BFR: easy does it for improved endurance

Sport scientists are always looking for ways to make training more effective and efficient. Because many of the basic principles of training are well understood and already applied in training, some of the latest and most innovative breakthroughs in training strategies actually come from other fields of science. The use of blood flow restriction (BFR) training is a prime example. Originally developed by physiotherapists and medical professionals to improve and accelerate the healing of muscle injuries⁽¹⁾, sports scientists soon realized that the same physiological principles in BFR could also help uninjured athletes to develop greater levels of muscle strength and power^(2,3).

What is BFR?

In the first SPB article on this topic, author Andrew Sheaff explained that blood-flow restriction training describes a form of training where restrictive wraps or cuffs are placed on the upper portion of the limbs to occlude venous (lower pressure, flowing towards the heart), but not arterial (high pressure flowing away from the heart) blood flow. Lower intensity training is then performed while these wraps remain in place. The popularity of BFR use has grown to such an extent that BFR is now more widely used for strength development than for injury recovery. That popularity is well deserved because review studies of the use of BFR for athletes seeking to develop maximum levels of power and strength show that it is highly effective⁽⁴⁾ – not just for athletes but also for completely untrained individuals who wish to improve muscle strength and size⁽⁵⁾.

Endurance and BFR

Given that the use of BFR training was pioneered in the context of injury, it’s not surprising that most of the research into BFR has explored the effects on muscle size, strength and the ability of athletes undertaking BFR to resume normal training and return to sport. However, some of the recent research into BFR training has begun to look at the potential benefits for endurance athletes such as runners, and the results seem to be promising.

In a previous 2022 SPB article, we looked at research published in the Journal of Strength and Conditioning Research investigating the effects of an 8-week program of three days per week running training combined with blood flow restriction aerobic capacity (and muscle strength) in endurance athletes⁽⁶⁾. This research found that the outcomes in the BFR-trained runners were superior to the runners who had performed normal running training. Compared with the normally training runners, the BFR-trained runners:

·        Experienced a significantly greater increase in VO2max (5.1% vs. -1.1%)

·        Exhibited greater isokinetic knee extensor strength (16.5% vs. −5.9%)

·        Showed higher leg muscle mass (10.3kg vs. 9.7kg)

Although some research published around the same time questioned the use of BFR training for endurance athletes⁽⁷⁾, more recent studies provide evidence in favor of the use of BFR as part of an endurance training program. For example, a study BFR use in interval training to develop endurance performance found that the addition of BFR enhanced physiological improvements in aerobic, muscular, and anaerobic performance⁽⁸⁾. Another systematic review and meta analysis (a study that reviews and pools all the previous data on a topic to come up with robust conclusions) found that compared to not using BFR training, the aerobic performance gains in a general fitness program in untrained individuals were enhanced when some BFR training was included⁽⁹⁾.

Box 1: Why does blood flow restriction training work?

There’s no doubting the effectiveness of BFR - certainly when it comes to building muscle size and strength - but why exactly does restricting blood flow during exercise produce these benefits? The truth is that we don’t know for sure. However, a number of mechanisms have been proposed for the positive and superior (compared to conventional training) adaptations, which include the following:

-       An increase in the recruitment of type II fast-twitch fibers (because the reduction of available oxygen impairs slow twitch [endurance] fiber recruitment)⁽¹⁰⁾.

-       Muscle cell swelling, which promotes a greater inflow of active substances required to stimulate growth⁽¹¹⁾.

-       An increase in the production of signalling molecules such as nitric oxide, which activate anabolic pathways⁽¹²⁾.

-       An increased secretion of anabolic hormones such as human growth hormone and testosterone due to a more anaerobic environment and increased lactate accumulation^(13,14).

New research

While there is very robust evidence that BFR training can be a valuable addition to a strength training program – regardless of whether the goal is strength for injury rehab or training for outright strength – the evidence that BFR training can enhance aerobic capacity and endurance performance over and above that produced by regular training is still pretty sparse. However, a brand new study on elite rowers by a team of German scientists makes for very persuasive reading on the aerobic benefits of BFR training⁽¹⁵⁾.

Published just a month ago in the Journal of Strength and Conditioning Research, this study examined the effects of low-intensity rowing using blood flow restriction (BFR) on the maximal oxygen uptake (VO2max) and peak power outputs in rowers during an incremental ramp test and time trial times over 2,000m (both key parameters of rowing performance). To do this, 11 elite, male rowers were recruited to participate in the study. With an average age of 22 years, these rowers had typically been in training for eight years, training for around 20 hours per week over 12 x separate sessions per week. All the rowers went through a 10-week study period. This comprised of:

·        Five weeks of normal training without any BFR sessions (which served as the control condition for the rowers).

·        Five weeks of a BFR training intervention, which consisted of normal training as above but where one hour per week of normal training was substituted with BFR training – the intervention condition.

In the intervention condition, BFR training took place three times per week. To achieve the blood flow restriction, elasticated cuffs were placed around the top of the legs of the rowers, using the typical pressure employed in previous studies (see this article for a complete discussion on cuff pressures and impacts on BFR outcomes). No cuffs were placed on the upper arms, the reasoning being that much of the power generated during rowing comes from the lower body.

Each of the three BFR sessions consisted of two sets of 10 minutes of low-intensity rowing (ie 20 minutes per session) during which intensity was kept at an easy and steady state pace – ie blood lactate levels remained at 2mmol/L or less (often referred to as ‘zone 1’ training – see this article). At the start of the 10-week study period, all the rowers were assessed for their VO2max levels, peak power outputs and sustained power over the 2,000 time trial. These measurements were then repeated after the 5-week control condition training period, then again after the 5-week intervention condition (ie at the end of the ten weeks). Statistical analysis was then carried out to see how these measured parameters of fitness changed following the 5-week normal training period and the 5-week period using BFR training, and in particular, whether the BFR training resulted in superior gains.