Effect of Blood Flow Restriction Training on Quadriceps Muscle Strength, Morphology, Physiology, and Knee Biomechanics Before and After Anterior Cruciate Ligament Reconstruction: Protocol for a Randomized Clinical Trial

Abstract

Background

Despite best practice, quadriceps strength deficits often persist for years after anterior cruciate ligament reconstruction. Blood flow restriction training (BFRT) is a possible new intervention that applies a pressurized cuff to the proximal thigh that partially occludes blood flow as the patient exercises, which enables patients to train at reduced loads. This training is believed to result in the same benefits as if the patients were training under high loads.

Objective

The objective is to evaluate the effect of BFRT on quadriceps strength and knee biomechanics and to identify the potential mechanism(s) of action of BFRT at the cellular and morphological levels of the quadriceps.

Design

This will be a randomized, double-blind, placebo-controlled clinical trial.

Setting

The study will take place at the University of Kentucky and University of Texas Medical Branch.

Participants

Sixty participants between the ages of 15 to 40 years with an ACL tear will be included.

Intervention

Participants will be randomly assigned to (1) physical therapy plus active BFRT (BFRT group) or (2) physical therapy plus placebo BFRT (standard of care group). Presurgical BFRT will involve sessions 3 times per week for 4 weeks, and postsurgical BFRT will involve sessions 3 times per week for 4 to 5 months.

Measurements

The primary outcome measure was quadriceps strength (peak quadriceps torque, rate of torque development). Secondary outcome measures included knee biomechanics (knee extensor moment, knee flexion excursion, knee flexion angle), quadriceps muscle morphology (physiological cross-sectional area, fibrosis), and quadriceps muscle physiology (muscle fiber type, muscle fiber size, muscle pennation angle, satellite cell proliferation, fibrogenic/adipogenic progenitor cells, extracellular matrix composition).

Limitations

Therapists will not be blinded.

Conclusions

The results of this study may contribute to an improved targeted treatment for the protracted quadriceps strength loss associated with anterior cruciate ligament injury and reconstruction.

Over 200,000 individuals sustain an anterior cruciate ligament (ACL) tear in the United States annually.^1–3 Impairments in quadriceps strength following ACL reconstruction are common^4–14 and are related to altered knee biomechanics.^15–20 Additionally, quadriceps weakness has been associated with negative short- and long-term outcomes, such as low return-to-sport rate,^21,22 reduced quality of life,^13,23 and early-onset osteoarthritis.²⁴ Thus, an emphasis on interventions to address impairments in quadriceps strength is needed to limit these negative consequences.

A growing body of literature shows that peripheral changes (morphologic and cellular) in the quadriceps occur following ACL injury and reconstruction that result in protracted quadriceps weakness.^8,25–28 Traditional physical therapy techniques have had limited effectiveness on mitigating these alterations in the composition of the quadriceps muscle.^8,25–27 However, blood flow restriction training (BFRT) has been suggested as a potential method to improve these deficits.^29–31 With BFRT, a pressurized cuff is applied to the proximal thigh that partially occludes blood flow as the patient exercises. It is believed that the accumulated effects of fatigue, mechanical tension, metabolic stress, and reactive hyperemia contribute to promoting adaptation of the quadriceps with minimal strain.^32–39 Therefore, patients are able to train at reduced loads and may receive the same training benefits as if they were training under high loads.

BFRT has been successfully used in healthy populations, demonstrating equivalent and/or greater strength gains than traditional forms of exercise.^32,40–42 Additionally, BFRT has been shown to preferentially improve muscle fiber cross-sectional area and increase satellite cell abundance in healthy patients.^{33,35,43–46} These properties of the quadriceps are negatively impacted after ACL injury and persist despite reconstruction and rehabilitation.⁴⁷ Whether BFRT is able to address these identified cellular properties is poorly understood.^{33,35,43–46} Establishing the efficacy of BFRT to positively alter these cellular properties, when detrimentally affected by injury, is crucial to translate the rehabilitative potential of BFRT.

To date, very few studies have tested BFRT following an ACL reconstruction.^48–50 The results of these studies have been conflicting in regards to quadriceps strength and cross-sectional area, and there have been no comparisons of cellular alterations and movement mechanics. Previous work is also limited by inconsistent experimental procedures, including length of total occlusion time, intermittent versus continuous occlusion, and length of rehabilitation. Therefore, further investigation is needed to determine if BFRT is able to mitigate strength loss in injured populations for whom high-load strengthening exercises are contraindicated.

The purpose of this study is to evaluate the effect of BFRT on quadriceps strength and knee biomechanics and to identify the potential mechanism(s) of action of BFRT at the cellular and morphological levels of the quadriceps. The primary objective is to evaluate the effect of BFRT training on quadriceps strength, assessed via peak quadriceps torque and rate of torque development, in patients who have had an ACL reconstruction. The secondary objectives are measuring changes in (1) knee biomechanics, (2) quadriceps muscle morphology, and (3) quadriceps muscle physiology after using BFRT in patients who have had an ACL reconstruction. Our hypothesis is that BFRT will restore quadriceps strength, knee biomechanics, and quadriceps morphology and cellular composition in patients who have had an ACL tear and reconstruction to a greater extent than standard of care.

Methods

Trial Design

This is a randomized, double-blind, placebo-controlled clinical trial that began in Fall 2017 and will continue for 5 years. All components of the study will be completed at the University of Kentucky except for the immunohistochemical assays of the muscle biopsies, which will be performed at the University of Texas Medical Branch. The trial will be reported according to SPIRIT, TIDieR, and CONSORT guidelines. Figure 1 outlines the trial phases.

Figure 1.

Flowchart of the clinical trial. ACL = anterior cruciate ligament; BFRT = blood flow restriction training; MRI = magnetic resonance imaging.

Participants

Participants will be recruited from the University of Kentucky Sports Medicine Clinic. We will enroll up to 60 participants in the study, with each participant providing their written consent prior to enrollment. With a potential attrition rate of up to 20%, we expect at least 48 participants to complete the study, with approximately 24 participants per treatment group. Specific inclusion and exclusion criteria for entrance into the study are detailed in Table 1.

Table 1.

Inclusion and Exclusion Criteria for Entrance Into the BFRT Clinical Trial for Patients With ACL Injury^a

Inclusion Criteria
Male or female (15–40 years of age); must be skeletally mature with closed physes
Diagnosis of ACL tear with planned surgery confirmed via clinical examination and MRI within the past 8–10 weeks
No previous ACL injury or reconstruction on either the involved limb or noninvolved limb
At least a 5/10 on the Tegner Activity Scale
Pass the PARQ questionnaire with the exception of question 5, “Do you have a bone or joint problem?”, because the participant pool being recruited for this study will have an ACL tear and having this injury does not change the risk for the participant
Exclusion Criteria
Complete knee dislocation
Surgical management requiring repair of other structures within the knee other than the ACL and menisci (such as PCL, MCL, LCL)
Any current or previous conditions or surgeries that might affect gait
Body mass index ≥35
Pregnant
Spinal fusion
Any implanted medical device or other contraindication for MRI
History of deep vein thrombosis and/or varicose veins or family history of deep vein thrombosis
Taking Warfarin/Coumadin, Clopidogrel/Plavix, Rivaroxabn/Xarelto, Dabigatran/Pradaxa, Apixaban/Eliguis, Edoxaban/Savaysa, Betrixaban, or any other anti-coagulant that may cause excess bleeding
Allergic to Betadine or Xylocaine HCl
Recent inflammation, bleeding disorders, active bleeding, or infection within the lower limbs
Diabetic or have uncontrolled hypertension
Diminished capacity to provide informed consent
Unfeasible to attend regular physical therapy and study visits

^aACL = anterior cruciate ligament; BFRT = blood flow restriction training; HCl = hydrochloride; LCL = lateral collateral ligament; MCL = medial collateral ligament; MRI = magnetic resonance imaging; PARQ = physical activity readiness questionnaire; PCL = posterior cruciate ligament.

Randomization/Allocation/Blinding

Upon enrollment into the study, participants will be randomly assigned to receive physical therapy plus active BFRT (BFRT group) or physical therapy plus placebo BFRT (standard of care group). All assessments will be done by the outcome assessors who are separate from the physical therapists applying BFRT to further maintain blinding and control for bias. The randomization schedule will be created by the study biostatistician, viewed and updated by the physical therapists, and implemented in permuted blocks of 4 for each sex using R (version 3.4.0 or above) to ensure adequate distribution of all groups across the collection period. The physical therapists cannot be blinded to the treatment, because they will administer the BFRT session and will be responsible for all discussions with the participants pertaining to BFRT.

Interventions

Physical therapy

Both groups will receive standard pre- and postsurgical physical therapy, which will focus on range of motion, muscle activation, functional mobility, hip strengthening, core stability, balance, and gait training. eAppendix 1 (available at https://academic.oup.com/ptj) shows the interventions for both groups throughout the plan of care.

Presurgical physical therapy will involve visits 3 times per week for 4 weeks. This phase will focus on effusion management, range of motion, strength, and gait (eAppendix 1, Part A).

Postsurgical physical therapy will involve visits 3 times per week for 6 to 7 months. This phase will be divided into 3 stages. Stage 1 will be initiated within 3 days postsurgery and will continue for 2 to 6 weeks postsurgery. This stage will involve effusion management, muscle activation, range of motion, and returning to walking without the use of crutches (eAppendix 1, Part B). Stage 2 will be initiated once all goals of Stage 1 have been achieved, typically 4 to 6 weeks postsurgery. This stage will involve progression of strengthening, proprioception exercises, and functional movement patterns (eAppendix 1, Part C). Stage 3 will be initiated once all goals of Stage 2 have been achieved, typically 3 to 5 months after surgery. This stage will involve introduction to dynamic agility, running, and impact training in preparation for return to activity and sport (eAppendix 1, Part D).

Blood flow restriction training

BFRT will use 1 of 2 systems (Brand 1 and Brand 2). Brand 1 will be utilized as the active unit for the BFRT group with the restriction pressure set per manufacturer's instructions, whereas Brand 2 will be utilized as the placebo unit for the standard of care group with minimal restriction pressure (less than 20 mmHg). For both groups, the band will be placed proximally on the thigh as per manufacturer's instructions and will be controlled by a central unit that continuously monitors the pressure in the leg. The central unit will be shrouded from the participants; therefore, the participants will not be able to see what the restriction pressure is set to. All participants will perform the same exercises with an emphasis on quadriceps strengthening. This step will control for the potential effect of differing exercise programs and allow us to more accurately determine the efficacy of BFRT.

Each BFRT session will last for approximately 20 minutes. Participants will receive BFRT for 4 weeks presurgery. BFRT will resume 2 weeks postsurgery and finish 4 to 5 months postsurgery at the time of Visit 4 (Fig. 1). Participants with an isolated ACL reconstruction will finish at 4 months postsurgery, whereas participants with an ACL reconstruction and meniscus repair will finish at 5 months postsurgery due to initial weight-bearing and ROM restrictions.

Presurgery exercises and later stage postsurgery exercises will consist of the seated knee extension machine, seated leg press machine, box step ups and step downs, and wall squats with exercise ball (eAppendix 2, Part A, available at https://academic.oup.com/ptj). Early stage postsurgery exercises will be non-weight–bearing exercises consisting of quadriceps sets, short arc quads, long arc quads, prone terminal knee extensions, and straight leg raises (eAppendix 2, Part B). Exercise intensity will be progressively increased if the participant reports less than a 7 on the rating of perceived effort scale (Fig. 2). All exercises will be performed at the end of each physical therapist visit to minimize the effect of quadriceps fatigue on the other exercises that the participants will be performing as part of their visit.

Figure 2.

Rating of perceived effort. The scale begins at 0, which is defined as no physical effort is taking place. This can be likened to the perception of effort sitting on an exercise machine but not having to exert any effort to complete the activity. The scale ends at 10, which is described as the maximum perceivable effort. This can be likened to the perception of effort when, despite putting forth as much exertion as you can, you cannot physically complete the activity being attempted.

Treatment Adherence

Participants must attend 80% of their physical therapist visits and cannot have more than 12 consecutive days between visits up until the time point of Visit 4. Participants with an isolated ACL reconstruction must attend 48 of 60 visits during this timeframe (1 month presurgery and 4 months postsurgery), and participants with an ACL reconstruction and meniscus repair must attend 58 of 72 visits during this timeframe (1 month presurgery and 5 months postsurgery). Any participant not meeting this requirement will be withdrawn from the study and omitted from the analysis. Participants will continue with standard physical therapy visits until conclusion of the study at 6 to 7 months.

Physical Therapist Training and Treatment Fidelity

The physical therapists underwent a structured period of training for 1 month, which involved instruction on the theory and application of BFRT, practice trials of the BFRT protocol with healthy individuals, and co-treating while following the standard rehabilitation program to ensure consistency among the physical therapists. Strict adherence and compliance with the study's manual of operations will be maintained with any protocol deviations being recorded and reviewed to assist in research adherence to study protocols. All potential adverse events will be reported within 24 hours to the Institutional Review Board and the Data and Safety Monitoring Board. An adverse event form will be completed, collected, and analyzed for all adverse events. Subsequent follow-up will then follow standard operating procedures as outlined in Figure 3. The physical therapists will complete standardized treatment notes for each visit, and treatment fidelity will be assessed using a treatment checklist and audits.

Figure 3.

Flow of adverse event reporting.

Outcome Measures

The primary outcome measure will be peak quadriceps strength and rate of torque development measured both isometrically and isokinetically. For the isometric measurements, each participant will perform 5 maximal isometric contractions at 90 degrees of knee flexion with 30 seconds of rest in between each contraction. For the isokinetic measurements, each participant will perform 10 repetitions of knee flexion and extension at 150 degrees per second. Quadriceps strength will be assessed with a Biodex Multi-Joint System 4 Isokinetic Dynamometer (Biodex Medical Systems, Shirley, NY, USA) at Visits 2, 3, 4, and 5 (Fig. 1).

The secondary outcome measures will include knee biomechanics as well as quadriceps muscle morphology and physiology. The participant's knee biomechanics, including knee extensor moment, knee flexion excursion, and knee flexion angle, will be assessed via 3-dimensional motion analysis. Participants will perform walking (Visits 2, 3, 4, and 5), step downs (Visits 2, 3, 4, and 5), running (Visit 5), and drop vertical jumps (Visit 5). To evaluate quadriceps muscle morphology, we will assess physiological cross-sectional area (PCSA) and T1 rho relaxation times with magnetic resonance imaging (MRI) at Visits 2 and 4. Lastly, we will evaluate quadriceps muscle physiology through assessments of muscle fiber type, muscle fiber size, satellite cell proliferation, fibrogenic/adipogenic progenitor cells, and extracellular matrix composition from muscle biopsies collected at Visits 2 and 4 (Fig. 1). A variety of immunohistochemical and histochemical assays will be used to evaluate these cellular features in muscle.^47,51,52 Assessing quadriceps muscle morphology and physiology will assist in understanding the mechanisms underlying the effects of BFRT.

Data Analysis

Power calculations

Based on data from our preliminary pilot study, a sample size of 48 participants (24 per group) was calculated to detect a between-group difference of 39 ± 47 Nm in quadriceps strength, with 80% power, at a 2-tailed significance level of .05. The power analyses for additional outcome measures are provided in Table 2. With a conservative estimate of a potential attrition rate of up to 20%, a total of 60 participants will be recruited (30 per group).

Table 2.

Power Analysis for Primary and Secondary Outcome Measures^a

Outcome Measure	Estimated SD	Difference to Detect	Powerb
Peak quadriceps torque (Nm)	47	39	80%
Knee extensor moment (Nm/kg)	0.50	0.42	81%
Knee flexion angle (degrees)	4.7	4	82%
PCSA (cm2)	32	27	82%
T1 rho relaxation time (s)	0.004	0.005	>90%
Muscle fiber type (type 2A fiber relative frequency)	0.09	0.08	85%
Satellite cell abundance (Pax7 + satellite cells/fiber)	0.05	0.05	>90%
Fibroblasts (Tcf4 + fibroblasts/mm2)	21	18	83%
FAPs (PDGFRα + FAPs/mm2)	1.3	1.1	82%

^aFAPs = fibrogenic/adipogenic progenitors; Pax7 = paired box 7; PCSA = physiological cross-sectional area; PDGFRα = platelet-derived growth factor receptor alpha; Tcf4 = transcription factor 4.

^bPower analyses performed with a sample size of 48 participants and an α = .05

Statistical analysis

Continuous variables will be summarized with descriptive statistics (eg, sample size, mean, standard error). Analysis of variance, analysis of covariance (ANCOVA), and multivariate analysis of variance will be used as appropriate depending on the outcome under analysis. Categorical variables will be summarized with inferential statistics (eg, counts, percentages) and will be analyzed with appropriate statistical methods, such as logistic regression models. We will include potential confounders (eg, age, sex, race, time to surgery) as covariates during analysis. For example, for the primary outcome, quadriceps strength, we plan to perform an ANCOVA on quadriceps strength change scores for all participants in the study, including explanatory variables such as age, sex, and treatment group. Next, if the ANCOVA shows significance, posthoc tests will be performed to identify differences over combinations of treatment group and/or sex. However, if model assumptions are not satisfied, remedial measures (eg, variable transformations, nonparametric methods) will be employed. All data will be analyzed using SAS or R, and data with P values of ≤ .05 will be considered statistically significant. Multiple testing corrections will be made as appropriate.

Role of the Funding Source

The funder (National Institutes of Health, ref. no. AR071398–01A1) played no role in the design, implementation, analysis, interpretation of results, or the decision as to if and where to publish papers.

Discussion

To date, little is known of the effects of BFRT within injured populations. To address this limitation, we will evaluate the effect of BFRT across scales from whole joint mechanics and strength to morphological features of the muscle as well as cellular alterations within the quadriceps. We expect to identify potential mechanisms of action of this intervention within an injured population. This would in turn provide foundational evidence for using this treatment in physical therapy.

Despite the growing use of BFRT, few studies have assessed its use following an ACL reconstruction.^48–50 For example, Takrada et al⁴⁸ reported a significant increase in knee extensor cross-sectional area in individuals receiving BFRT. However, the study followed an outdated rehabilitation protocol in which no exercise stimulus was provided with the BFRT. More recently, Iverson et al⁵⁰ followed a near identical protocol; however, they performed quadriceps exercises concurrently with BFRT. In contrast to Takrada et al,⁴⁸ they found no significant differences in cross-sectional area in participants receiving BFRT compared with a matched control group. Lastly, Ohta et al⁴⁹ reported a significant increase in muscular strength and cross-sectional area in individuals receiving BFRT after an ACL reconstruction compared with a matched control group. In contrast to the other studies, Ohta et al⁴⁹ used a standardized occlusion pressure (180 mmHg) for all participants. Further, none of the studies to date have examined whether the improved cross-sectional area or strength transfers to improved movement mechanics during common tasks such as walking, running, and jumping. The equivocal results and lack of detailed assessments of mechanics among these studies suggest a continued need to assess BFRT in individuals who have had an ACL reconstruction.

The combined use of 2 MRI techniques to evaluate PCSA and the percentage of contractile and noncontractile tissue could provide new insights on the effect of BFRT. Previously, PCSA has been shown to be strongly related to maximum muscle strength.⁵³ Although important insights have already been gained through assessment of muscle cross-sectional area and muscle volume,^54,55 these techniques do not consider the overall architecture of the muscle, such as the pennation angle and muscle fiber tract length. Additionally, through the use of T1 rho, we will delineate the change that is due to the addition of contractile tissues vs noncontractile tissue.⁵³ These sequences will provide additional insight on whole muscle morphology not previously known in the study of the effects of BFRT.

The effects of BFRT after an ACL tear will be evaluated at the cellular level through pre- and postassessment of muscle biopsies taken from the vastus lateralis muscle. Negative alterations such as muscle fiber type switching, expansion of the extracellular matrix, and reduction of muscle satellite cell abundance impede skeletal muscle plasticity.^56–59 We have, in our preliminary studies, reported that many of these alterations occur after the injury and before surgery.^47,51 The administration of BFRT before surgery will allow us to assess the potential it has to reverse or even stop previously reported negative alterations.

We recognize that there are several limitations and design constraints in this study. For example, the physical therapists are not blinded to the treatment groups. We have attempted to minimize potential bias by using standardized scripts for explaining the BFRT intervention and all assessments are performed by the outcome assessors who are blinded to group allocation. Another limitation is that if sample sizes are not large enough, it may not be feasible to conduct an intent-to-treat analysis or missing data imputation for participants who drop out of the study. However, based on our prior studies, dropout is anticipated to be low, and this has been accounted for in our power analyses. We will investigate the effect of omitting these participants in our future publications. In addition, we recognize that there are many potential settings of BFRT and that we are able to assess only 1 set of BFRT treatment parameters. Therefore, there is the potential that more effective criteria exist than those we have chosen.

To ensure that the results of our study will inform physical therapists and have an impact on patient care, results will be presented at scientific conferences and published in academic journals. We also will disseminate the results of this clinical trial to professional groups, including the American Physical Therapy Association (APTA), American College of Sports Medicine (ACSM), and National Institutes of Health (NIH).

Author Contributions and Acknowledgments

Concept/idea/research design: L.N. Erickson, K.C. Hickey Lucas, K.A. Davis, C.A. Jacobs, K.L. Thompson, C.S. Fry, B.W. Noehren

Writing: L.N. Erickson, K.C. Hickey Lucas, K.A. Davis, C.S. Fry, B.W. Noehren

Data collection: L.N. Erickson, K.C. Hickey Lucas, K.A. Davis, P.A. Hardy, A.H. Andersen, B.W. Noehren

Data analysis: L.N. Erickson, K.C. Hickey Lucas, K.A. Davis, K.L. Thompson, P.A. Hardy, A.H. Andersen, B.W. Noehren

Project management: C.A. Jacobs, P.A. Hardy, B.W. Noehren

Fund procurement: A.H. Andersen, B.W. Noehren

Providing facilities/equipment: P.A. Hardy, A.H. Andersen, C.S. Fry, B.W. Noehren

Consultation (including review of manuscript before submitting): L.N. Erickson, K.C. Hickey Lucas, C.A. Jacobs, P.A. Hardy, B.W. Noehren

Sponsor Contact Information: National Institutes of Health (NIH), 9000 Rockville Pike, Bethesda, MD 20892; Phone: 301–496-4000; Email: NIHinfo@od.nih.gov.

Executive Committee: primary investigator (Dr Brian Noehren), site directors (Dr Brian Noehren, University of Kentucky; Dr Christopher Fry, University of Texas Medical Branch), biostatistician (Dr Katherine Thompson), compliance and subject recruitment leader (Dr Cale Jacobs), and medical officer (Dr Darren Johnson).

Steering Committee: primary investigator (Dr Brian Noehren), site directors (Dr Brian Noehren, University of Kentucky; Dr Christopher Fry, University of Texas Medical Branch), biostatistician (Dr Katherine Thompson), compliance and subject recruitment leader (Dr Cale Jacobs), study coordinator (Ms Jessica Newton), research assessors (Dr Peter Hardy, Dr Anders Andersen, Ms Kylie Davis), research assistants (Dr Lauren Erickson, Dr Kathryn Hickey Lucas), and medical officer (Dr Darren Johnson).

Ethics Approval

This study was approved by the University of Kentucky's Institutional Review Board (15–0750-F6A).

Funding

The funding for this study was provided by a National Institutes of Health (NIH) research grant (ref. no. AR071398–01A1).

Clinical Trial Registration

This study was registered at ClinicalTrials.gov (ref. no. NCT03364647). Protocol modifications will be reported to the University of Kentucky's Institutional Review Board and to the trial registry. Participants are assigned a numeric code for all documentation. The electronic data are stored on password-protected computers and server. Limited study personnel will have administrative rights to access the data via password-protected files.

Disclosures

The authors completed the ICJME Form for Disclosure of Potential Conflicts of Interest. They reported no conflicts of interest.