Clinical Scenario

There is a high prevalence of lateral ankle sprains in sports, which accounts for between 7% and 15% of all athletic injuries.^1,2 A recurrence of ankle sprains with symptoms of prolonged ankle dysfunction characteristically represents the condition known as chronic ankle instability (CAI).³ Of the individuals sustaining lateral ankle sprains, an estimated 40% of these patients can progress to CAI.⁴ CAI has primarily been described as “an encompassing term used to classify a subject with both mechanical and functional instability of the ankle joint.”⁵ People often express their ankles as “giving way” and lacking functional stability.⁵ CAI is associated with muscle weakness, increased pain, decreased ankle range of motion, or decreased neuromuscular control of their ankle during functional movements.⁵ It has recently been reported that patients with CAI were weaker in isometric eversion, inversion, and plantar flexion compared with healthy controls.⁶ Muscle inhibition frequently occurs following ankle sprains.⁷ The subsequent inhibition can create strength deficits that may last up to 6 months and can increase the probability of lateral ankle sprains.⁸ Not only does neuromuscular control of the ankle decrease in CAI, but the quality of arthrokinematics is also decreased.⁹ Individuals with CAI have been shown to have altered activation of the fibularis longus and tibialis anterior.^1,2 These muscles have a critical role as dynamic stabilizers of the ankle joint during tasks such as walking, running, landing, and other sporting activities.¹⁰ As such, the presence of altered muscle activation may be a contributing factor to the bouts of instability and recurring ankle sprains often endured among individuals with CAI.

Time loss for sport is often due to injury, and therapeutic exercise is often used to rehabilitate following injury to return to activity or sports participation. Therapeutic exercise is often used to provide patients with enhanced muscle activation, muscular strength, increase neuromuscular control, improve patterns of dysfunction, or decrease pain. The use of blood flow restriction (BFR) training as a rehabilitation tool has been gaining interest. BFR uses an inflatable cuff tourniquet placed around the limb, decreasing arterial and venous flow, and causing a hypoxic environment in which the muscles have an increased demand for oxygen. The hypoxic conditions with BFR create increased muscular fatigue even under low load intensities. During this hypoxic environment, there are many training-related muscular adaptations that are not completely understood but include physiologic changes in muscle metabolism, metabolic stress, and neuromuscular recruitment strategies which are still being studied.^1,2 It is suggested that increased cellular swelling, upregulation of protein synthesis, and enhanced cell signaling are responsible for acute gains in strength and hypertrophy.¹¹ The induced muscle hypoxia created with BFR increases metabolic stress and may mediate a shift toward high-order motor unit recruitment that is normally not experienced with low-load exercise.¹²

BFR has been shown to increase adaptations from therapeutic exercise, including strength, endurance, and hypertrophy.¹ BFR training has consistently enhanced training adaptations in muscular strength and hypertrophy when utilized with low-load resistance exercise at 20%–30% of the one-repetition maximum intensity.¹⁰ BFR under low loads has been found to be equally effective to traditional strength training in patients with knee osteoarthritis,¹³ post anterior cruciate reconstruction,¹⁴ and anterior knee pain,¹⁵ among other conditions provided in the literature. BFR can be used as an intervention when heavier loads might not be tolerable or in instances of immobilization or nonweight bearing preventing muscular weakness and mitigating limb circumference from disuse atrophy.¹⁶ Individuals experiencing CAI might not tolerate increased loads and exercises that require dynamic stability appropriately and thus may benefit from BFR during recovery. Without appropriate intervention, these individuals can be at risk for recurrent injury, and may not progress effectively. Therefore, the purpose of this appraisal is to evaluate the effects of BFR on muscular outcomes in the treatment and management of CAI.

Focused Clinical Question

Is there evidence to suggest that BFR training improves muscle strength, muscle activation, and/or cross-sectional area (CSA) of the lower leg musculature in individuals with CAI?

Search Strategy

Results of Search

The initial search of literature resulted in four possible studies for inclusion. After investigation by the researchers, three studies were found to meet the inclusion and exclusion criteria.^1,2,10 The three studies were evaluated by the Levels of Evidence Oxford CEBM (2011) and the PEDro scale for randomized controlled trials. Figure 1 represents the summary of search history and inclusion studies. The three studies evaluated one of the muscular outcomes of either strength, CSA, or muscle activation as a result of BFR training. Each of the three studies reported a type of significant improvement in muscular outcomes using BFR for CAI. Table 1 represents the best evidence and conclusion by the authors.

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—Summary of search history and included studies.

Citation: International Journal of Athletic Therapy and Training 29, 5; 10.1123/ijatt.2023-0094

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Results of Evidence Quality Assessment

The studies included in this CAT used the defined inclusion and exclusion criteria. The studies chosen are the best current evidence to examine muscular outcomes of BFR in CAI and are presented in Table 2. The validity of the Killinger et al.¹ and the Burkhardt et al.² studies was determined by the authors to be of Level 2 quality using the CEBM (2011) definitions for the level of evidence.¹⁷ Both studies were similar in study design investigating muscle activation using EMG. Subject participant requirements were identical requiring that subjects have a history of lateral ankle sprain greater than 12 months before testing, with subjective indications of “giving way” and “feelings of instability.” Both studies required study participants to score >11 on the Identification of Functional Ankle Instability scale and both utilized the same BFR equipment in the collection of their data (Delfi Medical Innovations, Inc.).

The validity of the study by Werasirirat and Yimlamai¹⁰ was determined to be 8/10 consistently by two authors (Spencer and Sales) and considered to be of “good” quality. The lack of blinding of study participants and therapists administering the therapy was the only weakness of the studies. PEDro scores for Killinger et al.¹ and the Burkhardt et al.² studies were scored at 6/10 and also considered to be of “good quality.” Allocation concealment and blinding of subjects as well as blinding of therapists administering the therapy and blinding of assessors were consistent weaknesses of both studies.

Implications for Practice, Education, and Future Research

Continuous dysfunction of the lower leg muscles that dynamically stabilize the ankle joint is present in patients with CAI that may contribute to the chronicity of instability.¹ Individuals with CAI are reported to experience arthrogenic muscle inhibition of the fibularis longus^21,22 and soleus,²³ with subsequent inhibition of the muscles proximal to the injured ankle (hamstrings).²⁴ A recent meta-analysis has shown consistent deficits in concentric eversion strength in patients with CAI.²⁵ Evidence also suggests that patients with CAI exhibit altered muscle activation of the fibularis longus and the tibialis anterior during functional movements²⁶ with significant reductions in the CSA of the fibularis longus following ankle sprains.²⁷ Traditional rehabilitation for CAI has been focused on functional improvements in strength, balance, and functional activities in the surrounding ankle musculature. All three studies identified in this appraisal suggested that therapeutic exercise with BFR training enhances muscular adaptations in patients with CAI.^1,2,10 Two of the three studies investigated the immediate acute changes in muscle activation responses (EMG) to BFR^1,2 while the third study examined changes after training with BFR combined with a 4-week supervised rehabilitation program on improving muscle strength and size.¹⁰ In patients with CAI, muscle activation of the fibularis longus and tibialis anterior was greater using BFR compared with no BFR exercise during maximum voluntary isometric contractions¹ while muscle activation of the soleus muscle during dynamic balance tasks improved significantly.² From these studies, we can understand that there is moderate evidence that BFR can increase muscle activation and strength compared with groups not using BFR.^1,2 Significant improvements were also noted in peak torque strength of the ankle everters, dorsiflexors, and plantar flexors after a 4-week rehabilitation program using BFR compared with no BFR.¹⁰ In addition, CSA of the fibularis longus was significantly increased following an intervention of BFR with rehabilitation exercises.¹⁰ This CAT suggests that the use of low-load BFR training with CAI is more effective compared with exercise without BFR training to improve muscular outcomes of strength and muscle CSA.

Patients with CAI may be limited by the inability to tolerate stresses or utilize loads sufficient to induce gains in strength and hypertrophy thus limiting therapeutic progression. The American College of Sports Medicine recommends load intensities of 60%–80% of the one-repetition maximum to increase strength and hypertrophy.²⁸ BFR has been consistently shown to enhance training adaptations in muscular strength and hypertrophy when combined with low-load resistance intensities at 20%–30% one-repetition maximum effort.¹⁰ Much of the research investigating BFR has applied the low load with four sets at 30, 15, 15, and 15 repetitions each. This was a consistent dosing among the studies included in this analysis.^1,2,10 Rest periods are often short to create increased fatigue and load. Rest periods between sets ranged from 30 to 60 s in the Burkhardt et al.² study, with the other studies consisting of 30¹⁰ and 45 s of rest.¹ Inflation pressure of the cuff was also consistent at 80% arterial limb occlusion pressure^1,10 while the Burkhardt et al.² study utilized an arterial limb occlusion pressure ranging between 40% and 80%.

One of the first to investigate BFR in patients with CAI using EMG was Killinger et al.¹ In this study, BFR created moderate to large and clinically important increases in acute muscle activation of the fibularis longus and tibialis anterior muscles.¹ Average muscle activation of the fibularis longus in isometric eversion was 5.6% greater in subjects who utilized BFR (p = .03; effect size [ES] = 0.41; 95% CI [0.1, 10.3]). The BFR group also demonstrated 7.7% greater activation of the tibialis anterior muscle during dorsiflexion maximum voluntary isometric contractions (p = .01, ES = 0.72; 95% CI [2.7, 13.3]). The presence of greater acute muscle activation with BFR exercise suggests that higher threshold motor units and muscle fibers are activated to complete the exercise. Although there were no strength comparisons made in this study between the two groups, understanding that increased activation of motor units using BFR can correlate with increased adaptations of enhanced muscle strength. Future studies should examine clinically relevant outcomes and the effects of BFR on training adaptations in muscle strength, endurance, and hypertrophy particularly in muscles (fibularis longus and soleus) that commonly experience inhibition in CAI populations.

Findings of increased muscle activation in the ankle evertors and dorsiflexors conflict the results of the study by Burkhardt et al.,² in which the authors observed outcomes using BFR during the dynamic balance exercise. Participants performed four sets of reaching in anterior, posteromedial, and posterolateral directions (Y-balance) performing repetitions of 30, 15, 15, and 15 each. Participants only exerted 80% of their maximal reach for each repetition. Participants repeated the study a second time within 24–48 hr (washout period) following their initial trial in the other group. They found increased acute muscle activation in vastus lateralis and soleus muscles during dynamic balance exercises using BFR but did not observe a significant increase in muscle activation of the fibularis longus and anterior tibialis during the Y-balance dynamic balance exercise.² These findings could be contributed to the more functional exercise of single-leg balance versus a supine leg maximum voluntary isometric contraction.^1,2 Balance exercises might require a larger demand on the soleus compared with the anterior tibialis in this particular dynamic movement.² By contrast, the dorsiflexion isometric exercises performed in the Killinger et al.¹ study would recruit more activation in the anterior tibialis compared with the soleus in a functional stance position. Therefore, it is possible that the difference in findings can be due to the vast difference in exercise types and differing metabolic demands of the muscles for the alternative findings. Nevertheless, subjects using BFR during dynamic balance exercises exhibited greater muscle activation of the vastus lateralis (p < .001; ES = 0.86; 95% CI [0.28, 1.44]) and soleus muscles (p = .03; ES = 0.32; 95% CI [−0.24, 0.87]).

The study by Werasirirat and Yimlamai¹⁰ used BFR with rehabilitation exercises in CAI athletes and found that 4 weeks of supervised rehabilitation exercises with BFR was superior to traditional rehabilitation exercises alone in improving muscle strength and muscle size. Subjects participated in rehabilitation exercises three times a week for four consecutive weeks. All participants did the same 30 min of therapeutic exercise, consisting of double- and single-leg heel raises, double- and single-leg squats, Both Sides Utilized (BOSU) standing double and single leg, and Y-balance functional reaching.³ An isokinetic dynamometer was used to measure the highest peak torque in hip extension, hip abduction, ankle dorsiflexion, ankle plantar flexion, ankle eversion, and ankle inversion. CSA of the fibularis longus was measured with a diagnostic ultrasound. The BFR group showed significant improvements in mean peak torque values for the hip extension (p < .001; d = 0.13; 95% CI [−0.51, −0.33]), hip abduction (p = .001; d = 0.08; 95% CI [−0.18, −0.07]), ankle dorsiflexion (p = .004; d = 0.05; 95% CI [−0.12, –0.03]), ankle plantar flexion (p < .001; d = 0.06; 95% CI [−0.22, −0.13]), and ankle eversion (p < .001; d = 0.11; 95% CI [−0.36, −0.19]). The control group only had a remarkable increase in peak torque in hip extension and hip abduction. Although the increases in mean peak torque were significant, they were interpreted as very trivial due to the very low ESs. The BFR exercise group did though show greater improvements in the relative strength of the ankle plantar flexors (p = .30; d = 1.31; 95% CI [0.02, 0.34]) and ankle evertor muscles (p = .20; d = 1.36; 95% CI [0.04, 0.52]), both with large ESs indicating a likely beneficial change. In addition, a significant increase in CSA of the fibularis longus was noted in the BFR group compared with the non-BFR control (p = .004; d = 1.11; 95% CI [0.03, 2.30]). These results showed promising improvements of increased hypertrophy and strength over a 4-week period using BFR as an adjunct to traditional therapeutic exercise.

In general, those with CAI have repetitive episodes or perceptions of the ankle giving way, which is often accompanied by symptoms of persistent pain, muscle weakness, and recurrent ankle sprains.²⁹ Rehabilitation of these injuries can be extremely challenging due to the altered activation of the evertors and dorsiflexors, and notable muscular weakness seen in this injury. Failure to adequately activate and strengthen these muscles will continue to impair the patient’s ability to control rearfoot motion thereby permitting the ankle to “give way.”²¹ The findings of this appraisal support the clinical question that there is moderate evidence to support the use of BFR in improving strength, muscle activation, and muscle size in patients with CAI. Because BFR has been found to generate benefits utilizing low-load intensities, this makes BFR an excellent treatment strategy for clinicians desiring better muscular outcomes in treating patients with CAI.

In conclusion, BFR can be beneficial for those experiencing CAI to increase the demand placed on the muscles, resulting in increased muscle activation, increased strength, and CSA compared with traditional resistance exercise alone. Having a history of lateral ankle sprain and instability can predispose patients to recurrence of injury, with a predisposition resulting in CAI. Evaluating biomechanics, muscle deficiency, ankle range of motion, and neuromuscular control will be beneficial in addressing the rehabilitation of CAI with the use of BFR. There is a need for further research investigating these effects in BFR training for CAI. Well-designed studies should be done to obtain more adequate data on how BFR can increase strength in lower leg muscles. In addition, future studies are needed to investigate responses to BFR in functional movement and performance tasks, walking, and fall risks in older adult populations with CAI. Investigating sensorimotor and balance adaptations to BFR during balance training activities in patients with CAI are also warranted. Return to sport or functional activities following diagnosis of CAI can take months. Clinicians such as athletic trainers, physical therapists, physiotherapists, or rehabilitation specialists can use BFR to improve acute clinical outcomes and perhaps enhance recovery time in patients with CAI.

This critically appraised topic should be reviewed in 2 years, or when additional best evidence becomes available, to determine whether additional best evidence has been published that may change the clinical bottom line for the research question posed in this review.

CAT Kill Date: July 2025

CATs have a limited life span and should be revisited approximately 2 years after publication or as new evidence becomes available in that time frame (see https://doi.org/10.1123/ijatt.2018-0093).