What is BFR?

BFR involves applying a strap to the arms or legs, allowing oxygenated blood to flow into the limbs while restricting the return flow.

 - Hytro

What is BFR?

BFR involves applying a strap to the arms or legs, allowing oxygenated blood to flow into the limbs while restricting the return flow.

 - Hytro

Blood Flow Restriction (BFR) training is a scientifically proven technique used to rapidly increase muscle size and strength. It involves applying a tightened strap to the proximal point of the arms or legs. This allows oxygenated blood to flow to the muscles but restricts the amount that then returns to the heart. As a result, blood remains in the muscle, which causes it to swell. Once the muscle swells, a whole host of physiological reactions occur that ultimately result in enhanced muscle size and strength – along with the greatest muscle pump of your life.

The magic of BFR is in allowing athletes to work at lower intensities than usual. While lifting weights of 70% of one-rep max is widely considered to be required for optimal muscle growth, using BFR allows athletes to lift at 20-30% of their one-rep max and attain similar results. Through BFR, muscle hypertrophy can be increased beyond anything that conventional weightlifting alone is able to achieve.

BFR training even allows athletes to continue their progress on rest days due to the lack of significant muscle damage. There are further benefits to those recovering from injury or illness who are unable to train conventionally. BFR can be used as a clinical rehabilitation tool to speed up recovery, enabling muscle growth at an intensity that can be tolerated. BFR has the ability to help accelerate recovery, which is useful for athletes after competing, or on days off.

Ultimately, BFR training helps athletes to hit their targets like never before. With just two weeks of consistent use, countless studies have shown BFR to significantly increase muscle size, strength and endurance.

So how does BFR work?

  • 1. Oxygenated blood flows into the exercising muscle where oxygen is used to produce energy in a process called aerobic respiration.
  • 2. Pressure applied by the BFR strap restricts the return flow of now de-oxygenated blood to the body which causes muscle swelling and stress as blood continues to pool.
  • 3. As de-oxygenated blood builds in the muscle, decreasing oxygen concentrations create a hypoxic environment (low oxygen), triggering production of lactic acid which further stresses the muscle cells.
  • 4. The body responds to this stress through production and accumulation of metabolites responsible for muscle growth, while the brain significantly increases production of growth hormone.
  • 5. Slow twitch fibers (requiring oxygen for energy) rapidly fatigue in a hypoxic environment, and consequentially, fast twitch fibers (working without oxygen) are recruited to help the muscle contract.
  • 6. This condition is unique to BFR whereby all types of muscle fibers work together to produce force while performing low intensity exercise. Under normal conditions, fast twitch fibers are reserved for high intensity exercise and explosive movements. With BFR, they are forced to contract and grow.
  • 7. Together, these reactions upregulate muscle protein synthesis and increase the body’s ability to hypertrophy, similar to conventional weightlifting while using much lighter loads.


To understand how BFR increases muscle strength, it’s first important to understand how muscles contract. Under normal circumstances, oxygenated blood flows to muscles from the heart. Oxygen and glucose are delivered to muscle cells to produce energy which allows the muscle to contract. Once this has happened, the deoxygenated blood returns to the heart so it can be reoxygenated. Simple enough, right?

Here’s where things get a little more complicated. Inside the muscle, there are three types of muscle fibre that contract: Type I (otherwise known as slow-oxidative), Type IIA (fast-oxidative) and Type IIX (fast-glycolytic). Type I muscle fibres require oxygen and deliver energy for low intensity or endurance exercise. Type IIA needs oxygen but can function to some extent without. Together with Type IIX muscle fibres, they produce energy in short, intense bursts which cater for high energy activities such as sprinting and heavy weightlifting. Type IIX muscle fibres, however, contract without oxygen.

With BFR training, oxygenated blood still flows to muscles from the heart but deoxygenated blood is restricted from returning. This creates a build-up of deoxygenated blood in the muscle that leads to cell swelling and creates a hypoxic (low oxygen) state. Do you know the infamous muscle burn feeling? Yep. That’s what a hypoxic state feels like. This is the state where BFR really works its magic. The lack of available oxygen in the muscle means that Type I and Type IIA fibres rapidly fatigue. Type IIX muscle fibres are forcibly recruited to contract the muscle – even though the body is working at a low intensity.

It usually takes heavy lifting (and serious sweat) to engage Type IIX muscle fibres. However, BFR training is able to do this when working at just 20-30% of one-rep max. When Type IIX muscle fibres contract under a condition of blood flow restriction, they produce lactic acid and other metabolites which place the muscle under stress. This drives the release of growth hormone and increases protein synthesis, the physiological regulator of muscle growth, which leads to increases in muscle size and strength. When the straps are loosened, these hormones and metabolites then flood the rest of the body, allowing even nonrestricted muscles to benefit from the hypertrophic effects of BFR.

Discover the numbers behind the research into BFR’s effect on muscle strength.


Resistance training enhanced with BFR (BFR-RE) is an excellent way to build muscle size, strength and power. BFR-RE can produce gains that go beyond what is achieved through conventional training alone. When BFR-RE is then used in combination with resistance exercise, the resulting gains are supercharged.

To make the most of this formula, athletes practising BFR-RE should use resistance of 20-30% of their one-rep max, perform a high number of reps in each set (75 reps across four sets, consisting of 30, 15, 15, 15 is most common) and have little rest between sets – around 30 seconds. The straps should be applied at all times during a set. Because the recovery times from BFR training are quicker than those of high load resistance training, it is also possible to add extra BFR sessions into a typical weekly resistance training plan. These can be completed solely as BFR-RE sessions, or in a 20-minute slot at either the beginning or end of a conventional training session. Resistance bands, weights or even bodyweight are all effective ways of applying resistance.

When performed as a warm-up before a conventional training session, a small amount of BFR accelerates muscle deoxygenation, and can actually condition the muscles to work in a low oxygen state for the rest of the session. This reduces perceived effort once the straps are removed, which improves the quality of reps and increases the rep range, ultimately increasing muscle hypertrophy. As a cool-down, BFR is then an excellent finisher to lock-in the improvements made from the resistance session.

To gain the most benefits, BFR-RE sessions should take place 2-4 times per week and last no longer than 20 minutes. Although increases in muscle size and strength are seen within two weeks, we recommend continuing with BFR-RE for at least twelve weeks.


Studies have proven that BFR not only increases gains from resistance exercise but also improves aerobic endurance. In some instances, BFR-AE training has produced a 15% increase in VO2 max, a 7-27% increase in skeletal muscle mass. Those are far from the only impressive statistics from blood-flow restriction training. Significant improvements in aerobic capacity have also been noted due to the range of adaptations that occur from BFR.

The stress that BFR places muscles under increases their ability to take in and use glucose and oxygen, both of which increase the rate of energy production and therefore allow athletes to go further for longer. This is due to an increase in muscle fibre capillarisation, increasing the delivery of oxygen and glucose to the working muscle. And, increased mitochondrial biogenesis, resulting in greater numbers of mitochondria in the cells using oxygen and glucose to produce energy.

Although adaptations can be seen as early as three weeks into BFR-AE training, we recommend continuing with BFR-AE for at least six weeks for significant adaptations to occur.


The key element of BFR training is the application of a strap. However, the strap cannot just be applied anywhere. To maximise the efficiency of BFR, the strap needs to be placed at the most proximal part of a limb. The strap should never be placed around the middle of the limb, such as mid-bicep or mid-thigh. Such incorrect placement significantly raises the risk of nerve damage.

Once the strap is in the correct place, it needs to be tightened appropriately. If it is too tight then it will totally stop the flow of blood, rather than restrict it. If it is too loose then blood will not be restricted. On a scale that goes from 1 to 10 where 1 is the loosest and 10 the tightest, the strap should be tightened to a 7. If in doubt, it’s better to have the strap too loose than too tight.

If you start to feel numbness or tingling in the limb you’ve overtightened the strap and it needs to be loosened. If you feel no pressure at all then the strap needs to be tightened.

At first, BFR training may feel strange. It is normal to feel a slight discomfort. For those totally new to BFR, it’s best to start slow and build up gradually. As with any new training that is metabolically demanding, your muscles may initially need slightly longer to recover. Train with BFR for no longer than 20 minutes a day, ideally 2-4 times per week.

It won’t be long before adaptations occur and BFR training becomes second nature. Any initial discomfort will seem worth it once the widespread benefits that come from BFR truly kick in.


Much scientific research has gone into analysing the safety of BFR training, with the ultimate conclusion that BFR is a safe method of training.

Concerns have been raised about the impact of BFR on the nervous system. However, the research found no change in nerve conduction velocity following four weeks of BFR training, suggesting that there is no negative impact. The only concern comes from poorly applied straps, such as those placed around the mid-bicep of mid-thigh, rather than at the most proximal point of the limb.

Due to the restriction of blood flow, further research has gone into the effect of BFR on the cardiovascular system. Experiments have considered whether damage to veins and long-term blood flow occurs. It has since been proven that BFR actually improves vasodilation and increases blood flow – can we link to this resource?.

Questions have also been raised about blood clots. Rather than increasing the chances of blood clots, research has found that low-intensity BFR with straps appropriately tightened may actually reduce the chance of blood clots forming due to its effects on fibrinolytic activity. Dangers come only when the strap is tightened to maximum levels, preventing the flow of any blood.

Some initial muscle damage may occur when those new to BFR begin training, though research has found that this will not last longer than 24 hours. Any new exercise that the body has not adapted to before is likely to have the same effect. However, once adapted, there is no significant muscle damage from BFR, which is a key benefit of its application in comparison to conventional training. This means BFR can be used on rest days without hindering recovery. Incidences of exertional rhabdomyolysis (the excessive breakdown of muscle that can result in organ damage) are just as low as those from high-intensity resistance training.

As BFR is only mimicking the response of high-intensity resistance training at a lower intensity, the long-term effects on health and safety are similar.


There are over 1,000 scientific papers that have explored the safety and effectiveness of BFR training. Given the incredible results, such newfound attention is unsurprising. From the numerous papers, there is compelling evidence that BFR training is a hugely beneficial tool to increase strength, power and endurance across a wide demographic. Specifically, studies have found that BFR increases muscle mass and strength, muscular endurance, power output and aerobic capacity. Impressively, research has also found significant muscular improvements in well-trained athletes who would not normally benefit from low load exercise.

We’ve compiled an in-depth overview of blood-flow restriction training research – check it out!



The results speak for themselves. They can be significant, and they can be fast. In just two weeks notable increases can be seen in muscle size, strength and endurance thanks to adaptations within the body that come directly from BFR training.


BFR training restricts deoxygenated blood from flowing out of a muscle. This creates a hypoxic state within the muscle that increases the contractions of fast glycolytic muscle fibres, also known as Type IIX. When these contract under a condition of blood flow restriction, they produce lactic acid and other metabolites that, given the pooled blood and the stress the muscle comes under, produces the conditions necessary for increased release of growth hormone and muscle protein synthesis, the central regulator for muscle growth. BFR training therefore gives all of the advantages of working at high intensity with high loads while actually working at much reduced intensities. When used in conjunction with resistant exercise, BFR produces exceptional results.


To be most effective, BFR training should be done no more than 20 minutes a day, two to four times per week. Programmes should last for at least 12 weeks. Check out our BFR training programmes for detailed workouts utilising blood-flow restriction.


Muscle size increases as a result of the growth hormone and extra protein synthesis that comes about from BFR training. It is therefore a powerful stimulus that adds the biggest bang for buck to any workout you choose.


Although the strap is placed just above the bicep, gains can also be seen in nonrestricted muscle areas such as the shoulders and chest. This is because of the systemic effect of BFR. When blood flow is restricted in the strapped limbs, the body’s reaction is to produce metabolites and hormones within the muscle. When the straps are released, these are then flushed around the entire body, driving gains in areas such as the shoulders and chest. These areas also benefit while the strapped limbs are restricted. This is because the body realises that the strapped limbs are extremely fatigued and so increases muscle fibre recruitment in non-strapped areas. An example of this is the pectoral muscles being recruited during a bench press due to tricep fatigue from BFR.


Our BFR training programmes are a great starting point for those using BFR.


The benefits are widespread. By initiating a muscular response usually seen only from high intensity or resistance training, BFR increases muscle size, strength and endurance by working at low intensities, such as 20-30% one rep max. The training can be used for muscular hypertrophy, enhanced endurance, accelerated recovery, or to aid clinical rehabilitation and recovery.


Research has proven that BFR is a safe and effective training method. Users must read the warnings and know the correct techniques before starting BFR training. More information can be found in our BFR safety section.


Now you should have a good understanding of what BFR is and how it can help you achieve your fitness goals.

If you are ready to strap in, check out our range of Performance TechWear.

Not quite ready? No problem, find out more about BFR training in these articles;

Home bicep workout

Work out at home with BFR

Try BFR for Ultimate Rugby Performance

The proof is in the science