“You can get stronger and gain muscle by lifting light weights” seems like a clickbait-y type of claim, but would you be surprised to learn there’s some legitimate (and not-so-legitimate) science to support a training method to do just that?
Enter Blood Flow Restriction Training.
Blood Flow Restriction (BFR) dates back to the 1960’s, when Dr. Yoshiaki Sato used small, rubberized bands as tourniquets at the top of his arms and legs during the limited exercise he could do while recovering from an injury. By reducing the blood flow, he thought, he could stimulate the muscles effectively and not only limit muscle loss but also speed up his recovery. As the story goes, Dr. Sato reported that his method worked well and subsequently patented it as Kaatsu training. Over the last 20 years or so, BFR has become a popular training method in not only sport, but also in rehabilitation settings.
WHAT IS BLOOD FLOW RESTRICTION (BFR)?
Blood Flow Restriction (BFR) is a training technique that involves restricting venous blood flow from a working muscle while allowing arterial blood flow into the muscle. This is typically achieved using a cuff or band placed around the proximal portion of the limb.
Blood flow starts with the beating heart, which pumps out blood to the organs and tissue through arteries, and then returns to the heart through veins. Blood pressure measures the force of the blood in our arteries when the heart is contracting, e.g. the “systolic” phase of the cardiac cycle (the top number), and the blood’s force in our arteries when the heart is relaxing, e.g. the “diastolic” phase of the cardiac cycle (the “bottom” number). A blood pressure measurement of 120 mmHg / 80 mmHg refers to 120mmHg systolic over 80 mmHg diastolic. The higher pressure in the arteries compared to the veins facilitates flow of blood through the vessels of our body.
Now, suppose you placed a cuff or wrap to your arm or leg with a bit of pressure to squeeze the blood vessels in that limb. If applied tightly enough to squeeze only the veins, but not the arteries, more blood enters the limb than can exit. This causes a bit of a traffic jam in the blood vessels, delaying the oxygen and energy delivery to the working muscle in the occluded limb. This is the technique used with BFR in order to create an environment conducive to increases in muscle size and strength.
HOW DOES BFR WORK?
Let’s go over the 3 most likely ways BFR has an effect on muscle growth (hypertrophy) and muscle strength.
Metabolic Stress
Metabolic stress is essentially the accumulation of biochemical by-products like hydrogen and inorganic phosphate ions, which occurs when some of the energy stores within the muscle get low during exercise. It’s what is most commonly attributed to the “burn” you feel in muscles when they are working.
In general, anytime muscles are contracting during resistance training, they’re producing these metabolites, making it hard to determine whether metabolites contribute to hypertrophy or if it’s just the mechanical force from muscular contractions. Based on the present data, it appears the majority of muscular hypertrophy is caused by mechanical signals, whereas metabolites may have an indirect role.
Because low loads are used in BFR, the tension in the muscle is not as comparable to that of conventional resistance training and BFR has been shown to cause virtually no muscle damage. This actually may be one of the other reasons it works, as muscle hypertrophy, e.g. increased muscle fibre size, occurs when muscle protein synthesis exceeds muscle protein breakdown. In other words, hypertrophy seems to lag until muscle protein breakdown is minimized and muscle protein synthesis predominates.
Metabolic stress is one of the likely mechanisms by which BFR works, though it’s not a slam dunk. Existing evidence is conflicting regarding this topic, as there does not seem to be conclusive advantages of using continuous BFR versus intermittent BFR.
Motor Unit Recruitment
A motor unit is the nerve cell and all the muscle cells or fibres it has an effect on. When lifting heavier loads (above 65% of 1 RM), many of these motor units are recruited to help overcome the force required to resist or move the external load. However, with BFR, similar amounts of motor units are recruited with significantly lower loads. The ability to recruit more motor units significantly contributes to strength and is the most likely reason for gains in strength seen with the use of BFR.
Cellular Swelling and Cellular Hypertrophy Signalling
Cell swelling, in the case of BFR, is most likely the result of restricting blood flow out of the limb, causing muscle cells to swell. This swelling is thought to have a potential effect on muscle hypertrophy. What is proposed in theory, is that this swelling leads to several other cellular mechanisms that are anabolic in nature (muscle growing). Though not clear yet, cell swelling may contribute to the mechanisms leading to an increase in muscle size over time.
Hormonal Signalling
As one small bonus, there are potential hormonal contributions that may be worth mentioning here. For example, there is a significant increase in growth hormone release following BFR, though the impact on hypertrophy is unclear and shouldn’t be considered a primary factor at this time. For now, this is more of an interesting phenomenon but shouldn’t be considered as being a significant contributor to the effects seen with BFR.
HOW TO DO BLOOD FLOW RESTRICTION TRAINING?
The band or wrap used in BFR training is typically applied as high up on the limb as possible. In research and healthcare settings, a specialized cuff that can measure blood pressure is often used in an effort to reduce the risk of either the cuff being too tight and occluding arterial blood flow which could be harmful, or the cuff being too loose and not delivering the intended result. However, more practical applications can be applied and will be discussed later. It should be noted here that BFR has been shown to be quite safe across different populations, limb locations, and pressures in both relatively short and long-term applications.
The loads lifted while using BFR are low compared to conventional resistance training. As a general guide, conventional resistance training is performed with loads at or above 60-65% of a 1-repetition-maximum (1RM) for sets of 1- to 20-repetitions, followed by rest periods of 2- to 5-minutes.
In contrast, the most commonly used scheme in BFR is a set of 30 repetitions, followed by 3 sets of 15 reps with 30-60 seconds of rest between sets with loads between 20-30% of 1 RM.
BLOOD FLOW RESTRICTION FOR STRENGTH TRAINING
First, a conventional approach to strength training is still going to be the most valuable stimulus for hypertrophy and strength gains. While BFR has been shown to yield similar results in hypertrophy when compared to conventional resistance training, it is unknown exactly how long these results can be expected as it is possible there are diminishing returns with continuous use over time. In isolation, BFR has an overall smaller effect size than conventional resistance training, though this effect may be different when BFR is combined with conventional training.
Improvement in strength seems to be the same or even better when BFR is combined with conventional strength training interventions. This is most likely due to the recruitment of additional motor units when using BFR despite the lower relative loads. For this reason, BFR may likely be an appropriate addition to a conventional training program where heavier loads are being lifted for part of the training session and BFR with low loads is utilized the other part of the training session.
BLOOD FLOW RESTRICTION FOR INJURY REHABILITATION
BFR is often used in a rehabilitative setting as heavier loads may be unavailable, may be contraindicated following procedures, or cannot be tolerated well due to pain or instability. It has been shown that patients in these settings lose less muscle over time even when BFR is applied passively without exercise being performed. Following procedures or injury, BFR has been successfully used to minimize muscle loss, improve strength, and improve measurements related to returning to sport or activity. It should be noted that when compared to rehabilitation without BFR, the BFR groups do tend to outperform the non-BFR groups.
As mentioned before, it is important to remember that BFR has been shown to provide greater benefits in strength when combined with conventional resistance training. For this reason, BFR should not be a stand-alone intervention for too long following an injury if you are looking to maximize the benefits of a well-rounded rehabilitative program
BLOOD FLOW RESTRICTION FOR RECOVERY
A final potential benefit of BFR may be for its use as a recovery modality. Oddly, this idea stems from research on the heart. It has been shown that repeated bouts of starving the heart of oxygen and then reperfusing it with oxygen-rich blood has a positive effect on cardiac tissue health. This idea was then taken further to assess the effect of passively occluding blood flow to muscle tissue for 2 bouts of 3 minutes following intense exercise and letting the blood naturally flow through the limb for 3 minutes between sets. What’s great about this study is that they measured objective outcomes like jump and sprint performance 5 minutes and 24 hours after this intervention instead of something more subjective like soreness. In this study, it was shown that the passive BFR following intense exercise had some positive effects with the majority of these effects happening 24 hours after the intervention. Though this evidence may not be incredibly strong and caution should be taken in the interpretation of the results.
BFR SUMMARY
BFR seems to have a positive effect on hypertrophy and strength in both trained and untrained individuals. BFR is safe, causes little to no muscle damage, and is easy to recover from. In fact, it may aid recovery. For this reason, you don’t need to change your programming much to work BFR in. However, if your normal training is wearing you down, BFR can be a solid substitution for some of your training volume to continue maximizing strength and hypertrophy gains while backing off more frequent or higher volumes of heavier, conventional lifting.
Similarly, BFR can be extremely beneficial in the context of rehabilitation whether performing exercise with them on or passively having limbs occluded for several bouts. This can be beneficial when unable to lift heavier loads due to surgical precautions or due to discomfort with heavier loads. In general, when using BFR in a rehabilitative context, it is highly recommended to work with a healthcare professional when starting out.
Whether used for additional gains in regular training or in the rehabilitation of a musculoskeletal injury, BFR seems to be a safe and practical tool to improve strength and hypertrophy across diverse populations.
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