July 2016

Blood Flow Restriction Training: The slow-flow movement is fast becoming rehab’s hottest trend

Figure 1. A patient does a leg lift while wearing a BFR cuff. (Photo by Jason Risner, courtesy of Owens Recov- ery Science.)

Figure 1. A patient does a leg lift while wearing a BFR cuff. (Photo by Jason Risner, courtesy of Owens Recovery Science.)

Partially occluding blood flow to the limbs, in com­bin­ation with low-load resistance exercise, appears to facilitate hypertrophy and strength gains in athletes and others while protecting the joints from damaging stresses.

By Cary Groner

When Johnny Owens, MPT, was chief of human performance optimization at the Center for the Intrepid at Brooks Army Medical Center in San Antonio, TX, from 2005–2015, soldiers were returning from overseas deployments with severely damaged legs due to bomb blasts and other combat injuries. Many had amputations, and for the rest, rehabilitation was challenging; of those who initially kept their limbs, roughly 15% had them amputated within five years.

“We had a problem trying to get those with really weak or damaged limbs to gain strength because they couldn’t tolerate heavy loads,” Owens said.

To help them regain capabilities, Army researchers developed a prosthetic partial exoskeleton,1 but even this futuristic device required quadriceps strength beyond the capabilities of many patients.

Owens heard about a crazy-sounding approach pioneered in the 1980s by Japanese researchers, including Yoshiaki Sato, MD, PhD, and Takashi Abe, PhD, that involved partially occluding blood flow to the affected limb for short workouts.2,3 Owens did some reading, decided to give it a try, and found the results so promising that blood flow restriction (BFR) training soon became his specialty.4 He still puts in time at Brooks, but he believes BFR may hold potential not only for those with war wounds, but also for high-level athletes and patients recovering from surgery. He’s now co-owner and director of clinical education at Owens Recovery Science, which is based in San Antonio but has clients nationwide.

Possible mechanisms

Traditional guidelines suggest that to optimally gain size and strength in a limb, people need to perform resistance training using at least 70% of the concentric one-repetition maximum (1RM) force of which they’re capable.5 However, as a 2016 literature review concluded, low-load resistance exercise combined with moderate BFR appears to facilitate hypertrophy and strength gains in athletes and others.6 A key benefit is that the use of light loads can protect joints and other healing parts that might otherwise be damaged by the heavy loads typically used to build strength and mass.

In Japan the approach has been popular for years and is called Kaatsu,7 but it’s still catching on in the US. Part of the problem is lingering uncertainty about exactly how it works. Researchers theorize that BFR may create a hypoxic environment that is then associated with increased metabolic stress, increased muscle fiber recruitment, cellular swelling, enhanced intramuscular signaling for protein synthesis, and proliferation of myogenic stem cells, all of which likely promote muscle development.6

Partially occluding blood flow to the limbs, in combination with low-load resistance exercise, appears to facilitate hypertrophy and strength gains in athletes and others while protecting the joints from damaging stresses.

It’s important to emphasize, too, that practitioners are talking about partial occlusion—not cutting off all arterial flow to the limb, as would be done in catastrophic injury or surgery—and for periods of just five to 10 minutes at a time.

Jeremy Loenneke, PhD, an assistant professor of exercise science at the University of Mississippi in Oxford, became interested in BFR a few years ago and has since become one of the world’s leading researchers in the field. Loenneke, a coauthor of a couple of the review studies just mentioned, told LER that the buildup of metabolites in the muscle being targeted using BFR may be important, as are increases in muscle activation.

“When you apply BFR, there’s some suggestion that you can increase type-2 [fast-twitch] muscle fiber recruitment, and typically you’re not going to recruit those fibers unless you’re exercising to failure,” Loenneke said. “When we restrict blood flow, we see high levels of EMG [electromyography] amplitude, which is comparable to what we see with lifting a heavy weight, and that may affect fiber recruitment. It’s also possible that you’re creating a hypoxic-like environment inside the muscles. But to be honest, we’re not exactly sure.”

Brian Clark, PhD, executive director of the Ohio Musculoskeletal and Neurological Institute, and a professor of physiology and neuroscience at Ohio University in Athens, became interested in BFR when he was conducting NASA-funded research into neural and muscular factors in muscle weakness.

“We know muscles adapt to strain, and that if you overload a muscle mechanically it stimulates cellular processes that result in growth, repair, and regeneration,” Clark said. “I suspect that a similar process happens when, instead of overloading it mechanically, you overload it metabolically.”8

According to Daniel Hollander, EdD, a professor of kinesiology who specializes in strength and conditioning at Southeastern Louisiana State University in Hammond, there’s still much to learn about such processes.

“I don’t think we understand half of what’s happening with BFR training, but one possibility is that you get an increase in anabolic hormones to the working muscle at a much lower intensity than you would with normal weight resistance,” Hollander said.

Hollander and colleagues reported in 2006 that BFR training elicited a greater growth hormone response than traditional training, in fact, but did not affect levels of resting total testosterone, free testosterone, or cortisol.9

“That’s a positive adaptation, but the caveat is that you’re not talking about one growth hormone, but rather hormones, some of which have different biologic availability than others,” he said. “The benefit of those, though, is that you get muscle hypertrophy without the ligamentous stress of repetition at a higher intensity. There’s an adaptation to the hypoxic environment similar to training at altitude, but in a more locally adaptive way.”

Potential risks

Figure 2. Johnny Owens adjusts a setting on a tourniquet worn by Marine Corps Staff Sgt. Bran- don Kothman during BFR training at Brooke Army Medical Center, San Antonio, TX. (US Army photo by Robert D’Angelo.)

Figure 2. Johnny Owens adjusts a setting on a tourniquet worn by Marine Corps Staff Sgt. Bran- don Kothman during BFR training at Brooke Army Medical Center, San Antonio, TX. (US Army photo by Robert D’Angelo.)

Some researchers, including Brian Clark, have investigated potential risks associated with BFR, such as nerve damage or deep vein thrombosis (DVT).

“I wrote an NIH [National Institutes of Health] grant to study BFR, but the criticism—and rightfully so—was that we had no clue whether it was safe,” Clark said. “So I decided to look at those concerns.”

In a 2009 paper coauthored with Todd Manini, PhD, Clark reported findings suggesting that BFR training doesn’t affect blood clotting time or vascular function, and noted that vascular compression alone stimulates the fibrinolytic system without elevating the coagulation cascade.3 The authors noted, however, that BFR may place demands on the cardiovascular system beyond those seen with traditional high-load exercise.

In another 2009 paper, Clark and his coauthors reported that, compared with low-load exercise without restriction, BFR training was associated with more severe delayed-onset muscle soreness; they also found that BFR with concentric exercise led to greater soreness than when it was used with eccentric exercise.10

A 2011 study from Clark and colleagues found that, over four weeks, both high-load resistance exercise and low-load BFR exercise increased strength without altering nerve or vascular function, and that both approaches increased fibrinolytic activity without alter­ing markers of coagulation or inflammation.11 In a 2016 paper, however, Clark and Manini re­ported a case (the second recorded) of exertional rhabdomyolysis—a serious kidney disorder—in a BFR user, and urged caution about the approach.12 The first such case was reported in a 31-year-old Norwegian hockey player in 2010.13

Yoshiaki Sato, MD, PhD, one of the Japanese pioneers of the approach, has written that he himself had a serious side effect in the early days, as he was still refining the technique. Overzealous BFR training gave him a blood clot in his leg, which traveled to his lung and nearly killed him.

“It is quite difficult to reduce blood flow by the appropriate amount in order to achieve beneficial effects,” Sato concluded.14

Daniel Hollander reported in 2010 that BFR training was associated with alterations in pain and effort sense similar to those associated with higher loads but no occlusion.15 There’s a downside to inhibiting pain, he acknowledged; athletes could damage tissue without knowing it.

“If you attempt to partially occlude too soon during rehab, you could hurt yourself,” Hollander said. “But, if you do it at a level you know won’t be injurious, there could be a positive effect.”

In a similar vein, patients could damage connective tissue because it doesn’t gain strength in an environment of low mechanical loading; if muscles acquire disproportionate strength, patients could be putting their tendons at risk.6

Last year, Marty Spranger, PhD, an assistant professor of physiology at Michigan State University in East Lansing, published a paper in the American Journal of Physiology – Heart and Circulatory Physiology pointing out that reductions in blood flow to exercising muscle engage the exercise pressor reflex (EPR), which significantly contributes to the body’s autonomic cardio-vascular response to exercise.16 Spranger and his coauthors noted that, in patients with cardiovascular diseases, the EPR can generate exaggerated increases in sympathetic nerve activity; as such, they suggested that it be used with caution in such patients, and could even pose risks to healthy individuals.

A team of University of Mississippi researchers, including Jeremy Loenneke, replied with a letter to the editor acknowledging the concerns and noting the importance of customizing the amount of restriction to the patient.17 Spranger and colleagues answered that determining such optimal values would be complex due to individual physiological differences related to EPR processing.18

“I fully agree with everything Spranger said,” Clark told LER. “The autonomic response to exercise certainly needs to be considered.”

Loenneke agreed.

Figure 3. Low-load resistance training with BFR can be conducted more often than traditional higher-load training—as often as twice daily for two weeks with a 20% 1RM. (Photo courtesy of Jeremy Loenneke, PhD.)

Figure 3. Low-load resistance training with BFR can be conducted more often than traditional higher-load training—as often as twice daily for two weeks with a 20% 1RM. (Photo courtesy of Jeremy Loenneke, PhD.)

“Spranger brought up good concerns, things we need to think about before we prescribe BFR widely,” he added.

Most studies of BFR so far have included healthy participants and have not been designed to detect potential EPR effects, Clark noted.

“It’s all about the outcome variables you measure,” Clark continued. “We measured blood clotting factors, inflammatory factors, and vascular stiffness of the peripheral tissues. But it would not surprise me if the acute effects of BFR were different due to the exercise pressor reflex, in which a buildup of metabolites may feed back via sensory nerve fibers to the autonomic nervous system, which then increases sympathetic outflow that acutely raises blood pressure and total peripheral resistance. We didn’t see any issues in healthy young people, but you’ve got to be more careful when you move into other populations.”

That said, Clark noted that professional opinion about BFR varies widely.

“When I heard about BFR, my first concern was the risk of deep vein thrombosis, and when I talked to a general practitioner or a physical therapist, they had the same response,” he said. “But if I talked to an orthopedic surgeon or a hematologist, they would not have the same level of concern. Even the case of rhabdomyolysis was rare, and you could argue that pretty much any exercise can cause it. Although I’m not convinced that the adaptations associated with BFR are any different than with other muscle-growth activities, I do think it has potential viability in patient populations where traditional exercise modalities are contraindicated because of joint-integrity issues or related problems.”


Research suggests a few such possibilities, in fact. For example, BFR has been shown to be effective for early muscular training after anterior cruciate ligament reconstruction.2,19 There’s also evidence to suggest that the modality may induce adaptation not just in muscle but in bone, possibly through altered fluid distribution.20 Moreover, BFR appears useful for building strength in middle-aged people who are unable to perform resistance training with heavy loads.21

BFR also appears promising for female patients with osteo­arthritis (OA); a 2015 paper from researchers at the University of Kansas and the University of Iowa found that it increased leg strength in women at risk for symptomatic knee OA compared with the same exercise program without BFR22 (a parallel study in men showed no improvement over standard exercise23).  In a Brazilian study published this May, 34 women with knee OA were randomized to do quadriceps strengthening either using traditional loading or lighter loads with BFR.24 The two approaches led to similar improvements in pain, function, and quad strength, but those in the BFR group reported less anterior knee pain while training.

BFR could help older patients with problems other than OA, too.

“There are a lot of scenarios in the elderly where high-intensity resistance exercise can’t be done,” said Brian Clark.

“We’ve found that older people don’t like to lift heavy weights,” added Jeremy Loenneke. “This would allow them to get the benefits of lifting those weights without having to do it.”

The gains may not just pertain to weightlifting, for that matter. Researchers have shown that when elderly people trained five days a week for six weeks using low-workload walking with BFR, their knee extension, flexion torque, and thigh muscle cross-sections all improved to a greater extent than in a control group who walked without BFR.25

Of course, older patients are more prone than their younger counterparts to the potential cardiovascular risks associated with BFR training that have been acknowledged by Loenneke and others. Much of Loenneke’s work has focused on how to adapt this still-uncertain science for maximal benefit and minimal risk. In fact, all the clinicians LER spoke with emphasized the importance of patient-specific training protocols.

“How do we individualize this?” Loenneke asked. “What’s the lowest pressure we can apply and still see muscle adaptation? I think now that if you apply thirty percent of one RM at a moderate pressure, you’re probably maximizing the stimulus for muscle.”

In other words, if the maximum the person could lift in a given exercise was 100 pounds, the traditional target for training would be 70 pounds; with BFR, by contrast, it would be 30 pounds, though with more reps.

In a 2015 paper in Sports Medicine, Loenneke and his coauthors suggested guidelines for BFR training volume and frequency.7 In terms of volume, they noted that one popular approach consists of four sets of an exercise, with repetition goals of 30 in the first set, then 15 each in sets two through four, for a total of 75 reps. These are higher than the repetition volumes typically associated with high-load resistance training, but Loenneke pointed out that recent evidence suggests skeletal muscle hypertrophy may be more influenced by volume than by intensity.26 In any case, doubling those numbers by repeating the protocol didn’t increase the benefits, and the authors speculated that there may be a volume threshold beyond which muscles don’t develop further.

Loenneke also noted that, early in training, participants may not be able to complete all of the reps dictated by such a protocol. In such cases, clinicians should decrease the patient’s intensity (eg, from 30% to 20% of 1RM) or increase inter-set rest periods beyond the usual 30 seconds; BFR thus modified still shows significant benefits. For that matter, patients new to BFR shouldn’t regularly train to muscle failure until they become accustomed to it.

In terms of frequency, low-load resistance training with BFR can be conducted more often than traditional higher-load training—as often as twice daily for two weeks with a 20% 1RM, according to Loenneke’s article. Such frequencies have reportedly not been associated with increased markers of muscle damage and oxidative stress.

Figure 4. National Football League players who have tried BFR training include the Houston Texans’ Jadeveon Clowney, who was recovering from microfracture surgery on his right knee following a lateral meniscus tear.

Figure 4. National Football League players who have tried BFR training include the Houston Texans’ Jadeveon Clowney, who was recovering from microfracture surgery on his right knee following a lateral meniscus tear.

For postsurgical patients, Loenneke and his colleagues have proposed progressive implementation of BFR from early rehab through a resumption of high-load resistance training. The protocol consists of BFR only during bed rest, followed by BFR plus low-workload walking, then BFR plus low-load resistance exercise, and finally BFR plus high-load resistance exercise.

Athletes seeking to prolong their careers could benefit from mixing some BFR training in with their usual regimens, the authors suggest, given that their joints, too, would be spared by decreased mechanical stress.

“Most people will use this to get better, then transition to normal exercise, but I don’t see any reason to discourage them from using it long term,” Loenneke said.

Some research supports the idea. For example, in one study from Japan, six-week strength gains were similar for individuals who did high-load resistance training with no BFR and those who did a combination of low-load resistance training with BFR and high-load training without BFR; both groups had greater increases than those who used BFR alone.27 Another study found that, when college football players supplemented traditional strength training with low-load BFR training, they significantly improved their bench press and 1RM squat strength.28 Other researchers, however, have reported insignificant additional effects by adding BFR to traditional training in fit individuals.29


Some evidence suggests the response to BFR training may depend on the type of athlete, interestingly enough.7 For example, researchers have observed that distance runners experienced significantly greater metabolic stress with BFR training than sprinters did;30 this may occur because distance runners are more dependent on oxygen delivery than sprinters, who tend to be more accustomed to the anaerobic environment typical of BFR. More work remains to clarify such matters, but the findings highlight the advisability of customizing BFR training protocols to individual patients, as already noted.

Johnny Owens knows this from personal experience. After he’d received some attention for his work with injured veterans, he got a call from the Houston Texans, who asked him to try BFR techniques on several of their players, including linebacker Jadeveon Clowney, who was recovering from microfracture surgery on his right knee following a lateral meniscus tear. Reports from the team were positive; the Texans’ director of sports medicine, Geoff Kaplan, told ESPN that the athletes who received BFR training were exhibiting better muscle control and progressing faster than team trainers were expecting.31 Owens has since begun working for more than 20 college and professional teams in football, basketball, baseball, hockey, and soccer, and is involved in several ongoing clinical trials.

“We have to apply BFR training to different clinical problems,” he said. “The amount of load, how you do it, how much you’re doing, all might be different. Someone with a microfracture in their knee will have a very conservative protocol; he can’t put any load through his knee at first. So we do exercises that are protective of his repairs and try to manipulate those with the tourniquet on to get an increase in the protein synthesis response. Then we move to a biking protocol; then we load in a protective range of motion based on the location of the lesion in the knee. It’s a way to tackle the strength and hypertrophy phase of rehab, and to mitigate the effects of atrophy.”

Owens eschews the tourniquets common to Kaatsu training, which aren’t US Food and Drug Administration-approved as medical devices; instead, he utilizes a Delfi system similar to that used in surgery, which measures occlusion pressure with Doppler.

“Clinically, it’s the safest we could find,” Owens said. “Our target is 80% occlusion, but there are many variables in estimating that—limb size, tissue density, systolic blood pressure, cuff placement and width, the patient’s gender—and the only way we can account for them all is with Doppler.”

Down the road

As clinical trial results continue to become available, and more clinicians explore the potential benefits of BFR training, it should become clearer how to use the technique in different patients for optimal strength and safety. For now, it looks like proponents have a natural motto at hand: less strain, more gain.

Cary Groner is a freelance writer in the San Francisco Bay Area.

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One Response to Blood Flow Restriction Training: The slow-flow movement is fast becoming rehab’s hottest trend

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