Cross-Education Balance Effects After Unilateral Rehabilitation in Individuals With Chronic Ankle Instability: A Systematic Review

Acute lateral ankle sprains are one of the most common ailments in the physically active population.¹ An estimated 40% of lateral ankle sprains lead to chronic ankle instability (CAI) at least 1 year after the initial injury.² Chronic ankle instability is primarily characterized by episodes of giving way, recurrent sprains, and feelings of instability.³ Another frequent characteristic of CAI is impaired sensorimotor control.⁴ Pain, perceived instability, kinesiophobia, and reduced somatosensation, self-reported function, and health-related quality of life are evidence of sensory-perceptual impairments in individuals with CAI.⁴ These changes contribute to and can be exacerbated by motor-behavioral impairments, such as altered reflexes, neural inhibition, muscle weakness, altered movement patterns, and decreased physical activity.⁴ Reduced balance performance is among the most commonly identified sensorimotor impairments in the population with CAI.⁴ This was further supported by previous authors^5–10 who reported balance training was effective in improving both patient-reported and clinician-oriented outcome measures in those with CAI. Other modes of exercise, such as strength training, have also exhibited positive effects on balance performance.¹¹

Most often, rehabilitation protocols aimed at improving balance focus on the involved extremity with the goal of restoring normal function to that leg.^5–10 However, recent findings¹² indicate that among individuals with CAI, balance training of the uninvolved limb is an effective means of improving balance performance of the involved limb; this phenomenon is known as cross-education. More broadly, cross-education is defined as adaptation of an untrained limb after unilateral training of the contralateral limb. Cross-education is possible because of the interhemispheric communication in the brain, primarily through the corpus callosum.¹³ This interhemispheric communication causes unilaterally performed activities to stimulate the ipsilateral motor cortex, which corresponds to the untrained limb.¹⁴ Cross-education effects have been widely observed in strength and conditioning research.^15,16 Patients with neurologic disorders that result in unilateral neuromuscular deficits (eg, stroke) have also shown benefits from cross-education training, particularly when paired with mirror therapy.¹⁷ Despite the benefits to healthy and injured populations, the applicability of cross-education to CAI rehabilitation is less established.

Balance cross-education could potentially aid individuals with CAI in multiple ways. Providing therapeutic exercises for the uninvolved limb could offer a method for improving sensorimotor control in an involved limb that may be unable to engage in such activity because of swelling, range-of-motion deficits, or pain with weight-bearing. Furthermore, training the involved limb could improve balance deficits in the uninvolved limb.¹⁸ Although numerous authors have studied the effects of therapeutic exercise on balance performance in individuals with CAI,¹⁹ none have thoroughly reviewed the literature to determine how cross-education affects balance performance in the population with CAI. Therefore, the purpose of our study was to conduct a systematic review of the literature that has examined the cross-education effects of unilateral exercise on balance performance in those with CAI.

METHODS

We conducted a systematic review in accordance with guidelines established by the 2020 Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA; https://www.prisma-statement.org/) statement to examine the effect of unilateral exercise on balance performance in individuals with CAI. On October 27, 2020, we searched the PubMed, SPORTDiscus, and CINAHL online databases for relevant studies using the following terms: ankle instability AND rehabilitation OR therapy OR exercise OR intervention OR training.

After the duplicates were removed, 2 authors (B.L.-P. and S.C.) eliminated additional studies based on the titles and abstracts. The same authors reviewed the full texts of the remaining studies to ensure they met the inclusionary criteria. Any disagreements were settled by a third author (R.S.M.). To be included, each study must have met the following criteria:

1. Study was published in English.

2. Study participants had either CAI or a similar condition, such as functional ankle instability or mechanical ankle instability.

3. Study participants must have undergone a unilateral lower extremity exercise program.

4. Study participants must have undergone testing of balance performance of their untrained limbs before and after the intervention.

Because a preliminary search resulted in few studies that met our criteria, we opted to include randomized controlled trials and nonrandomized intervention studies.

RESULTS

Search Results

Full results of the literature search are outlined in Figure 1. The database searches yielded 3 texts that fit the inclusion criteria, and we added 3 articles from a hand search to bring the total to 6. The hand search consisted of reviewing the reference lists of related articles to find articles that were not identified by the database search. Our database search returned 86 additional studies that explored the effects of therapeutic exercise in populations with ankle injuries. Most failed to meet multiple inclusionary criteria; 2 studies initially appeared to be eligible but were ultimately excluded. One investigation²⁵ examined the balance performance of the involved and uninvolved limbs after rehabilitation, but the intervention was applied bilaterally. Other researchers²⁶ evaluated the balance performance of the involved and uninvolved limbs after unilateral rehabilitation among participants described as having a history of acute ankle sprain, not CAI. Two studies were funded by external sources,^27,28 1 received no external funding,¹² and 3 groups did not report whether funding was received.^29–31

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Certainty of Evidence and Risk of Bias

Each article's score on the Downs and Black checklist is presented in Table 1. The articles' risk of bias ranged from high to low. One article²⁹ exhibited a low risk of bias with a score of 19, 1 article¹² exhibited a moderate risk of bias with a score of 17, and the remaining 4 articles^27,28,30,31 exhibited a high risk of bias with scores <14.

Table 1 Certainty of Evidence Assessment

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Study Findings

Information extracted from each study is reported in Table 2. Hedges g effect sizes for the untrained limb ranged from small to large, and 9 of 14 comparisons had CIs that crossed 0 (Figure 2). Three studies^12,27,29 demonstrated large effect sizes and CIs that did not cross 0. Each of those comparisons indicated a positive effect on balance in the untrained limb.

Table 2 Study Characteristics

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DISCUSSION

We aimed to conduct a systematic review of the existing literature to examine the effects of unilateral lower extremity rehabilitation on balance performance of the untrained limb in the population with CAI. Our main finding was that in 5 of the 6 studies, unilateral lower extremity rehabilitation resulted in improved balance performance of the untrained limb from baseline to postintervention testing. Furthermore, researchers^12,27,29 in 3 studies observed that cross-education effects were associated with large, statistically significant effect sizes, indicating that a clinically meaningful change likely occurred. Although comparisons of these results with those of an untrained control group would strengthen the evidence of a cross-education effect, 4 of 6 studies^27,28,30,31 did not include an untrained control group. In 1 investigation¹² that included an untrained control group, balance performance did not improve from baseline to postintervention testing. Similarly, the control group in another study²⁹ failed to exhibit the improved balance performance demonstrated by the experimental group. We are unaware of any other systematic review that has examined cross-education effects on balance performance in participants with a lower extremity musculoskeletal injury. However, our findings support those of other researchers^32,33 who found that unilateral balance training in healthy individuals resulted in improved balance performance of the untrained limb. Our results also agree with those of a systematic review³⁴ whose authors concluded that cross-education effects can occur in healthy adults after unilateral resistance training, but the results might ultimately depend on the training protocol.

The studies in the current systematic review varied in terms of which limb was chosen for training. Generally, the authors found cross-education effects whether the involved limb or the uninvolved limb was trained. Clinically, patients with CAI could benefit from cross-education balance effects in the involved or uninvolved limb. In patients with CAI who have experienced a recurrent ankle sprain, balance training of the uninvolved limb could be an effective means of enhancing postural control before resuming weight-bearing activity on the involved side. Conversely, balance training of the involved side might be beneficial to a patient with CAI who presents with bilateral balance deficits. Although the authors³⁵ of a previous systematic review and meta-analysis determined that individuals with CAI did not experience bilaterally impaired balance, subsequent work¹⁸ suggested that some might experience central nervous system alterations that reduce postural stability of the uninvolved limb.

Most of the cross-education effects on balance performance were generated by a form of balance training. Because we only evaluated investigations that analyzed cross-education effects, naturally each balance-training study was focused on single-legged stance exercises. Another important similarity was that all balance-training protocols were long term, lasting 4 to 12 weeks with 2 to 5 sessions per week. Aside from these similarities, the protocols varied in their use of dynamic movements, unstable surfaces, and visual feedback. Balance training in various modes, frequencies, and durations has consistently demonstrated benefits to the balance performance of individuals with CAI.¹⁹ However, variations in exercise intensity, fatigue, and movement pacing might affect the cross-education of the untrained limb after unilateral training.³⁴ The reasons why the authors²⁸ of 1 study did not find a cross-education effect remain unclear, especially because it included a 12-week progressive balance-training protocol. A possible explanation is that the participants did not appear to have bilateral balance deficits at baseline, and thus, had little room for improvement of the untrained limb. Isokinetic strength training of the ankle joint was explored in only 1 study³¹ and improved the balance performance of the untrained healthy limb. Others³⁶ found that strength training resulted in improved balance performance of the trained limb in people with CAI. Therefore, enhanced motor control might be experienced bilaterally in this population. This assertion is further supported by a study³⁷ of healthy participants in which unilateral isokinetic hip strengthening improved static balance performance of the contralateral limb.

Multiple balance outcomes, including stability indices, postural sway, 1-legged standing test, BESS, and SEBT, were effective in detecting cross-education effects for balance performance among individuals with CAI. However, the effectiveness of some outcomes for identifying cross-education effects remains uncertain. Although investigators^27,28 in 2 studies examined postural sway using center-of-pressure data, only 1 noted a cross-education effect. Additionally, the authors²⁹ of the study that included the BESS and SEBT combined the data for limbs, groups, and times, which might have obscured the true strength of the cross-education effects. Postural sway and the BESS and SEBT have all demonstrated the ability to detect balance performance improvements after rehabilitation,¹⁹ but their value in assessing cross-education needs to be explored further.

CONCLUSIONS

Our primary finding in this systematic review was that unilateral therapeutic exercise can improve the balance performance on the untrained limb of individuals with CAI. The majority of included studies indicated that balance training was an effective technique, but additional evidence suggests that unilateral ankle-joint strengthening can also improve contralateral balance performance. Because of the limited volume of research on this topic, future investigators must determine which training protocols are most effective in this population.