Eversion Force Sense Characteristics in Individuals with Functional Ankle Instability
This article aims to determine whether there are eversion force sense deficits in individuals with functional ankle instability. Database searches resulted in 7 studies that met the inclusion criteria of testing ankle eversion force sense in participants with functional ankle instability. For each study, effect sizes were calculated separately for ipsilateral and contralateral reference ankles, and for each possible outcome variable: absolute error, constant error, and variable error. For ipsilateral variable error, 8 of 9 measures favored participants with functional ankle instability (effect size range= 0.217 to 0.706); however, only 3 confidence intervals did not cross zero. All 3 measures of ipsilateral constant error favored the uninjured group (effect size range = −0.532 to −0.254), although all confidence intervals crossed zero. There were no consistent trends for ipsilateral absolute error or for any contralateral error measure. Individuals with functional ankle instability may have different ipsilateral eversion force sense characteristics than uninjured individuals.
Ankle sprains are one of the most common injuries experienced by individuals involved in physical activity, accounting for 15% of all reported injuries in the National Collegiate Athletic Association over the past 16 years.1 Especially concerning is the high prevalence of reinjury and chronic symptoms following acute ankle sprain. Freeman2,3 coined the term functional ankle instability to describe those patients experiencing sensations of “giving way” and instability following an acute lateral ankle sprain. The incidence of functional ankle instability following an acute lateral ankle sprain ranges from 32% to 47%.4–6 These recurrent symptoms can limit physical activity and activities of daily living for years post-injury5,6 and decrease quality of life.4
Although several pathological factors have been associated with functional ankle instability, no single mechanism in the development of functional ankle instability after an acute ankle sprain has been identified. One of the factors commonly linked to functional ankle instability is impaired kinesthesia.7 One measure of kinesthesia is force sense, which describes an individual’s ability to detect muscular force generation. It is hypothesized that an impaired ability to accurately detect ankle eversion force, either the effort needed or the actual tension developed, may contribute to instability.8
Peripherally, force sense is thought to arise primarily from the golgi tendon organs located within muscle tissue.9 Golgi tendon organs are aligned in series with muscle fibers and sense tension, whereas muscle spindles sense stretch. Damage to these structures causes force sense impairment.10 Lateral ankle sprains constitute the majority (80%) of all ankle sprains,11 and concurrent peroneal muscle strain has been documented in approximately 15% of lateral ankle sprains.12 This damage is hypothesized to occur as a result of overstretching during excessive inversion or as a result of a strong reflexive peroneal muscle contraction following inversion.13
Eversion force sense is measured by having individuals evert their ankle against a load cell at a load equal to a specific percentage of their maximum voluntary isometric contraction (MVIC) twice in succession.8,14,15 For the first eversion (called the reference or target), the participant is provided oral or visual feedback regarding the accuracy of their force production. This first target may be created using either the injured (ipsilateral) ankle8,14 or uninjured (contralateral) ankle.14,15 During the second eversion, the participant is asked to recreate the exact same amount of force as the first (target) load. This re-creation is done with the injured ankle and without any visual or oral feedback. Trial error is the difference between the target and reproduction force.
Typically, 3 trials (target-reproduction couples) are collected. Although some have argued that the use of 3 trials is insufficient for threshold-based proprioceptive tasks due to their psycho-physical properties and expected fluctuations,16 force sense is a matching (not threshold) task and has been reliably measured with 3 trials.17 Specifically, 3 outcome measures are assessed based on trial error: variable error, absolute error, and constant error. Variable error is a measure of consistency and is calculated as the standard deviation of trial error.8,14 Constant error is the average of trial error, where absolute error is the average of the absolute value of trial errors.14,15 Absolute error and constant error both measure overall performance. Constant error specifically relates to directionality (whether the individual overshoots or undershoots the desired target), whereas absolute error measures the magnitude of error without regard to direction. For example, if a target is 20 N and an individual actually reproduces a force of 24 N, the individual trial constant error would be −4 N (20 N minus 24 N) and the absolute error would be 4 N.
It has been hypothesized that individuals with functional ankle instability may have force sense deficits that account for their symptoms of instability and giving way of the ankle joint. Several studies have shown eversion force sense deficits in individuals with functional ankle instability,8,14,15 but these findings have not been consistent.17–20 Thus, it is unclear whether true force sense deficits exist in individuals with functional ankle instability. The purpose of this systematic review is to answer the question, “Are there eversion force sense deficits in individuals with functional ankle instability?”
Literature Search and Selection
To identify research on ankle joint force sense, the following databases were searched from the earliest record through February 2010: Medline/PubMed, CINAHL, and Cochrane. The following key terms were combined and searched: ankle, propriocept*, kinesth*, effort, tension, force sense, and heaviness (Table 1). The asterisk indicates a wildcard allowing different endings. No language restrictions were applied. Studies were eligible for inclusion if they included a measure of eversion force sense in a human ankle joint (ie, a force matching procedure where the participant was asked to recreate a target load both with and without feedback).
|STEP||SEARCH||NO. OF HITS|
|14||#12 OR #13||25a||7a||3a|
|13||#1 AND #6||18||4||3|
|12||#1 AND #9 AND #8||18||5||2|
|11||#1 AND #9||675||144||67|
|10||#1 AND #8||702||311||82|
|9||#4 OR #5 OR #6 OR #7||133,286||14,995||8413|
|8||#3 OR #2||11,220||2022||697|
In addition, each study had to have stated inclusion criteria requiring injured ankles to have episodes of giving way, frequent sprains, or either “chronic ankle instability” or “functional ankle instability” described as the target pathology. Either chronic ankle instability or functional ankle instability were allowed because the terms are sometimes used interchangeably in the literature. We did not restrict our analysis to any particular study design. Also, in an attempt to minimize publication bias, we included both abstracts and manuscripts. It has been shown that negative results are less likely to be published in full manuscripts21 and, although there is no absolute consensus, many meta-analyses agree that these materials should be included in scientific reviews.22 The references of each study retrieved for detailed review were hand-checked for additional studies, and the primary author was contacted to identify any additional studies in publication for each study that met the inclusion criteria.
The quality of the included studies was determined by the primary author (C.J.W.) using a 20-item questionnaire developed specifically to assess the quality of studies of individuals with ankle instability. Each item was scored as yes or no, with 3 items potentially scored as not applicable. The overall score was calculated as the percentage of items scored as yes after any not applicable items were removed. The development of this questionnaire to assess threats to construct, internal, and external validity in ankle instability research is reported in Arnold et al.23 In keeping with the methods of Arnold et al,23 quality assessment was applied to data available in the published abstract or manuscript only. Study quality was used as a descriptive measure, not as an inclusion or exclusion criteria, because previous work found no relationship between study quality and statistical outcome.23,24
Data Extraction and Statistical Analysis
Group means and standard deviations, or the Pearson correlation coefficient between ankle instability and force sense error, were extracted for each outcome variable in each study. Effect sizes (Cohen’s d) with 95% confidence intervals were calculated separately for ipsilateral and contralateral reference ankles and for each of 3 outcome variables (absolute error, constant error, and variable error). Where insufficient information was available to calculate effect sizes in the published manuscript or abstract, the primary author was contacted and additional information was requested. In all such cases, sufficient information was obtained to retain the article in the systematic review.
Literature Search and Selection
The database search identified 35 studies, of which 6 were duplicates (Figure 1). The remaining 29 articles were screened based on the title and abstract. Two were excluded because they were not human research, 5 did not address the ankle joint, and 15 did not measure force sense. Seven articles were retrieved for detailed evaluation.8,14,15,25–28 Contact with primary authors and hand-checking of the references led to the addition of 6 studies for detailed review.17–20,29,30
Of the 13 studies under detailed review, 2 abstracts were excluded because data were duplicated in subsequent articles.28,29 Two articles used parts of the same dataset and thus were not independent.14,27 Of those 2, the most recently published article was included in our review.14 In addition, 1 study was excluded because it reported eversion force sense data for healthy individuals only and could not speak to our primary objective.30 Finally, 2 studies were excluded because they tested force sense in the direction of plantarflexion rather than eversion and included only healthy participants.25,26 A total of 7 studies were included in this review.8,14,15,17–20
Participant characteristics are shown in Table 2. The participant population was predominantly college-age individuals (average age ranged from 19.0 to 23.1 years). Reported inclusion criteria for the functional ankle instability group varied between studies. Six studies required a minimum of 1 ankle sprain, and 4 required a history of giving way. Describing included participant characteristics, only 3 studies reported a measure of severity of the initial ankle sprain, and no study reported on the presence or absence of mechanical instability. Control participants for all studies reported no history of ankle sprain, and 3 studies reported no history of lower extremity fracture or surgery. Only 1 study specifically reported that control participants had no history of giving way in either ankle.
|UNI-OR BILATERAL INSTABILITY||HISTORY OF ANKLE SPRAIN||INJURY SEVERITY||GIVING WAY||FREQUENCY OF GIVING WAY||ACUTE SYMPTOMS EXCLUDED||AGE, Y||HISTORY OF ANKLE SPRAIN||HISTORY OF LE Fx OR SURGERY||GIVING WAY|
|Arnold et al19||Metropolitan area||22.7±2.6||Not specified||Minimum of 1 (average 3.2±2.5)||Initial injury required immobilization||Yes||0.57±0.54 per week||Yes||23.1±3.8||No||Not specified||Not specified|
|Arnold & Docherty14||College students||22.9±5.8||Unilateral||Minimum of 1 (average 1.9±2.6)||2 mild, 7 moderate, 4 severe, 7 ungraded||Yes||11.5±23.4 per year||Yes||NA||NA||NA||NA|
|Arnold & Docherty17||Not specified||21.3±3.2||Not specified||Minimum of 1 (average 2.3±2.6)||Not specified||Not specified||Not specified||Not specified||20.8±2.3||No||Not specified||Not specified|
|Docherty et al20||Not specified||19.8±1.6a||Not specified||Minimum of 1||Not specified||Yes||Not specified||Not specified||19.8±1.6a||No||No||Not specified|
|Docherty & Arnold8||College students||19.0±1.1||Both||Minimum of 1||2 mild, 7 moderate, 7 severe, 4 ungraded||Yes (at least 1 per month)||1 daily, 5 weekly, 14 monthly||Yes||19.3±1.6||No||No||No|
|Docherty et al15||College students||22.4±4.9a||Unilateral||Minimum of 1||Not specified||Not specified||Not specified||Yes||22.4±4.9a||No||No||Not specified|
|Taylor et al18||Military athletic cadets||20.3±1.2a||Unilateral||History of FAI (not defined)||Not specified||Not specified||Not specified||Yes||20.3±1.2a||No||Not specified||Not specified|
The methods for each study are summarized in Table 3. The ankle creating the reference (target) force was the ipsilateral ankle in 4 studies,8,17,19,20 contralateral ankle in 2 studies,15,18 and both ankles were used in 1 study.14 Loads ranged from 10% to 75% of MVIC. The force sense measures calculated were absolute error (n = 7),8,14,15,17–20 variable error (n = 6),8,14,15,17,19,20 and constant error (n = 4).14,15,17,19 Four studies8,18–20 analyzed differences in force sense between functional ankle instability and control groups, and 3 studies14,15,17 analyzed correlations between force sense performance and descriptors of functional ankle instability (such as the frequency of giving way at the ankle or the Ankle Instability Index31).
|STUDY||QUALITY SCORE (%)||STUDY DESIGN||INJURED (N)||UNINJURED (N)||REFERENCE ANKLE||LOAD (% OF MVIC)||FS MEASURE CALCULATED||ANALYSIS TYPE|
|Arnold et al19||45||2 group, 2 treatment, cross-over||15||15||Ipsilateral||10||AE, VE, and CE||Between group differences|
|Arnold and Docherty14||50||Cohort||20||0||Ipsilateral & contralateral||10 and 30||AE, VE, and CE||Correlations between FS error & AII or GW|
|Arnold and Docherty17||15||Cohort||17||27||Ipsilateral||50 and 75||AE, VE, and CE||Correlation between FS error & injury severity, roll-overs, or GW|
|Docherty et al20||21.1||Case-control with repeated measures||18||18||Ipsilateral||30||AE and VE||Between group differences|
|Docherty and Arnold8||52.6||Case-control||20||20||Ipsilateral||10, 20, and 30||AE and VE||Between group differences|
|Docherty et al15||40||Cohort||47||13||Contralateral||10 and 30||AE, VE, and CE||Correlation between FS error & AII|
|Taylor et al18||20||Case-control||20||20||Contralateral||50||AE||Between group differences|
Quality assessment for all included studies averaged 34.8±15.69 (Table 3). The average quality score for a full article was 47.5±6.65,8,14,15 whereas the average quality score for abstracts was lower at 25.3±13.42.17–20
Three studies reported absolute error using the contralateral ankle as the target or reference ankle (Figure 2).14,15,18 The force loads in these studies ranged from 10% to 50% of MVIC. All confidence intervals crossed zero. Five studies reported absolute error using the ipsilateral ankle as the reference ankle (Figure 3).8,14,17,19,20 The force load in these studies ranged from 10% to 75% of MVIC. Only one significant effect size was calculated,14 favoring greater absolute error with greater amounts of ankle instability when producing loads equal to 10% of MVIC.
Three studies reported constant error using the contralateral ankle as the reference ankle (Figure 4).14,15,18 However, for 2 studies,14,15 insufficient information was available to determine the magnitude and direction of force sense error from the magnitude and direction of the reported correlation. These studies could not be used to systematically review constant error but were not excluded from review of absolute error and variable error. Thus, we were only able to calculate and include the effect size for 1 study.18 The force load of the included study was 50% of MVIC. The confidence interval crossed zero.
Three studies reported constant error using the ipsilateral ankle as the reference ankle (Figure 5).14,17,19 However, 1 study gave insufficient information to determine either the magnitude or direction of constant error based on the reported correlation and was excluded from the analysis.14 The force loads in these studies ranged from 10% to 75% of MVIC. All 3 measures of constant error favored a greater magnitude of constant error in the uninjured group. Specifically, uninjured individuals were more likely to undershoot the target. However, all confidence intervals crossed zero.
Three studies reported variable error using the contralateral ankle as the reference ankle (Figure 6).14,15,18 The force loads in these studies ranged from 10% to 50% of MVIC. Only 1 study reported significant group differences,15 favoring greater variable error in the injured group when producing loads equal to 10% of MVIC.
Five studies reported variable error using the ipsilateral ankle as the reference ankle (Figure 7).8,14,17,19,20 The force loads in these studies ranged from 10% to 75% of MVIC. Three of the 9 measures were significant (confidence intervals that did not cross zero) and favored greater variable error in the injured group.8,17
Eversion force sense may be an important component of ankle stability. Several studies have been published evaluating force sense error in individuals with functional ankle instability and healthy controls.8,14,15,17–20 However, no conclusive answer has been given to the question, “Are there eversion force sense deficits in individuals with functional ankle instability?” Overall, this systematic review found a trend toward increased ipsilateral constant error in the uninjured group and ipsilateral variable error in the injured group. However, this trend was weak because the majority of measures had confidence intervals that crossed zero. All other measures did not appear to differ between groups.
Increased ipsilateral variable error indicates that the performance of injured particpants was less precise. Thus, at any point in time individuals with functional ankle instability might produce either more or less force than they intended to produce. These miscalculations may explain decreased feelings of stability noted by individuals with functional ankle instability. As discussed in Docherty et al,15 the variability of these errors fits with the sporadic nature of symptoms of giving way at the ankle reported by individuals with functional ankle instability. At times, force sense may be adequate to prevent feelings of instability, but the variability means that, on occasion, force sense error may be great enough to contribute to an episode of giving way of the ankle.
Constant error captures both magnitude and directionality of force sensing error. It is a function of both the precision and accuracy of performance. A trend toward a greater magnitude of ipsilateral constant error in the uninjured group was found (although no individual study reached significance). Specifically, uninjured individuals were more likely to undershoot the target. A greater magnitude of constant error may simply indicate that the uninjured participants were more consistent in the direction of their error. For example, participants would have a larger constant error score if they overshot the desired force at a consistent magnitude than if they alternated overshooting and undershooting the target by similar magnitudes.
Ipsilateral Versus Contralateral Testing
One study found group differences in force sense error (specifically variable error) while using the contralateral ankle as the reference.15 No other differences between groups were found for absolute error, variable error, or constant error using the contralateral ankle. The importance of the distinction between contralateral and ipsilateral reference ankles should be understood in relation to the different type of information given by these 2 methods. With ipsilateral testing, an individual both creates and recreates the target force with the same ankle; thus, the same proprioceptive structures from the target aide in the reproduction. However, these proprioceptors are hypothesized to be damaged, which could introduce error into both the target and recreation. If those errors were equal in amount, no deficit would be found because force replication would be precise, although potentially inaccurate. However, Arnold and Docherty,14 who conducted the only study to test the use of both contralateral and ipsilateral methods, found that this was not the case. Large variable errors during ipsilateral testing indicated that individuals with functional ankle instability were imprecise in their force sensing ability.
Contralateral testing attempts to control for limitations of ipsilateral testing by using the uninjured ankle as a control. The assumption is that local proprioceptive information from the target force created by the uninjured ankle is accurate and any deficit in recreation is due to error of the injured ankle only. However, transferring information from side to side may increase the contribution of the central nervous system, increase the difficulty of the task, and lead to more variability of performance for both injured and uninjured particpants, as found in the study by Arnold and Docherty.14
Increased central nervous system involvement may also introduce error because bilateral deficits in postural control have been noted in this population.32 Also, some may argue that contralateral methods are less clinically relevant because a person may not get the opportunity to sense force loads using the opposing ankle in real life. However, in some continuous motor skills, such as walking or running, the opposite limb does have the opportunity to sense force loads, and that information may be useful in planning and executing joint forces on the opposite side. The relative importance of ipsilateral and contralateral force sense remains unknown.
Differences Between High and Low Loads
Our review noted that all but 1 significant effect size used loads of ≤30% of MVIC. One potential explanation for the general lack of significant differences at larger loads is related to motor unit recruitment. At low loads, only a few motor units may be recruited to produce the target force. If those motor units include damaged proprioceptors (mainly golgi tendon organs or muscle spindles), the erroneous input from those damaged proprioceptors will be weighted heavily. Whereas at higher loads, the influence of each individual proprioceptor will be weighted less because a greater number of motor units are recruited and, thus, a larger number of proprioceptors are available to provide adequate feedback.17 Another potential explanation is that lower loads eliminate potential confounding variables that could mask results at higher loads. For example, assistance from other muscles may occur at higher loads. Moreover, muscle fatigue may occur at higher loads; muscle fatigue has been shown to cause muscle damage in and of itself, adding another error source.10
One potential point of concern is that the quality scores for several studies were low. For example, although sensations of giving way at the ankle following acute lateral ankle sprain are the hallmark symptom of functional ankle instability,3 studies did not specifically report requiring symptoms of giving way in their functional ankle instability group. However, we feel this was most likely due to space limitation imposed in the abstract format and, therefore, a case of inadequate reporting as opposed to poor study design. In the future, enhanced reporting is needed. In addition, it is important to note that our study quality scores are typical based on previous work using this instrument, which has reported scores ranging from 0% to 43.7%.23,24
One limitation in this body of literature is that all of the studies were convenience samples of college age individuals, limiting the generalizability of findings to other populations. In addition, all of the studies came from a related group of researchers. Their findings have been replicated at multiple sites over the course of several years; however, replication of findings by outside researchers would increase credibility of results. Two studies by other researchers were identified by the literature search; however, they could not be included in this review because both only evaluated plantarflexion force sense and did not include individuals with functional ankle instability.25,26 However, this work on force sense at the ankle joint, as well as previous research at the elbow joint,9,33 show that force sense is an important factor in motor control and may be altered in the presence of fatigue or injury.
Vuillerme and Boisgontier26 reported increased plantarflexion force sense error during a fatigued state. Because more injuries are sustained during the second half of athletic competitions when athletes are fatigued,34 it may be important to know the effect of fatigue on eversion force sense. Because they are unable to accurately sense the forces they are producing, individuals with functional ankle instability may have a decreased ability to compensate for fatigue-related deficits. This may partially explain the increased risk of reinjury and chronic symptoms in these groups. Future research investigating the relationship between fatigue and eversion force sense in individuals with and without functional ankle instability is needed.
In addition, there is a need for prospective studies in this area because it is unknown whether increased force sense error is a result or a cause of functional ankle instability. Prospective investigations into the relationship between force sense and fatigue may help further explain the mechanisms behind functional ankle instability.
Individuals with functional ankle instability may have different ipsilateral eversion force sense characteristics than uninjured individuals. Increased performance variability could partially explain the sporadic nature of giving way symptoms in functional ankle instability individuals. However, effect sizes are small and their clinical significance is not clear.
Implications for Clinical Practice
Given the evidence for altered force sense characteristics in individuals with functional ankle instability, exercises that emphasize ankle proprioception may be important components of rehabilitation. Future research should attempt to establish what type of intervention or training results in optimal improvement of force sensation.
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