The Science and Evidence: Blood Flow Restriction Training

Johnny Owens, PT, first developed blood flow restriction (BFR) training at Brooke Army Medical Center to improve limb strength of wounded warriors without compromising vulnerable soft tissue and joint structures.¹ BFR training is typically performed with a tourniquet system that is similar to a blood pressure cuff on the proximal upper and/or lower extremities.^2-6 The cuff is typically pressurized from 120-230 mmHg, so as to impede venous blood return while maintaining arterial flow, leading to an accumulation of exercise metabolites and an exaggerated physiologic training response at lower loads.^2-6

While the American College of Sports Medicine (ACSM) and the National Strength and Conditioning Association (NSCA) recommend training at 70-80% of one repetition maximum (1RM) to optimize muscle strength and hypertrophy, BFR is typically performed at 20% to 50% of 1RM.⁶ Similarly, aerobic fitness is traditionally prescribed at 75% of VO2 max, but BFR aerobic training is typically performed at 20% of VO2 max.^{7, 8} As such, BFR training may provide a significant advantage for maximizing physical therapy outcomes and athletic performance. As Fry et al. points out, “In the last 8 to 10 years, low-load training with blood flow restriction has attracted a lot of attention, both as a possible alternative to heavy resistance exercise in the rehabilitation setting and as a training method to increase muscle strength and size in healthy individuals.“³

SAFETY CONSIDERATIONS

Given that tourniquets control blood flow via general occlusion, the risk of vessel, nerve and skin injury must be considered. Odinsson et al. reviewed 63,484 surgical procedures, whereby tourniquets were required to manage blood flow.⁹ Despite tourniquets being inflated to over 100 mmHg (on average) above systolic blood pressure, most surgeons safely apply them for 2 hours (on average) with a minimal complication rate of 0.04%.⁹ As a comparison, BFR training is typically performed with less pressure for a duration of about 10-15 minutes,¹⁰ which is approximately one eighth of the surgical occlusion times reported by Odinsson et al.⁹ To further increase safety during BFR training, a number of investigators recommend wider cuffs than those used during surgery, as there is an inverse relationship between cuff width and occlusive pressure.^{11, 12}

Clark et al. also measured vascular stiffness and abnormal blood clotting via D-Dimer testing during a single bout of BFR exercise and over a 4-week course of BFR training and reported no change.¹⁰ Moreover, the authors found no difference in nerve conduction velocity during BFR training.¹⁰ Rather, Poton and Polito found lower values of systolic blood pressure, diastolic blood pressure and heart rate following low-intensity, BFR training compared with more traditional, high-intensity strength work.¹³ In a systematic review of safety issues related to BFR training, Loenneke et al. evaluated cardiovascular stress, oxidative stress, muscle damage and nerve conduction and concluded that BFR is a safe training alternative for individuals regardless of age and training status when used in a controlled environment by trained and experienced personnel.¹⁴

STRENGTH TRAINING

Low-intensity, blood flow restriction training is thought to create an environment of low-oxygen availability, causing the activation of type II muscle fibers and anaerobic metabolism.^{8, 15} The anaerobic environment leads to a build-up of muscle metabolites, which stimulate a number of changes required for muscle and bone strengthening.¹⁶ Together, the hypoxic environment and metabolites stimulate neural afferents, causing a significant increase in growth factor hormone (GFH).¹⁷ While Takarada reported a 290 times increase in GFH after a single bout of BFR exercise (5 sets of bilateral knee extensions to fatigue, at 20% of 1RM with an average 214 mmHg of cuff pressure) compared to baseline,¹⁸ Pierce et al. used a similar protocol and found a lower, but still physiologically relevant, 9-fold increase.^{8, 19}

Fry et al. also reported a 9-fold increase in GFH compared to baseline following a single session of BFR exercise session (one set of 30 repetitions of bilateral leg extensions at 200mmHg cuff pressure, followed by 3 sets of 15 repetitions with 30 second rest intervals between sets).³ While a number of studies have also measured changes in insulin-like growth hormone¹⁷ and testosterone²⁰ following blood flow restriction training, the impact of BFR training on these hormones remains controversial.⁸ Moreover, West et al. did not find a correlation between testosterone, growth hormone and insulin-like growth hormone and muscle protein synthesis following BFR training.²¹ This finding suggests that local instead of systemic events may predominantly drive physiologic changes post BFR training.⁸

Local hypoxia and metabolites secondary to BFR training result in a 3-fold increase in ribosomal protein S6 kinase beta-1, a product of the rapamycin (mTOR) pathway, which is responsible for stimulating muscle protein synthesis (MPS), and subsequently, hypertrophy.²² 8-hours of BFR training has also shown to decrease proteolytic gene pathways 2-fold compared to traditional high-load exercise, suggesting an increase in muscle remodeling.²³ In addition, after an 8 week, low-intensity BFR program, Laurentino et al. reported a down regulation of myostatin mRNA, consistent with traditional, high-intensity strength training programs.²⁴ Given that myostatin is a negative growth factor that inhibits satellite cells and is inversely related to MPS, low-intensity-BFR may be just as effective as traditional, high-load strengthening program for muscle growth.⁸

It is perhaps worth noting that the effect size of for muscle hypertrophy and strength secondary to low-load, BFR training (as compared to low-load strength training, alone), is .39 and .58, respectively.²⁵ Moreover, it takes an average of 6 weeks of traditional, high-intensity strength training to transition from neural adaptation to muscle hypertrophy.²⁶ However, muscle strength and hypertrophy following low-load, BFR has been shown to occur during the first four weeks of training, suggesting that working smarter instead of harder might truly be a better strategy.^{8, 26}

MUSCLE ATROPHY

Following an injury or surgery, muscle weakness and atrophy are often an issue secondary to disuse and immobilization. After 5 and 14 days of leg immobilization, Wall et al. reported a 3.5% and 8.4% decrease in muscle cross sectional area, a 1.4% and 3.1% reduction in strength, a 68% and 54% decrease in strength and a 68% and 54% increase in  myostatin expression, respectively.²⁷ While pain and inflammation make traditional therapeutic exercise challenging, if not impossible, in this patient population, BFR in the absence of muscle contraction may provide an alternative training strategy. Following 2 weeks of lower extremity immobilization, subjects that received BFR (5 repetitions of 5 minutes at 200mmHg with 3 minutes rest between sets) demonstrated less knee and ankle muscle strength loss than non-active subjects and subjects performing isometric exercises.²⁸ Patients that undergo anterior cruciate ligament reconstruction surgery typically experience a significant reduction in quadriceps cross sectional area. However, Takarada et al. reported a 9.4% cross sectional area reduction following BFR (5 repetitions of 5 sets of lower extremity limb occlusion at 238mmHg for 5 minutes with 3 minutes rest between sets) compared to a 20.7% reduction in patients receiving placebo BFR.²⁹ Thus, BFR training has the potential to significantly improve therapeutic outcomes following a traumatic injury and/or surgery.

BONE HEALTH

There is also some evidence that BFR training may be beneficial for bone health.  Following 2 days of BFR consistent with Takarada et al. and at a cuff pressure 40% greater than the systolic blood pressure measured in the upper extremity, young-male subjects experienced a significant reduction in the bone reabsorption marker serum N-terminal cross linking telopeptide of type 1 collagen compared to those receiving only low-intensity exercise.³⁰ In addition, the authors reported a 21% increase in serum bone-specific alkaline phosphatase, a marker of bone formation, in older male subjects, compared to low-intensity exercise alone.³⁰ While the mechanism of bone strengthening secondary to BFR is not fully understood, Loenneke et al. suggests that it may be due to “increased interstitial fluid flow-induced sheer stress within the osteocyte membrane”.^{8, 25, 31}

AEROBIC TRAINING

BFR aerobic training is typically performed via walking or cycling with pressure cuffs set to 200 mmHg over the proximal lower extremities, bilaterally.⁸ After 3-weeks of a BFR walking program at 20-40% of VO2 max, performed twice daily, Abe et al. reported a   significant improvement in lower extremity size and strength compared to subjects performing the same program without BFR.³² In a separate study, Abe found that participants completing a 15-minute BFR cycling program at 40% VO2 max, 3 times per week, experienced improved lower extremity muscle size and strength along with a 6.4% improvement in VO2 max.^{8, 33} Yet, participants that cycled at the same intensity for 40 minutes, 3 times weekly, did not improve on any of the fitness-related variables.³³

CONCLUSIONS

Clearly, BFR training may be a valuable tool in the physical therapy profession, as it may facilitate a safer and more efficient strategy for making patients stronger.⁸ It also may help improve cardiovascular performance and overall bone health.⁸ Thus, BFR training may lead to enhanced clinical outcomes in patients suffering from acute, subacute and chronic musculoskeletal issues.  While not appropriate for every patient, BFR training may also by a particularly powerful tool for patients suffering from disuse, atrophy and/or general deconditioning.³⁴

The American Academy of Manipulative Therapy Fellowship is fortunate to have a strong relationship with champion power lifter and world record holder, Donnie Thompson.  During the AAMT Fellowship in Orthopaedic manual Physical Therapy, Mr. Thompson provides training on strength progression, nutrition and a number of alternative manual therapy strategies such as blood flow restriction.  As Mr. Thompson often reminds the AAMT fellows-in-training, the best way to treat an injury is to prevent it.  In this regard, the use of BFR training to maximize the strength of muscle, bone and connective tissue—so that it can withstand more force and be less susceptible to injury—should be considered part of injury prevention and/or rehabilitation programs.

AUTHORS

Dr. Cameron Moore, DPT, Cert. SMT, Cert. DN, FAAOMPT, Dip. Osteopractic
Fellow, AAMT Fellowship in Orthopaedic Manual Physical Therapy

Dr. Joe Ciccone, DPT, SCS, Cert. SMT, Cert. DN, FAAOMPT, Dip. Osteopractic
Fellow, AAMT Fellowship in Orthopaedic Manual Physical Therapy
Columbia University Medical Center, New York, NY

Dr. Raymond Butts, PhD, DPT, MSc, Cert. SMT, Cert. DN, Dip. Osteopractic
Senior Faculty, American Academy of Manipulative Therapy Fellowship, Atlanta, GA

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