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Blood flow restriction (BFR) training, also known as Kaatsu training or occlusion training, involves using a tourniquet, elastic band or pressure cuff placed at the proximal attachment of a limb to the torso to allow the inflow of arterial blood to working muscles while restricting the outflow of venous blood back to the torso during exercise.
A growing body of research has demonstrated both the safety and effectiveness of combining low-intensity strength training with BFR for the purpose of achieving hypertrophy; in addition, products have been introduced to the market that make the process easier to perform in a non-clinical setting. The result is that BFR is now more accessible and used by professionals such as physical therapists and athletic trainers to help individuals rapidly regain muscle mass after an injury. But is it safe? Here’s what you and your clients need to know about this controversial form of training.
While emerging research validates the benefits of BFR, specific guidelines for its application are still being developed. For this reason, the American Council on Exercise is not endorsing the use of BFR. The purpose of this article is to help health and exercise professionals learn more about the method, understand the proposed mechanisms of how it works, and to review the existing research literature to determine whether it is a technique that could be used to help clients achieve goals related to muscle growth.
A Rundown on the Research on BFR
Multiple studies have demonstrated that when combined with low-intensity strength training [between 20 and 40% of one-repetition maximum (1-RM)], using BFR could help increase muscle hypertrophy without having to use moderate-to-heavy resistance. These findings suggest that BFR may be an option worth considering for clients interested in achieving muscle growth without the soreness that usually accompanies high-volume, high-intensity strength training (Mattocks et al., 2018; Hughes et al., 2017; Pearson and Hussain, 2014; Pope, Willardson and Schoenfeld, 2013).
As Hughes and colleagues noted in their review of the research on BFR, “Augmentation of low-load resistance training with BFR to the active musculature can produce significant hypertrophy and strength gains, using loads as low as 30% 1-RM. BFR training has been found to yield hypertrophy responses comparable to that observed with heavy-load resistance training.” The researchers go on to suggest that, when combined with low-load resistance training, “BFR can attenuate the effects of sarcopenia and may be effective at improving bone health.”
How to Apply BFR
The practice of BFR requires the application of a pressure-inducing wrap at the proximal attachments where the arms and legs connect with the torso; specifically, at the inguinal crease of the upper thigh and immediately distal to the attachment of the medial deltoid on the upper arm (Pope, Willardson and Schoenfeld, 2013). The pressure should partially restrict the flow of arterial blood carrying oxygen to the muscles distal to the occlusion site and be tight enough to restrict the flow of venous blood back to the heart and lungs. Compression of the vasculature results in hypoxia—a lack of oxygen in muscle tissue—which, when combined with low-intensity resistance training, can accelerate the amount of metabolic stress in the involved muscles. This is what researchers believe is responsible for inducing the hypertrophy observed in study participants (Mattocks et al., 2018; Hughes et al., 2017; Pearson and Hussain, 2014; Pope, Willardson and Schoenfeld, 2013).
Dr. Jim Stray-Gundersen is a sports-medicine specialist and exercise physiologist who uses BFR with clients ranging from Olympic athletes to older adults. “Circulation in an extremity starts with arterial inflow to arterioles to capillaries and then to venous outflow,” says Dr. Stray-Gundersen, who is also the creator of the Bstrong brand of BFR bands. “What is occurring with safe and effective BFR is changing the characteristics of that circulation and creating a situation where insufficient blood and oxygen is getting to the working muscles, thus creating a metabolic disturbance.”
When a band is applied while the muscle is at rest, venous return is temporarily occluded, either severely or completely. “Then, when exercise is started,” explains Dr. Stray-Gundersen, “the muscle pump squeezes blood from the distended capillaries and veins, past the venous obstruction and into the central circulation. The patent arteries readily refill the distended capillaries and veins. The venous flow goes from a reasonably steady flow with non-BFR exercise, to one where there are periods of no flow (muscle relaxation) and high flow (during muscle contraction), which causes a metabolic crisis in the muscles. It’s this crisis that results in the anabolic processes responsible for hypertrophy.”
Like any method of exercise, it is not known exactly how BFR causes hypertrophy; however, according to numerous studies conducted on the technique, occluding venous blood flow combined with resistance training at a relatively low intensity appears to create the specific mechanisms responsible for hypertrophy.
Cell Swelling
The pressure on the proximal limb keeps blood in the muscle tissue, which can increase the volume of cells. The prevailing theory is that the pressure of the occlusion forces more fluids into the intracellular spaces of muscle fibers.
“There is a large body of evidence indicating that cell swelling promotes an increase in protein synthesis,” write Pope, Willardson and Schoenfeld. “It has also been hypothesized that an increase in myocellular hydration may trigger the proliferation and fusion of satellite cells to promote hypertrophy."
Additionally, Pearson and Hussain believe that, over time, cell swelling could contribute to hypertrophy. “Intracellular swelling is believed to threaten the structural integrity of the cell membrane,” the researchers write, “which causes the cell to initiate a signaling response that chronically leads to a reinforcement of its ultrastructure.”
Activation of Type II Muscle Fibers
According to the size principle of muscle activation, when a muscle contracts, the smaller type I muscle fibers are recruited first, followed by the larger, type II muscle fibers that can generate higher levels of force. A hypoxic condition (where oxygen is not available) means that the larger type II fibers capable of anaerobic metabolism are recruited in greater numbers. In addition, it appears as if the alpha-motor neurons responsible for activating type I muscle fibers are inhibited, which results in a higher level of activation of the type II fibers. The long-term adaptation is that the type II fibers become capable of storing more glycogen, allowing them to generate higher levels of force (Hughes et al., 2017; Pearson and Hussain, 2014; Pope, Willardson and Schoenfeld, 2013).
Enhanced Satellite Cell Activity
Muscle growth can occur when satellite cells are produced to repair muscle fibers damaged during exercise. Pearson and Hussain determined that BFR, when combined with low-intensity resistance training, increased satellite cell production, which could contribute to observed muscle growth.
Metabolite Accumulation and Anabolic Hormone Production
When adenosine triphosphate (ATP) metabolism is greater than ATP production, which occurs during exercise at higher intensities, a rapid accumulation of by-products (metabolites) occurs, which can change blood acidity. Restricting the venous blood flow increases the metabolic acidosis associated with the production of anabolic hormones including growth hormone (GH) and insulin-like growth factor (IGF-1). As Pope, Willardson and Schoenfeld noted in their review of the research, “Studies that have compared resistance exercise protocols with or without BFR have demonstrated a significantly greater systemic GH release subsequent to the BFR protocol.”
Kathy Smith is a fitness writer, producer and star of numerous workout videos, and is host of the Art of Living podcast, which advocates for the benefits of exercise throughout the aging process. Ms. Smith is in her late sixties and credits the use of BFR with low-intensity strength training for helping her to maintain the strength (and muscle definition) she needs for her active lifestyle, which includes hiking, surfing and downhill skiing. “What I love about BFR is that I’m able to lift with lighter weights, but still experience that extreme fatigue signal responsible for muscle growth created by heavier weights,” said Ms. Smith.
Todd Durkin, a San Diego-based ACE Certified Personal Trainer and the owner of Fitness Quest10, recently began using BFR for his own personal workouts. “What I’ve found with BFR is that several days a week, I can use lighter weights and higher reps to still achieve a phenomenal workout without stressing my joints. Wearing the cuffs while lifting lighter weight can help me to achieve the same muscle pump that I would normally experience after more sets with heavier weights,” says Durkin. Kathy Smith is a fitness writer, producer and star of numerous workout videos, and is host of the Art of Living podcast, which advocates for the benefits of exercise throughout the aging process. Ms. Smith is in her late sixties and credits the use of BFR with low-intensity strength training for helping her to maintain the strength (and muscle definition) she needs for her active lifestyle, which includes hiking, surfing and downhill skiing. “What I love about BFR is that I’m able to lift with lighter weights, but still experience that extreme fatigue signal responsible for muscle growth created by heavier weights,” said Ms. Smith. Todd Durkin, a San Diego-based ACE Certified Personal Trainer and the owner of Fitness Quest10, recently began using BFR for his own personal workouts. “What I’ve found with BFR is that several days a week, I can use lighter weights and higher reps to still achieve a phenomenal workout without stressing my joints. Wearing the cuffs while lifting lighter weight can help me to achieve the same muscle pump that I would normally experience after more sets with heavier weights,” says Durkin.
The amount of intensity it takes to stimulate hypertrophy with BFR is relatively consistent between studies. What differs, however, is the type and amount of pressure needed to successfully restrict blood flow (this is also why there is no current standard guideline for its application). In their comprehensive review, Bielitzki and associates identified three specific methods of creating BFR: perceived pressure technique, absolute and relative overlap technique and the maximal elasticity technique. While each method varies slightly, the common denominator is the application of external pressure to reduce arterial and venous blood flow during exercise to create a hypoxic environment that can accelerate the level of metabolic stress.
The perceived pressure technique is based on an individual’s response to the applied pressure of the occlusion. This process can be performed in several ways, such as applying an elastic wrap that is stretched taut to create pressure; winding a non-elastic material to a perceived tightness; or inflating a pneumatic cuff with air to create pressure (similar to a blood pressure cuff). Each method is applied around the upper thigh or upper arm, using enough pressure to allow arterial inflow of blood while occluding venous outflow. This method requires the exerciser to provide feedback that the wrap is “snug, but not painful,” according to Bielitzki and associates. On a 0-10 scale of pressure, the goal is to achieve a painless pressure between 7 and 10.
The absolute and relative overlap technique uses either an elastic or inelastic material wrapped around an arm or leg as close to the torso as possible. The objective is to pull the wraps tight enough so that it creates a circumference that is approximately 10 to 20% less than the upper arm or upper thigh. As Bielitzki and colleagues noted, this method uses an objective measurement of the circumference of the limbs as the guide for setting the pressure, so it may be a more effective means of applying BFR.
Finally, the maximal elasticity technique uses elastic wraps that are pulled to maximum tightness. Once the wraps are removed, they are reapplied with only 75% of their original tension, as identified during the initial wrapping process (Bielitzki et al., 2021).
How Much Pressure Should Be Used?
In their review of the research on BFR, Pope and associates identified a technique using pressurized cuffs at the occlusion sites. They found that pressures ranging from 50 to 180 mmHg were effective for inducing muscle growth. And, in a study comparing low-intensity resistance training with BFR to high-intensity resistance training alone, Korkmaz and colleagues used a cuff inflated to 130 to 150 mmHg. They observed that the BFR group experienced a “significantly greater increase” of muscle growth when compared to the non-BFR training group.
While a wide range of devices are currently available for applying BFR, Dr. Stray-Gundersen believes inflatable devices are safer and more effective than non-inflatable straps, bands or cuffs (which are often less expensive) “One can reproductively produce the correct amount of restriction from one time to the next in the same person, as well as from one individual to the next,” he explains. “If personal trainers are considering using BFR, they should look for systems based on inflatable pressure that can be measured and replicated from use-to-use.”
The reviews conducted by both Bielitzki and associates and Pope, Willardson and Schoenfeld revealed that cuff thickness was related to the amount of pressure applied. They observed that a narrower cuff required more pressure, while a wider cuff was able to occlude a greater volume of blood flow with less applied pressure. They also found that elastic cuffs may experience a length change over time. To apply a more consistent pressure, they recommend using cuffs made of materials that do not experience strain and length change after repeated use. They also recommend using either a perceived pressure of 7 out of 10 (with 10 being the highest), or the overlap technique with specific markings to ensure consistency in different exercise sessions.
Exercise is physical stress imposed upon the body; thus, any method of exercise contains some inherent risk. As Hughes and colleagues noted in their review of the research on BFR, “Despite concerns of disturbed hemodynamics and ischemic reperfusion injury, BFR training has been reviewed in-depth and correct implementation has been affirmed to present no greater risk than traditional exercise models.”
It is natural to wonder whether you should avoid using BFR with clients who have high blood pressure or vascular disease. However, Pope, Willardson and Schoenfeld concluded that, based on the currently available research, “BFR is a safe and novel method for training athletes, healthy persons and potentially those individuals with varying comorbidities.”
Even so, if you have any concerns about whether BFR might be dangerous for a client, check with their medical provider or forgo the use of the technique and instead use a traditional resistance-training protocol based on intensity and volume.
When using BFR with clients, Pope, Willardson and Schoenfeld recommend a simple system for identifying an appropriate amount of occlusion pressure: “The pulse should still be palpable distal to the occlusion site.”
Dr. Stray-Gundersen agrees. “The issue with correct pressure is twofold,” he says. “First, one needs to use the system at a low enough pressure to be safe. Second, one needs to use the system at a high enough pressure to be effective. How do we know when we get these conditions? For safety, regardless of pressure in the band or cuff, if you have a pulse distal to the band during exercise, you know you are using a safe pressure.”
In addition to determining an appropriate pressure, other variables include the amount of resistance used and the frequency of application. Available research suggests that it is best to keep the intensity low (between 20 and 40% of 1-RM) and perform BFR training two to three times per week. However, there are no current standardized guidelines regarding the type of wrap that should be used or the amount of pressure that should be applied. In this case, the recommendations regarding equipment choice and appropriate pressure described earlier should guide your use of this technique.
Clients of all ages are often interested in exercise for the purpose of muscle growth. For older clients, particularly those at risk of sarcopenia, it may be even more important. The use of heavy resistance for hypertrophy is effective, but it may cause a degree of soreness and discomfort that many clients may want to avoid. When considering exercise methods to help clients achieve muscle growth while avoiding muscle soreness, BFR may be worth considering, but be sure to identify an appropriate course that can teach you how to use it in a safe and effective manner. If there are any concerns about whether the technique is appropriate for a client, check with their medical provider or err on the side of caution and implement traditional methods of resistance training instead. And, always be prepared to offer clients various recovery strategies, which could help to mitigate any exercise-induced soreness.
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