Norwegian 4x4 intervals raised VO2max more than moderate runs in landmark trial

Forty moderately trained men, three sessions a week for eight weeks. That was the design Jan Helgerud and ten coauthors used in a 2007 randomized trial at the Norwegian University of Science and Technology, and the result is what put the protocol on the map. The men assigned to four-minute high-intensity intervals raised their VO2max by 7.2 percent. A matched group running at a moderate pace for the same total work showed essentially no change. The paper ran in Medicine & Science in Sports & Exercise, and its findings are why coaches still call this the Norwegian 4x4.

The protocol is back in the cultural conversation this week because the actress Jessica Biel told an interviewer she had built her training around it. ScienceAlert ran a feature, and the inevitable round of explainer videos followed. None of which is the actual story. The research is.

How the original 4x4 trial was designed

Helgerud’s team started with 40 healthy men who had a training history. Each was randomly assigned to one of four protocols, all matched for total energy expenditure: continuous moderate-intensity running, long-interval running at lactate threshold, short-interval work, or the 4x4. The 4x4 is the one that stuck. Four bouts of four minutes at 90 to 95 percent of measured maximum heart rate, with three minutes of jogging in between at around 70 percent. Sessions ran three times a week for eight weeks. Each came in at about 35 minutes once you counted the warm-up and cool-down.

The 4x4 group raised VO2max by 7.2 percent. The 45-minute continuous-running group, doing the same total amount of work, showed essentially no change. Stroke volume (the amount of blood the left ventricle pushes with each beat) climbed roughly 10 percent in the interval groups and stayed flat in the moderate group. Helgerud’s read was that intensity, not duration, appears to be the rate-limiting variable for aerobic adaptation in adults who already train.

Why intensity matters more than minutes

VO2max is the ceiling on how much oxygen working muscles can pull from circulating blood. Raising it requires two things: a heart that pushes more blood per beat, and muscle that can extract more oxygen from each beat. Both adaptations seem to respond better to brief, hard efforts than to long, comfortable ones.

Some of the cleanest evidence on the muscle side comes from Martin Gibala’s lab at McMaster. In a 2008 review for Exercise and Sport Sciences Reviews, Gibala and Sean McGee laid out a striking finding: six sessions of high-intensity interval work spread over two weeks raised muscle oxidative capacity by roughly the amount you would otherwise expect from weeks of conventional endurance training. They tied the response to PGC-1 alpha signalling and other mitochondrial biogenesis pathways, which appear to fire most aggressively during short, repeated bouts of near-maximal effort. Gibala’s 2014 follow-up with Jenna Gillen in Applied Physiology, Nutrition, and Metabolism pushed the case all the way down to sprint intervals: three 20-second all-out efforts, three times a week, produced cardiometabolic improvements comparable to much longer moderate sessions.

What 4x4 looks like in practice

The mechanics are simple, even if the doing is not. After a warm-up of five to ten minutes, you do four bouts of four minutes each, hard enough that talking in full sentences is off the table by the second minute. Three minutes of easy jogging, walking, or cycling between bouts. Repeat four times. Helgerud’s heart-rate target was 90 to 95 percent of measured maximum, which is well past what most people experience on a typical hard run.

You do not have to run for it to work. Helgerud’s group used a treadmill, but follow-up trials have replicated the VO2max gains on rowing ergometers, stationary bikes, and uphill walking, which is useful for adults who cannot run at the required intensity without risk. Total time, warm-up and cool-down included, runs 35 to 40 minutes.

The catch

The catch is that 90 to 95 percent of maximum heart rate is genuinely uncomfortable. Helgerud’s cohort was a group of already-active men in their early twenties. Later trials have run the protocol with older adults and with clinical populations, including post-myocardial-infarction patients in cardiac rehab, but those programs come with medical oversight, not a YouTube tutorial.

For someone who has not been training, jumping straight into four-minute bouts at 95 percent of HRmax is a bad idea. The standard cardiology and exercise-physiology advice is to spend two or three weeks building a moderate aerobic base first, then add one shorter interval session a week before working up to the full 4x4. If you have cardiovascular risk factors, talk to your doctor before starting.

Bottom line

Helgerud’s eight-week trial does not prove that intervals beat moderate exercise across the board. The sample was small, the cohort homogeneous, and outcomes outside VO2max (injury rates, sleep, long-term adherence) were not what the study set out to measure. What the paper supports is a narrower claim that has held up across almost two decades of replication, including in clinical populations: minute for minute, work at the top of your heart-rate range tends to raise cardiorespiratory fitness more than the same total time at a comfortable pace.

The recent celebrity attention will pass. The underlying physiology will not.

References

1. Helgerud J, Hoydal K, Wang E, et al. Aerobic high-intensity intervals improve VO2max more than moderate training. Medicine & Science in Sports & Exercise. 2007;39(4):665-71. https://doi.org/10.1249/MSS.0b013e3180304570
2. Gibala MJ, McGee SL. Metabolic adaptations to short-term high-intensity interval training: a little pain for a lot of gain? Exercise and Sport Sciences Reviews. 2008;36(2):58-63. https://doi.org/10.1097/JES.0b013e318168ec1f
3. Gillen JB, Gibala MJ. Is high-intensity interval training a time-efficient exercise strategy to improve health and fitness? Applied Physiology, Nutrition, and Metabolism. 2014;39(3):409-12. https://doi.org/10.1139/apnm-2013-0187