Jim Stray-Gundersen and the Evolution of BFR

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Since the mid-1980s, I’ve had the good fortune of working with the late Dr. Jim Stray-Gundersen on several projects. Our first conversation was at a training camp in Hatcher Pass, Alaska, either in October 1985 or 1986. We were discussing how we could better assess cross-country ski training beyond just counting hours per week, month, or year. The key, we agreed, was physiological analysis, that included metrics to assess duration and intensity, not just hours.

Roughly six years later, we were both attending the American College of Sports Medicine conference and connected with Dr. Eric Bannister, who introduced us to his TRIMP model for training load monitoring. That TRIMP algorithm is something I still use and have written about elsewhere.

Over the years I spent time in Dallas with Jim in his lab at UT Southwestern testing athletes, talking with experts like Peter Snell and Ben Levine about training and physiology. I also co-authored with Jim a study assessing the metabolic demands of common training methods used in cross country skiing. I observed testing and training of runners who participated in Jims’ and Ben Levine’s seminal high low altitude studies. All in all, a great education in coaching and the science of the sport.

About 12 years go, I was working in Stowe, Vermont, and drove down to Hanover to meet Jim while he was visiting to watch his son Sten play soccer at Dartmouth. Jim had started working with KAATSU, the original Blood Flow Restriction (BFR) system. He walked me through how it worked and took me through a short session. I’ll admit, I was skeptical.

Then in 2017, Jim launched a new BFR system using pressurized elastic bands—much simpler, safer, and critically, untethered from bulky pneumatic pumps. That change, alone made it far more practical for athletes, particularly in a sport like cross-country skiing.

My brother Joe and I bought a few sets to test. Joe used BFR with his daughter, a basketball player, and I began testing it with myself and a few masters athletes. Over time, we both found the response to BFR amazingly effective. For me, after just 6–7 BFR sessions in a two-week span, I went out for a run. What I felt was unmistakable, my legs felt significantly stronger, and my sustained pace at low intensity had improved by about 3-4%. That was the light bulb moment. This BFR training clearly worked. Over the last decade I have used several BFR protocols on myself and athletes in cycling, xc skiing, and the improvements are significant and measurable. It took several years of testing and personal use to really commit to sharing it with others.

My final project with Jim was over a two year span using Firstbeat HRV metrics to assess the stress of BFR workouts—very similar to normal free weight strength workouts,—and also to assess workout protocols and recovery parameter from BFR workouts. Unfortunately, we never got to the end zone with this work. But we learned a lot in the process.

How BFR Works

Jim’s patented elastic pneumatic bands restrict venous return while still allowing arterial inflow. That distinction is essential; he used Doppler ultrasound to confirm that even at maximum pressure, full occlusion does not occur. The goal is not to cut off blood supply but to safely slow venous return to create metabolic stress.

The training stimulus leads to elevated production of growth hormone, testosterone, and other anabolic hormones, enhancing strength, bone growth, bone density, and tissue repair. In short, BFR triggers the same cardiovascular and musculoskeletal adaptations we associate with high-intensity strength and endurance training, but with significantly lower loads, less injury risk and faster recovery. For endurance athletes, this is a real breakthrough. You get the benefits of strength training, specific strength, and recovery stimulus without the stress and extended recovery time of traditional heavy lifting.

And it’s fast: most BFR workouts are 20–40 minutes. Compared to traditional strength training, we’ve seen 18–20% gains in strength with minimal soreness or muscle damage. In fact, BFR causes muscles to fatigue through safe restriction and low oxygen availability, prompting a full hormonal cascade and strength response, without the structural damage that normally follows failure in a gym setting.

What Happens in a BFR Session

1. Bands are placed around the upper arms or thighs.
2. Pressure is applied to slow venous return.
3. The athlete performs low-load, high-rep exercises.
4. Local fatigue sends signals to the brain.
5. The brain releases growth hormone and other anabolic compounds.
6. Hormones circulate systemically and target working tissues.
7. Muscle, bone, and vascular tissue adapt and rebuild stronger.

It’s important to note: this isn’t just a local effect. The hormonal response is systemic: your whole body benefits.

What We Learned and How We Use BFR in Endurance Training?

Improving strength is essential for improving pace, power, and efficiency in endurance sport. But the volume required to build endurance—especially for juniors and masters athletes—often squeezes out time for strength work, or compromises recovery when it’s added in.

BFR changes that. Now, strength training can be done in 20–40 minutes, many times per week, using light loads and recovering quickly enough to stay on schedule with aerobic work.

Applications of BFR: Strength, Specific Strength, Adaptation, and Recovery

1. Anatomical Adaptation Phase

Before any serious strength training, BFR or otherwise, we need to prepare connective tissues. I typically recommend 6–8  “passive”  BFR sessions at very low intensity: jogging, biking, swimming, skiing, or ski erg.

Use all four bands and perform 1–5 minute efforts at sub-threshold intensity, not exceeding 15–20 minutes per session. These sessions won’t produce heavy muscle fatigue but will allow time for the ligaments and tendons to strengthen to avoid future injury.

2. General Strength Training

• Base Phase (16–20 weeks): 3–4 sessions/week
• Pre-Comp Phase (8–12 weeks): 2–3 sessions/week
• Competition Phase (12–16 weeks): 2 sessions/week (only in non-competition weeks), 1 sessions in competition weeks

Each session should be 20–40 minutes depending on frequency.

• 3–4 exercises/session
• 30–35 reps per set, 3 sets/exercise
• Rest: 30 seconds between sets
• Load should be very light—something you could do 100+ reps of without bands. The goal is fatigue, not load.

Focus on large muscle groups and compound movements. For upper body, use arm bands. For lower, use leg bands. For compound, use both.

Initially alternate with upper and lower body days. Frequency matters more than load, frequent moderate hormonal stimuli are better than infrequent maximal ones.

Sample:

• Day 1 – Lower body: squats, lunges, hip hinge
• Day 2 – Upper body: pushups, band rows, overhead press
• Repeat for 3–4 sessions/week

After 6–8 sessions, you’ll see measurable improvements in activation and strength.

For high-performance athletes, I recommend every third or fourth session be a maximal strength session (8–12 RM loads). Nothing extreme—just enough to maintain neuromuscular adaptation.

3. Sport-Specific Strength Endurance

In the base and pre-competition phases, we’ve had good results finishing endurance workouts with short BFR intervals.

2x/week, after an extended aerobic session:

• Put on the bands and inflate.
• Do 4–8 × 60–90 seconds of running, biking, roller skiing, or swimming at or below aerobic threshold (Zone 1 <70–75% max HR)
• Add terrain or light resistance to increase force output

We’ve also used 15–20-minute steady efforts post-workout to improve aerobic capacity and enhance the training effect.

4. Recovery & Adaptation

One of the more exciting applications: using short BFR sessions immediately after high-intensity workouts to enhance recovery and adaptation.

After speed or interval sessions:

• Put on the bands and inflate.
• Do 1–2 simple exercises (e.g., squats + rows)
• 3 × 30 reps, light load, just enough to create mild fatigue.

This drives an improved adaptation and recovery-focused hormonal response without adding real training stress.

Conclusion

After a decade of testing BFR, I’m convinced it can help athletes across all levels. I’ve seen cyclists increase sustainable race power by 5–10% in just three weeks of BFR. Yet, it’s still surprisingly underutilized in elite circles, or at least athletes are not talking about it.

That might be changing. I recently saw a post of Tadej Pogačar using BFR bands. It may be hitting the mainstream now, but the physiological benefits have been there for a while.

If you’re an endurance athlete looking to train smarter, not just harder, BFR is worth serious consideration.