BFR for Endurance: Smart Path to Boost VO2max – Learn How

Yes, we know — it sounds strange and unreasonable. It might seem even more unreasonable when we say that restriction is not a completely crazy idea here, at least locally around your legs. We're diving into what Blood Flow Restriction (BFR) is, how it's used in intervals and easy sessions, and what studies on cyclists and rowers have revealed. The main question is whether BFR can boost your aerobic capacity without leaving you totally exhausted after every session. For those who don't want to deep dive: the conclusion is that BFR can increase VO2max and MAP relatively quickly when implemented smartly – for instance, during rest between intervals or during light blocks – but the effects need to be confirmed in larger and longer studies.

What is BFR?

Blood flow restriction (BFR) is the English term for blood flow restriction. We'll use the abbreviation BFR throughout the article. In Swedish literature, the term occlusion training is more common, but it's the same thing.

In short, the method involves partially restricting blood flow to the working muscles to create an increased oxygen debt in the muscle. The result is a greater metabolic response and faster adaptation to low oxygen levels in the muscle — primarily through signals that could lead to more capillaries, adaptations in mitochondria, and improved handling of by-products from anaerobic work.

The most common way to achieve BFR is by applying a pressure band around the muscles and tightening it to reduce blood flow in and/or out of the muscle. In studies, a system from Delfi is often used, tailored for BFR training where you can set times for restriction, relaxation, repetitions, sets, and more. This system is well validated in research, but the price is around 50,000 kr. Don't worry in this economic downturn — there are also more manual methods, more on that further down ;)

So, BFR is not entirely new; the method has been used in healthcare and strength sports and has strong scientific support for providing increased metabolic stress and subsequent training adaptation. One example is a study from 2018 that compared mitochondrial protein synthesis during strength training with light weights + BFR, heavy weights, or light weights without BFR. Researchers saw an increase in mitochondrial protein synthesis by 119% with light weights + BFR, 115% with heavy weights, and 92% with light weights without BFR. So something extra happens when you create a local oxygen deficit, especially when you combine it with work.

We quickly summarize what happens during BFR below.

What happens in the muscle during BFR?

When you constrict the vessels slightly, both arterial and venous blood flow decreases. This results in lower oxygen transport to the muscle and reduced removal of metabolic by-products. The muscle responds by making adaptations to handle the oxygen deficit: more mitochondria, redistribution of mitochondria to where they're needed, and increased capillary density — similar adaptations to high-intensity anaerobic training.

This is particularly interesting for endurance athletes. Nowadays, there's quite a bit of research on high-performing athletic groups, and I mention a review article from 2021: “Blood flow restriction training and the high-performance athlete: science to application” which speaks directly to you Perform Better readers/listeners. (The link is earlier in the text.)

The Studies

Study 1 & 2 – BFR During Rest

First, an exciting setup from two studies conducted in 2016 and 2019 by the same group. In both studies, about 20 well-trained cyclists (maximal oxygen uptake, VO2max) 60–65 ml/kg/min and threshold around 300 W (287–296 W)) performed two extra interval sessions per week over a total of four weeks.

Participants maintained their regular training volume but were divided into two equivalent groups: one with BFR and a control group with the same extra 30-second intervals but without BFR. The load in the intervals increased over the weeks: first week 4 sprints, second 5, third 6, and fourth 7 sprints per session.

The setup looked like this:

• Sprint: 4–7 x 30 s max sprint
• Rest: 4.5 min lying on a couch with pressure cuffs high on the thighs inflated to 120 mmHg. Then they lay 2 min with BFR before the pressure was released and they got on the bike again.

Total rest was 4.5 min where 2.5 min were without BFR (passive rest) and 2 min with pressure band around the thighs. The control group rested the same duration but without BFR. The session was performed twice a week for four weeks.

Results

The results showed that the group with BFR increased their VO2max by 5–6% and their Maximal Aerobic Power (MAP) by 3–4% in the two different studies conducted over four weeks. In one study, they also measured Critical Power (threshold) and it increased equally (+10 W) between BFR and control group.

There was no difference in performance between the groups after these four weeks measured via a subsequent 15 km time trial. Clarification: MAP is the average power in the last minute before stopping the test — meaning the average power participants produced in the last minute before the VO2max test was terminated due to exhaustion. VO2max is often reached during approximately 30 s when the participant reaches a plateau. VO2max and MAP typically follow each other in these tests.

In one of the studies where markers for training adaptation (like capillary density or levels of mitochondrial protein) were measured, no significant differences in actual increase were found between BFR and control group. However, in the BFR group, a significantly increased signaling for capillary growth was observed — but over four weeks, this did not translate into a measurable increase in capillary density. Perhaps the study duration was too short.

The researchers in the review article also note that a 15 km time trial is not particularly sensitive for capturing small performance differences. Increased oxygen uptake does not automatically mean more muscle strength, but it allows you to produce slightly more power per interval — at least for somewhat longer intervals — and over time achieve a higher training response. These studies are short, so we hope for longer studies in the future.

Below is a table of the results from the 2019 study.

These two studies were the first to test the BFR concept during rest — smart because it doesn't reduce potential during the actual interval. Your muscles get to work optimally with all the oxygen available, but you extend the oxygen debt between repetitions, so the total stimulus on the muscle increases.

Using BFR during the intervals, however, is less effective: you won't be able to produce the same amount of work, your legs will fatigue faster, and the total strain on both muscles and heart/lungs will be lower.

Study 3 – BFR during easy activity

For lighter intervals, there are interesting results. In a study, a group of elite rowers (VO2max ≈ 63 ml/kg/min) was asked to do three sessions with BFR per week for five weeks. The intervals were light: 2 x 10 min with a maximum of 2 mmol lactate (i.e., around the aerobic threshold), with 10 min rest in between.

These were extra sessions beyond the regular training. The results were compared with a control group doing the same sessions but without BFR.

Results

The rowers with BFR increased their VO2max by a full 9% from 63 -> 70 ml/kg/min, compared to the control group's increase of 2.5% over the same five weeks. That's about 6.5% more improvement than the control group in just five weeks — which is on par with the increase in VO2max in studies where BFR was used during rest (5–6% after four weeks).

Their MAP (Maximal Aerobic Power) also increased significantly: 15% from 383 -> 442 W in the test group, while the control group saw no significant increase (396 -> 408 W).

Practically interesting is that the researchers in this study used elastic knee wraps rather than electronic pressure cuffs. Specifically, 2-meter long and 13 cm wide wraps (Best Body in Germany) were used, wrapped around the thighs. The band was standardized to be stretched to 75% of its maximum length — deemed sufficient to reduce blood flow adequately. This method has been validated with ultrasound in a previous study, which explains why the researchers used it.

Both the study on elite rowers and the two cyclist studies show that BFR can be effective in increasing oxygen uptake in the muscles over relatively short periods (4–5 weeks). It's tempting to call it a super trick for faster development. Why does it work so well so quickly? Two reasons:

• The limitation in your VO2max is often local in the muscles — the muscle's ability to uptake oxygen, use it, and transport waste products. With BFR, you target the muscles directly and stress them oxygen-wise without the mechanical power development being limiting.
• The method tricks the brain. When it's really tough, the head often wants to slow down to save the body — even in the elite. With BFR at easy intensities, you experience lower general effort, but locally in the muscle, the oxygen deficiency is as high as at >90% of VO2max.

A practical approach

If we dive deeper into all studies that measured markers and messenger-RNA (mRNA) in the muscle, we see that BFR in itself does not give a different or stronger response than doing intervals to exhaustion. It is therefore not magically better than creating oxygen deficiency conventionally.

BFR also doesn't provide any extra during moderate-intensity activity compared to a control group also going moderately hard but to the level where they experience exhaustion.

In practice, the most interesting aspect is that you can stress the muscle oxygen-wise hard and stimulate increased oxygen uptake even on lighter sessions — without getting muscularly exhausted. If you get an extra hour of oxygen debt per week for the muscles, either by doing BFR during rest or through BFR during light activity, it is highly likely over time that you will increase your VO2max somewhat without sacrificing the truly intense intervals that you still need for mechanical work capacity.

You can also restructure training: skip one or two of the toughest VO2max sessions and gain some of the effect through lighter sessions with BFR. The time you save by avoiding a couple of maximal sessions can be spent on threshold, tempo, race pace, strength intervals, or strength training.

Conclusion

Interesting for everyone — effects have been observed in both untrained and well-trained individuals. The effects are often measured as increases in oxygen uptake between 5–10%, and especially 5–7% more than a control group over a span of 4–8 weeks. The increase has been observed both with the method of 2 min BFR during interval rest and with 2 x 10 min BFR during low-intensity sessions.

The research is not new, but much remains to be explored. The studies are often small, and we still don't know how the differences develop over 6–8 weeks or longer. But perhaps it will be your task to test and evaluate 😉 In theory, there's much to gain here — the question is how significant the effects are in practice and if they actually notice on race day. That part remains to be explored :)