sustaining an acute first-time lateral ankle sprain) and a group of noninjured controls.

Methods Twenty-eight participants with CAI, 42 lateral ankle sprain “copers,” and 20 noninjured controls completed three 20-s single-limb stance trials in conditions 1 and 2. An adjusted coefficient of multiple determination statistic was used to compare stance limb three-dimensional kinematic data for similarity to establish patterns of interjoint coordination. The fractal dimension of the stance limb center of pressure path was also calculated.

Results Between-group analyses revealed that participants with CAI displayed notable increases in ankle–hip linked coordination compared with both lateral ankle sprain “copers” (−0.52 (1.05) vs 0.28 (0.9), P = 0.007) and controls (−0.52 (1.05) vs 0.63 (0.64), P = 0.006) in condition 1 and compared with controls only (0.62 (1.92) vs 0.1 (1.0) P = 0.002) in condition 2. Participants with CAI also exhibited a decrease in the fractal dimension of the center-of-pressure path during condition 2 compared with both controls and lateral ankle sprain “copers.”

Conclusions Participants with CAI present with a hip-dominant strategy of eyes-open and eyes-closed static unipedal stance. This coincided with reduced complexity of the stance limb center of pressure path in the eyes-closed condition.

1School of Public Health, Physiotherapy and Population Science, University College Dublin, Dublin, IRELAND; 2Sport and Exercise Sciences Research Institute, Ulster Sports Academy, University of Ulster, Newtownabbey, County Antrim, UNITED KINGDOM; 3Department of Kinesiology, University of Virginia, Charlottesville, VA; 4Emergency Department, St. Vincent’s University Hospital, Dublin, IRELAND; 5Insight Centre for Data Analytics, University College Dublin, Dublin, IRELAND; and 6Institute for Sport and Health, University College Dublin, Dublin, IRELAND

Address for correspondence: Cailbhe Doherty, B.Sc., A101 School of Public Health, Physiotherapy and Population Science, University College Dublin, Health Sciences Centre, Belfield Dublin 4, Ireland; E-mail: cailbhe.doherty@ucdconnect.ie.

Submitted for publication November 2014.

Accepted for publication March 2015.

Lateral ankle sprain (LAS) injury pervades a variety of activities, with between 0.88 (95% confidence interval (CI), 0.73–1.02) and seven (95% CI, 6.82–7.18) injury events occurring per 1000 exposures depending on the activity type (11). The prevalence of this injury in a wide range of sports and activities is further complicated by its capacity to deteriorate into an array of chronic sequelae and injury recurrence, collectively termed chronic ankle instability (CAI) (7,15–17), which has been linked to limitations in future physical activity participation (1).

Although CAI is considered a multifaceted condition with a range of consequences, persistent deficits in single-limb stance (SLS) postural control strategies are well established in individuals with CAI (18,26,36) and may be consequent upon a change in neural signaling after the initial ankle joint trauma (14). This theory has since been tested in previous studies comparing individuals with history of LAS with noninjured controls (13), with a new hypothesis emerging whereby the long-term outcome after LAS is dependent on the success or failure of the newly adopted post-LAS postural control strategies (34,35). This has yet to be confirmed however, as there is currently an absence of longitudinal investigations, which prospectively track the restoration or degradation of postural control strategies after an initial LAS.

More recently, LAS “copers,” who have history of LAS and have experienced a restoration of preinjury levels of function in the year after initial injury (15–17,34), have been compared with individuals with CAI during SLS (36); this is considered to provide a stronger, more relevant comparison in laying the foundation for longitudinal analyses and the development of clinical outcome models for the CAI paradigm (34). Recently published material from our laboratory was developed according to this paradigm; individuals with acute, first-time LAS were evaluated in comparison with a noninjured control group during eyes-open and eyes-closed SLS using kinematic and kinetic measures of joint position and platform stabilometry, respectively (9). A follow-up analysis of these same individuals 6 months after initial assessment revealed a hip-dominant postural control strategy prevailing during the prescribed tasks of SLS, again in comparison with noninjured controls (10). In this latter investigation, an adjusted coefficient of multiple determination (ACMD) statistic was used to evaluate waveform similarity between lower extremity three-dimensional (3D) joint angular displacements in the determination of interjoint “coupling” strategies during 20 s of eyes-open and eyes-closed SLS (10). We believe novel insight was gained by combining these laboratory measures; the increase in observed coupling between sagittal plane hip and frontal plane ankle motion in LAS participants underpinned a hypothesis that these individuals adopt a hip-dominant strategy in the maintenance of single-limb postural control, perhaps to compensate for a dysfunctional ankle joint (10). This theory is in agreement with the model of human postural control proposed by Nashner and McCollum (29), in which an “ankle strategy” is appropriated to the fine tuning of static postural control and a “hip strategy” is used to tackle more substantial postural control disturbances; the LAS group in the aforementioned studies were considered to have reduced capacity to use their ankle strategy, thus adopting the more proximal hip strategy in its place (9,10).

The measure of platform stabilometry used in the aforementioned investigations from our laboratory was the fractal dimension (FD) of the center of pressure (COP) path. The FD is a unitless measure that conceptualizes the complexity of the COP path using a value between 1 (a straight line or low complexity) and 2 (a convoluted line or high complexity) (24). In addition to a hip-dominant kinematic strategy, LAS participants were also shown to display a bilaterally reduced FD of the COP path during eyes-closed SLS within 2 wk of incurring their initial LAS injury (9) and on their involved limb only 6 months after their initial sprain (10). This was interpreted as reduced ability to use the available base of support on removal of visual afferents (8–10).

The current study is a continuation of those previously described and forms part of a larger longitudinal analysis of the LAS cohort. Specifically, we sought to complete the 12-month follow-up of the individuals we previously alluded to who completed the 2-wk and 6-month evaluations, thus allowing for participant segregation as CAI or LAS “coper” status. Kinematic and stabilometric measures were combined to compare stance limb interjoint coordination and COP path complexity during eyes-open and eyes-closed SLS between individuals with CAI, LAS “copers,” and a separately recruited noninjured control group of participants. We hypothesized that individuals with CAI would exhibit the same hip-dominant coupling strategies for completing eyes-open and eyes-closed SLS, which were documented 6 months previously, whereas LAS “copers” and control participants would not because of superior capacity to use an ankle-based balance strategy in isolation. Furthermore, we hypothesized that during eyes-closed SLS, CAI participants would exhibit poorer postural control ability, as evidenced by reduced FD of the COP path.

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METHODS

Participants

As part of the larger longitudinal study conducted in our laboratory, 82 individuals presenting with a first-time acute LAS were recruited from a university-affiliated hospital emergency department. All LAS participants were provided with the same basic advice on applying ice and compression upon discharge from the hospital emergency department; they were individually encouraged to weight-bear and walk within the limits of pain. Whether participants sought additional formal medical health care services for counsel or rehabilitation of their LAS was recorded upon arrival at the testing laboratory but not controlled as part of the current study.

These individuals were required to attend three test sessions and complete a number of movement tasks within 2 wk of sustaining their initial injury, with further follow-up at 6 and 12 months. Testing procedures for these participants in the acute phase of their injury has previously been reported (9). A total of 71 of the originally recruited 82 participants returned for the third test session (i.e., 12-month follow-up); the current investigation relates to the data collected for these individuals at this time point. An additional convenience group of 20 participants without a history of LAS was also recruited from the hospital catchment area population using posters and flyers to act as a control group. The characteristics of the individuals included in the current analysis are presented in Table 1. The following exclusion criteria were used for both limbs (where applicable) at the time of recruitment: 1) no history of LAS injury (excluding the initial acute LAS episode for the CAI and LAS coper groups), 2) no other severe lower extremity injury in the last 6 months; 3) no history of ankle fracture, 4) no history of major lower limb surgery, 5) no history of neurological disease, vestibular or visual disturbance, or any other pathology that would impair their motor performance. Participants provided written informed consent, and the study was approved by the university human research ethics committee.

TABLE 1

TABLE 1

LAS participants’ designation as CAI or LAS “coper” status was completed according to recently published guidelines (15–17). Self-reported ankle instability was confirmed with the Cumberland ankle instability tool (CAIT) (19); individuals with a score of <24 were designated as having CAI, whereas LAS “copers” were designated with a score of ≥24 to avoid the potential for false positives in this group (39). In addition, to be designated as a coper, participants must have returned to preinjury levels of activity and function (34). Finally, the activities of daily living and sports subscales of the Foot and Ankle Ability Measure (FAAMadl and FAAMsport) were used as a means to evaluate general self-reported foot and ankle function (6). All participants completed the CAIT and subscales of the FAAM upon arrival at the testing laboratory.

On the basis of these criteria, 28 of the LAS participants were designated as having CAI and 42 as LAS “copers” (Table 1). One LAS “coper” participant was excluded because he did not return to preinjury levels of activity participation.

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Protocol

Collection methods for this study have been previously documented (10). Briefly, after the collection of anthropometric measures required for the calculation of internal joint centers of the lower extremity joints, each participant was instrumented with the Codamotion bilateral lower limb gait setup according to the manufacturer’s guidelines (Charnwood Dynamics Ltd., Leicestershire, United Kingdom). A neutral stance trial was used to align the subject with the laboratory coordinate system and to function as a reference position for subsequent kinematic analysis (40). Participants then performed three 20-s trials of quiet SLS barefoot on a force plate with their eyes open on both limbs, each separated by a 30-s rest period. After another 2-min rest period, participants then attempted to complete three 20-s SLS trials with their eyes closed. Participants were required to complete a minimum of three practice trials on each limb for each condition before data acquisition (8,22). Participants who were unable to complete a full trial of unilateral stance after five attempts on the relevant limb were not included in the analysis for that limb. The test order between legs was randomized. For both conditions of the SLS task, participants were instructed to stand as still as possible with their hands resting on their iliac crests while adopting a postural orientation most natural to them; the position of the nonstance limb was not dictated in the sagittal plane as part of experimental procedures. Trials were deemed invalid if the subject lifted their hands off their iliac crests, placed their nonstance limb on the support surface, moved their nonstance hip into a position >30° abduction, adducted their nonstance limb against their stance limb for support, or if the foot placement assumed by the participants relative to the support surface changed in any way over the course of a trial. In addition, a trial was deemed as “failed” in the eyes-closed condition if the participant opened their eyes at any point.

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Kinematic and kinetic data processing

Three Codamotion CX1 units were used to acquire data on 3D angular displacements at the hip, knee, and ankle joints for both limbs during the SLS tasks. Two AMTI (Watertown, MA) walkway embedded force plates were used to acquire kinetic data. Kinematic and kinetic data acquisition were made at 100 Hz. The Codamotion CX1 units were time-synchronized with the force plates. Kinematic and COP data were analyzed using the Codamotion software and then converted to Microsoft Excel file format. Temporal data were set with the number of output samples per trial at 2000 + 1 in the data export option of the Codamotion software, which represented the complete unilateral stance trial as 100%, for averaging and further analysis.

Pairwise comparisons of 3D temporal angular displacement waveforms for the hip and ankle joints of the stance limb were made using the ACMD statistic (23) to determine the similarity of a given pair of waveforms during both conditions of SLS. The pairing of ankle and hip motion was completed in three dimensions, with nine resultant ACMD values for each SLS trial. The mean ACMD from three trials of unilateral stance was used as a representative ACMD for each participant for the eyes-open and eyes-closed conditions separately, with subsequent calculation of group (CAI, LAS “coper,” and control) means. ACMD values ranged from 0 (no similarity) to 1 (two identical curves) (23).

The kinetic data of interest was the COP (the location of the vertical reaction vector on the surface of a force plate) path (31). COP data acquired from trials of the unilateral stance were used to compute FD of the COP path using an algorithm previously published and described by Prieto et al. (31). FD was calculated on the basis of the 20-s interval for each SLS trial and averaged across the three trials for each participant on each limb and grouped accordingly. The COP time series were passed through a fourth-order zero phase Butterworth low-pass digital filter with a 5-Hz cutoff frequency (38).

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Data analysis and statistics

For both LAS groups (CAI and LAS “coper”), the limb injured at the time of recruitment was labeled as “involved” and the noninjured limb was labeled “uninvolved.” With regard to the control group, limbs were randomly assigned as “involved” and “uninvolved” in all cases.

For all outcomes, we calculated mean (SD) scores for the involved and uninvolved limbs of the CAI, LAS “coper,” and control groups.

A principal component analysis (PCA) was performed to reduce the dimensionality of the kinematic data. Specifically, the nine “latent” variables of interjoint coordination were reduced into significant components. This was performed separately for the eyes-open and eyes-closed conditions. Preliminary analyses (scree test and parallel analysis) informed our decision to retain three components for the eyes-open condition and two components for the eyes-closed condition. One variable relating to coupling of the hip in the transverse plane to the ankle in the sagittal plane in the eyes-closed condition displayed a low communality to both extracted components, and as such was considered as an independent input variable for subsequent analyses.

To test our hypothesis that the CAI group would display hip-dominant strategies of interjoint coordination, the components derived from the ACMD “latent” variables were compared between groups using a two-way MANOVA for each condition (eyes open and eyes closed). The independent variables were group (CAI, LAS “coper,” and control) and limb (involved or uninvolved). The dependent variables were the three extracted components for the eyes-open condition and the two extracted components for the eyes-closed condition, in addition to the independent coupling variable between transverse hip- and sagittal-plane ankle motions. Preliminary assumption testing was conducted to check for normality, linearity, univariate and multivariate outliers, homogeneity of variance–covariance matrices, and multicolinearity with no serious violations noted. An α level of P < 0.05 was used to determine significant differences for each analysis (20). Post hoc comparisons were completed using the Tukey honest significant difference test where appropriate. The significance level for post hoc analyses was set with a Bonferroni-adjusted α of P < 0.017 for the eyes-open condition (0.05/3 components) and P < 0.017 for the eyes-closed condition (0.05/2 components + 1 independent input variable) (21).

To test our hypothesis that the CAI group would display reduced COP path trajectory FD during the SLS task compared with LAS “copers” and controls, a two-way between-group ANOVA was conducted separately for each condition (eyes open and eyes closed). The independent variables were group (CAI, LAS “coper,” and control) and limb (involved or uninvolved). The dependent variable was FD of the COP path. The significance level for this analysis was set a priori at P < 0.05. Post hoc comparisons were completed using a Tukey honest significant difference test where appropriate. The significance level for post hoc analyses was set at P < 0.05 for both conditions.

All data were analyzed using Predictive Analytics Software (version 18; SPSS, Inc., Chicago, IL).

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RESULTS

All participants completed the eyes-open SLS task on both limbs. Thirty-six percent of CAI participants (10 of 28), 76% of LAS “copers” (33 of 42), and 85% of controls (17 of 20) completed the SLS task with their eyes closed on both their “involved” and “uninvolved” limbs.

Regarding interjoint coordination, there was a statistically significant main effect for group in the eyes-open (F3,322 = 2.585; P = 0.018; Wilks λ = 0.91) and eyes-closed (F3,220 = 3.58; P = 0.008; Wilks λ = 0.88) conditions. When the results of the dependent variables were considered separately, the only variables to reach statistical significance were components 3 (which loaded heavily on the interjoint coordination between sagittal plane hip and frontal plane ankle motion, and sagittal plane hip and transverse plane ankle motion) in the eyes-open condition (F2,321 = 6.508, P = 0.002, ηp2 = 0.074) and 2 (which loaded heavily on the interjoint coordination between sagittal plane hip motion and ankle motion in all three dimensions, and frontal plane hip motion and sagittal plane ankle motion) in the eyes-closed condition (F2,219 = 4.125, P = 0.019, ηp2 = 0.069). Post hoc analysis and inspection of the mean scores revealed that CAI participants exhibited lower mean scores for component 3 in the eyes-open condition, most notably on their involved limb (mean, −0.52; SD, 1.05) compared with both LAS “copers” (mean, 0.28; SD, 0.9; P = 0.007) and controls (mean, 0.63; SD, 0.64; P = 0.006). Because of the negative correlation between component 3 and its latent variables, this represented an increase in ankle–hip linked coordination in the CAI group. With regard to the eyes-closed condition, post hoc analyses revealed that CAI participants exhibited greater mean scores for component 2 compared with controls only (P = 0.002). This was evident on both their involved (CAI: mean, 0.62; SD, 1.92; control = 0.1; SD, 1.0) and uninvolved (CAI: mean, 0.07; SD, 1.19; control = −0.34; SD, 0.66) limbs. Because of the positive correlation between this component and its latent variables, this too represented an increase in ankle–hip linked coordination in the CAI group.

Descriptive statistics for the “latent” ACMD variables for the CAI, LAS “coper,” and control groups before PCA are presented in Table 2. Pattern and structure matrices for the PCA relative to the eyes-open and eyes-closed conditions are presented in Table 3.

TABLE 2

TABLE 2

TABLE 3

TABLE 3

Regarding the kinetic variables of interest, there was a statistically significant main effect for the group in the eyes-closed condition (F2,219 = 8.11, P = 0.001, ηp2 = 0.12) only. Post hoc analysis and inspection of the mean scores revealed that CAI participants exhibited lower FD of the COP path trajectory on their involved limb (mean, 1.78; SD, 0.11) compared with both LAS “copers” (mean, 1.90; SD, 0.1; P = 0.045) and controls (mean, 1.94; SD, 0.13; P < 0.001).

In an exploratory analysis, the concurrent validity of four variables deemed “significantly important” (eyes-closed SLS task completion, component 3 in the eyes-open condition on the involved limb, and both component 2 and the FD of the COP path on the involved limb in the eyes-closed condition) in determining the extent of disability was established by calculating their respective Pearson correlation coefficients to CAIT score. This was performed for LAS participants only. The ability of each of these variables to determine outcome (CAI vs LAS “coper”) was then tested for sensitivity and specificity. A cutoff value of 0.7 was adopted for the C statistic in the sensitivity and specificity analyses.

There was no correlation between CAIT score and eyes-closed SLS task completion (r = 0.004, P = 0.97), component 3 (r = 0.109, P = 0.39), component 2 (r = 0.213, P = 0.19), or FD of the COP path (r = 0.11, P = 0.39).

However, eyes-closed SLS task completion was moderately predictive of outcome (CAI vs LAS “coper”), with a C statistic of 0.71 (P = 0.003); the resultant prediction equation yielded a sensitivity of 0.64 and a specificity of 0.78, with a positive likelihood ratio of 2.93.

To explain these findings, post hoc analysis using independent-samples t-tests were performed to compare the CAIT scores of the subgroups of CAI and LAS “coper” participants who succeeded and failed at the eyes-closed SLS task. The P value for this post hoc analysis was set a priori with a Bonferroni adjustment at P < 0.025. This analysis revealed that LAS “copers” who were able to complete the task actually had significantly greater disability than those who could not and likewise for the CAI participants, thus explaining the capacity of task completion to predict outcome (CAI or LAS “coper”) despite the absence of a correlation to CAIT score. The results of this post hoc analysis for both subgroups of CAI and LAS “coper” participants are presented in Table 4. None of the other variables (components 2 and 3 and FD of the COP path) were predictive of outcome based on the C statistic.

TABLE 4

TABLE 4

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DISCUSSION

The primary finding of this motion analysis investigation was that individuals with CAI exhibit greater “coupling” of hip and ankle motion compared with both LAS “copers” and noninjured controls during an SLS task. This increase in ankle–hip “coupling” may represent a compensatory strategy to accommodate what is now a chronically unstable ankle in the CAI group (as determined using CAIT). Furthermore, the CAI group also demonstrated reduced FD of the COP path on their involved limb compared with both LAS “copers” and controls in the eyes-closed condition of SLS. These findings are consistent with those previously published on this group as a whole within 2 wk of their injury (9) and 6 months after (10). Therefore, the abatement of a hip-dominant postural control strategy may be conducive to superior outcome. The design of the current study, however, means that this cannot be confirmed.

To our knowledge, this is the first documented evaluation of postural control in a first-time LAS population exactly 12 months after initial injury using kinematic measures of lower limb interjoint coordination and platform stabilometry. The advantage of the experimental design is that all LAS participants (CAI and LAS “coper”) were recruited at the time of their first ever LAS injury, thereby securing the homogenous subgroups of LAS outcome. As we have alluded to, this study is part of a longitudinal analysis designed to develop an outcome model for the predictors of CAI after LAS injury.

The use of LAS “copers” provides a superior comparison group to individuals with CAI than noninjured controls because LAS “copers” have had the same exposure but are not characterized by the same symptom sequelae as those in individuals who develop CAI (34). The addition of a noninjured control group in this report, however, has allowed us to identify that based on the parameters used in the current investigation, LAS “copers” are no different from noninjured controls in their postural control strategies for eyes-open and eyes-closed SLS. This is evidenced by the absence of between-group differences for LAS “copers” and controls in this analysis, which is in agreement with previous findings during a similar task protocol (32,36). It has recently been identified that this tripartite comparison among CAI, LAS “coper,” and control participants is needed in the context of LAS research (34). Indeed, there are only a limited number of previous analyses that have evaluated movement patterns in these groups (4,5,32,36,37), with fewer still providing an analysis of SLS postural control using measures of platform stabilometry (32,36). Wikstrom et al. (36) identified that LAS “coper” participants’ stance limb COP paths exhibit lower velocity in both the anteroposterior and the mediolateral axes of the foot than individuals with CAI during a similar task. Shields et al. (32) demonstrated that the SD of the COP path and its range were significantly lower in LAS “copers” compared with those in subjects with CAI, a finding the authors interpreted as being demonstrative of better postural control predictability.

The issue regarding the application of these “traditional measures” of COP excursion, which quantify the length, area, and velocity of the COP path, apart from their questionable reliability (12), is that they have previously yielded inconsistent or even contradictory findings in LAS populations (27). By contrast, the FD measure used in the current analysis is a reliable measure (12), which has previously been successful in characterizing a degeneration in stability of the postural control system in the transition from eyes-open to eyes-closed stance (3). Furthermore, because we have adopted the FD calculation in analyzing the COP paths of these same participants during SLS within 2 wk (9) of incurring their initial injury and 6 months later (10), its use enables us to directly compare our findings across time points relevant to the development of CAI or LAS “coper” status.

Consistent with the investigations of these participants 2 wk and 6 months after injury occurrence (9,10), the findings of the current study revealed that individuals with poorer outcome (<24 on the CAIT in this study, “injured” status in those previously described), exhibit reduced FD of the COP path compared with individuals with superior outcome (noninjured controls and LAS “copers”), albeit in the eyes-closed condition only. This was previously interpreted as reduced ability to use the available base of support during SLS, isolated to instances where the task condition dictated the removal of visual afferents (8). Similarly, the CAI participants in the current study also exhibited greater “coupling” of hip–ankle joint coordination in the completion of eyes-closed SLS compared with controls, a finding consistent with the acute (2 wk) and injury “twilight” (6 months) data.

That a lower proportion of the CAI group was able to complete the balance task in the eyes-closed condition prompted an exploratory analysis, whereby this dichotomous outcome and the other group-defining variables (components 3 and 2 and the FD of the COP path) were separately correlated with CAIT score. Their capacity to predict outcome (CAI vs LAS “coper”) was also evaluated. Whereas the group-defining variables exhibited no correlation with CAIT score and did not predict outcome, task completion was determined as predictive of CAI or LAS “coper” status. The moderate specificity and sensitivity that an ability to complete eyes-closed SLS had in predicting outcome, in the absence of a correlation to CAIT score, may be underlied by a disability “cutoff”; the correlation between CAIT score and task ability is probably not linear, wherein it is possible that at a certain point, an individual’s ability to perform a difficult balance task (such as eyes-closed SLS) deteriorates drastically. Individuals below this cutoff have the potential to be equally likely to be unable to complete the task, whether they have “more” or “less” disability. Future analyses are required to elucidate such “cutoffs,” however.

The apparent difficulty CAI participants had in completing eyes-closed SLS may represent an impaired capacity to compensate and recoordinate the available sensory afferents or to rely on the remaining somatosensory and vestibular afferents when visual ones have been removed (25). It is generally accepted that there is redundancy of these three afferents in maintaining SLS (30), whereby a selective priority is placed on the basis of the availability of reliable information (28). This allows the fully functioning somatosensory system to maintain postural control and stability in the presence of altered afferent signals (25). However, prescribing an eyes-closed constraint during the SLS task imposes somatosensory demands beyond the capacity of even healthy individuals (as evidenced by the fact that 15% of controls were unable to complete our eyes-closed task protocol), impairing their ability to exploit available redundancies in the maintenance of static postural control (9). This impairment is seemingly magnified in individuals with musculoskeletal injury on the basis of the current findings and in light of the evidence previously outlined of participants with recent history of LAS (9,10). Thus, a decay in somatosensory afferents, which may occur with acute LAS injury and which is considered to contribute to instability persistence (14), combined with loss of visual input, can challenge the ability of the central nervous system to recoordinate the available information with an appropriated postural control response (13,28) in individuals with CAI. This then manifested in deterioration of eyes-closed unilateral standing postural control and stability in the CAI group, with less effective use of the supporting base on the involved limb (9). It is also plausible that the somatosensory deterioration associated with CAI development manifested in a “hip-dominant” compensatory strategy as evidenced by the significantly greater ankle–hip coupling compared with both LAS “copers” and controls in the eyes-open condition and compared with controls in the eyes-closed condition. Whereas the ankle strategy of human postural control is more suited to subtle corrections, the hip strategy is considered ideal for substantial disturbances of equilibrium (25). Tropp and Odenrick (33) previously used kinematic measures of sway amplitude at the ankle, hip, and trunk to confirm the existence of these strategies. They also identified the impaired postural control capacity of individuals with ankle instability in using their ankle strategies for SLS on the basis of an increased number of postural corrections at the trunk required by this group (33). In another kinematic analysis of participants with history of LAS during a SLS task, Huurnink et al. (22) failed to identify differences in kinematic outcome measures (ankle and hip angular velocities) between participants with and without history of ankle sprain. We believe that the use of the ACMD statistic in the current study has enabled specific identification of increased reliance on the more proximal hip strategy in the CAI group on the basis of the greater waveform similarity between the hip and ankle joints. During normal control of SLS, the foot’s narrow base of support makes it necessary to use the hip strategy in controlling substantial mediolateral disturbances of postural stability, whereas ankle movements may only achieve fine tuning of mediolateral sway (2). The basis of CAI may be underpinned by an impaired capacity to fulfill this mediolateral fine tuning, with subsequent transition to the more proximal hip. Herein lies a significant limitation of the current analysis; these and any other hypotheses regarding the sensorimotor predictors of CAI are still unclear, although the current study is part of a project designed to investigate this issue. Another significant limitation of this analysis is that we were unable to experimentally control whether LAS participants sought additional rehabilitation for their injury. However, to do so would have been unethical and no treatment data “clusters” were evident during data management and analysis.

The clinical implications of this study are twofold; first, in light of the evidence presented on these individuals during their “recovery,” it would seem that the capacity to perform static postural control tasks will challenge the individual to perform subtle corrections with ankle movements. An SLS task and derivations of such may therefore possess value as part of a rehabilitation program. On the basis of previous evidence, we would recommend, however, that the patient only progresses to such tasks when they are sufficiently able to complete them (8). Second, the use of eyes-closed SLS as a clinical test to quantify disability and functional capacity should be considered. There is further potential for future research to confirm this.

In conclusion, the results of the current study suggest that participants with CAI are separated by LAS “copers” and noninjured controls in their exhibition of a hip-dominant balance strategy during a task of eyes-open and eyes-closed unilateral stance.

This study was supported by the Health Research Board (HRA_POR/2011/46), as follows: principal investigator, Eamonn Delahunt; coinvestigators, Chris Bleakley and Jay Hertel; Ph.D. student, Cailbhe Doherty.

No conflicts of interest were associated with the authors and the results of this research.

The results of the present study do not constitute endorsement by the American College of Sports Medicine.

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Keywords:

ANKLE JOINT; BIOMECHANICAL PHENOMENA; KINEMATICS; KINETICS; POSTURAL BALANCE; JOINT INSTABILITY

© 2015 American College of Sports Medicine