NTNU researchers explain that it usually takes 2 to 3 minutes for the heart to start pumping at its maximum rate, so 4-minute intervals allow time at a high cardiac output within each bout.

It is demanding. Generation 100 participants were healthy and active, and Harvard Health reported only 3 adverse events, all injuries from slipping. A Harvard physician advises that people who are older or have heart disease should check with their doctor before trying HIIT.

The Norwegian 4x4: the interval workout with the best evidence

Q: What is the Norwegian 4x4 workout?

The 4x4 is an interval workout developed by researchers at the Norwegian University of Science and Technology (NTNU). In the version used in the Generation 100 study, as described by Harvard Health, it lasted 43 minutes: a 10-minute warm-up at about 60 percent of maximum heart rate, four cycles of 4 minutes at 85 to 95 percent of maximum heart rate with 3 minutes of recovery, and a 5-minute cool-down.

Q: What did the Generation 100 study find?

Generation 100 randomly assigned 1,567 Norwegians aged 70 to 77 to twice-weekly high-intensity intervals, twice-weekly moderate continuous training, or national activity guidelines for five years. Combined, the exercise groups showed no difference in all-cause mortality compared with controls. The interval group showed a lower mortality trend (3 percent died, versus 6 percent in the moderate group), but that difference was not statistically conclusive.

The Norwegian 4x4 is an interval workout: four bouts of 4 minutes at 85 to 95 percent of maximum heart rate, separated by 3 minutes of easier recovery. Its reputation rests largely on Generation 100, a five-year randomized trial of 1,567 Norwegians aged 70 to 77, in which 3 percent of the interval group died compared with 6 percent of a moderate-exercise group. The trial did not prove that intervals extend life, but it is some of the best long-term evidence any single workout has.

For executives, the appeal is obvious. The 4x4 takes about 40 minutes, twice a week, and comes with a research pedigree that most workouts on social media do not. That combination has made it a favorite of the longevity crowd.

The research deserves a careful reading, though, because the headline and the actual findings are not quite the same thing. This piece covers what the 4x4 is, what Generation 100 found, where it falls short, and how to think about it if your week is already full.

What is the Norwegian 4x4 workout?

The 4x4 comes out of the Cardiac Exercise Research Group (CERG) at the Norwegian University of Science and Technology (NTNU) in Trondheim, which has studied high-intensity interval training for more than two decades.

In the form used in the Generation 100 study, Harvard Health describes a 43-minute session:

A 10-minute warm-up at 60 percent of maximum heart rate
Four cycles of 4 minutes at 85 to 95 percent of maximum heart rate, each followed by 3 minutes of recovery
A 5-minute cool-down

For reference, the American Heart Association estimates maximum heart rate as "about 220 minus your age," so a 50-year-old's estimated maximum is 170 beats per minute. The formula is a population average, not a measurement of your heart.

Why four minutes?

The length of each interval is deliberate. NTNU's research news service explains that "it usually takes 2-3 minutes for the heart to start pumping at its maximum rate." A 4-minute bout gives you time working near that ceiling before the recovery period, which is what the method is designed to train.

Ulrik Wisløff of NTNU's CERG summarizes the rationale: "Interval training is thought to be the most effective method of training the heart to pump lots of oxygen-rich blood."

1,567

Norwegians aged 70 to 77 randomized in the Generation 100 trial

BMJ (via PEDro)

5 years

of supervised training and follow-up

BMJ (via PEDro)

of the interval group had died after five years, vs. 6% in the moderate group

Norwegian SciTech News (NTNU)

What did the Generation 100 study find?

Generation 100 was published in The BMJ in 2020 by Dorthe Stensvold and colleagues at NTNU. It is unusual in exercise science because it was randomized and ran for five years, far longer than most training studies.

According to the trial abstract, participants were randomized to "two sessions weekly of high intensity interval training at about 90% of peak heart rate (HIIT, n = 400), moderate intensity continuous training at about 70% of peak heart rate (MICT, n = 387), or to follow the national guidelines for physical activity (n = 780; control group); all for five years." NTNU's report notes that the moderate sessions lasted 50 minutes.

The results, as reported by NTNU: "In the interval training group, 3% of the participants had died after five years. The percentage was 6% in the moderate group." In the control group, 4.5 percent had died. For comparison, NTNU notes that "Among most 70-77-year-olds in Norway, 90% will survive the next five years."

The interval group had the lowest five-year death rate, but all three groups did far better than expected

Share who died within five years, %. Expected rate for Norwegians aged 70 to 77 shown for comparison

4x4 intervals

4.5%

Guidelines (control)

Moderate continuous

10%

Expected, age 70-77

Source: Norwegian SciTech News (NTNU), on the Generation 100 study (2020)

View data

	Value
4x4 intervals	3%
Guidelines (control)	4.5%
Moderate continuous	6%
Expected, age 70-77	10%

Quality of life also favored the interval group. Stensvold told NTNU: "Both physical and mental quality of life were better in the high-intensity group after five years than in the other two groups."

What the evidence says, and what it doesn't

This is where precision matters, because the 4x4 is often presented online as "the workout proven to extend your life." That is not what the trial concluded.

The mortality difference was not conclusive

The formal analysis in The BMJ reported a hazard ratio of 0.63 for the interval group compared with controls, with a 95 percent confidence interval of 0.33 to 1.20. Compared with the moderate group, the hazard ratio was 0.51, with a confidence interval of 0.25 to 1.02. Both intervals cross 1, which means the results are compatible with no real difference.

The authors' own conclusion, per the abstract: "This study suggests that combined MICT and HIIT has no effect on all cause mortality compared with recommended physical activity levels. However, we observed a lower all cause mortality trend after HIIT compared with controls and MICT."

A trend is worth taking seriously. It is not proof. Harvard Health summarized the result simply as no survival differences among the three groups.

The control group did not stay still

NTNU reported that "One in five people in this group trained regularly at high intensity." When a meaningful share of the comparison group adopts the behavior you are testing, differences between groups shrink. This makes the study harder to interpret in both directions.

The participants were unusually healthy

All three groups died at rates well below the expected 10 percent. NTNU is candid about why: "The people who signed up to participate in Generation 100 probably had high training motivation to begin with. They also started with a relatively high level of activity, and most of them considered themselves to be in good health." Stensvold added that "training is probably not the only reason why so few of the Generation 100 participants died."

What is well supported

Beyond Generation 100, NTNU's group has a long line of research on intervals in clinical populations. Their 2004 study, led by Øivind Rognmo, found that high-intensity intervals improved aerobic capacity more than moderate exercise in patients with coronary artery disease, and a 2007 study led by Wisløff found that older patients with chronic heart disease could perform high-intensity exercise safely, with improvements in heart function and quality of life. Those were supervised clinical studies, not do-it-yourself training.

Read it like a pitch deck

Founders learn to read a startup's headline metric with some suspicion: what is the denominator, what is the comparison, and what happens to the story once you look at the confidence interval? Exercise research deserves the same treatment.

The Generation 100 headline, half the death rate of the moderate group, is real. It also sits on a small number of events. When only a few percent of each group dies, a handful of deaths in either direction can move the percentages a lot, which is exactly why the confidence intervals are wide. A second long-term trial showing the same pattern would be far more persuasive than any amount of enthusiasm about the first.

None of this is a reason to dismiss the 4x4. It is a reason to adopt it for the right reasons: it is an efficient, well-studied way to build aerobic fitness, and the long-term data leans in its favor. That is a strong case on its own. It does not need to be inflated into a guarantee.

The fair summary: the 4x4 is a well-studied, efficient way to train aerobic capacity, with a long-term randomized trial that points in a favorable direction without settling the mortality question.

A close-up of a stopwatch against a black background — Four minutes on, three minutes off. The structure is simple. The effort is not.Photo: William Warby on Unsplash

Is the 4x4 right for a busy executive?

It depends on what you already do and how you recover.

It is efficient. Two sessions a week is a manageable commitment, and Generation 100 tested that schedule over five years rather than a few weeks.

It is hard. Intervals at 85 to 95 percent of maximum heart rate are uncomfortable by design. On a week of short sleep, travel and a board meeting, a hard session can add stress to a system that already has plenty.

It is not the only answer. The same NTNU piece on the 4x4 includes a dissenting view from Thomas Haugen of Kristiania University College: "The best progress is achieved with different combinations of training sessions." Relying on one workout forever tends to lead to plateaus.

Safety comes first. Generation 100 participants were healthy and active, and Harvard Health reported only three adverse events, all injuries from slipping. That does not transfer automatically to a sedentary 55-year-old. Harvard's Dr. I-Min Lee puts it plainly: "HIIT is a great regimen for people who are young and healthy. If you're older or have heart disease, check with your doctor before trying it." If you have heart disease, risk factors for it, chest symptoms with exertion, or have been inactive for a long time, talk to your physician first.

What you can do this week

Get clearance if you need it. If any of the risk factors above apply, book the conversation with your physician before your first hard session.

Build a base first. If you are not currently exercising regularly, weeks of easier aerobic work make hard intervals safer and more tolerable.

Try one session, not a program. Put a single 4x4 session on your calendar this week, with a proper warm-up and cool-down, and notice how you feel that night and the next morning. Your sleep and mood the next day are useful data.

Pair it with easier days. Generation 100 tested twice a week, not every day. The rest of the week can be easy aerobic work, strength training and genuine recovery.

Watch your stress load. Hard training is a stressor, and so is running a company. Our piece on what chronic stress does to a founder's body explains why stacking stressors without recovery backfires.

The recovery side of high performance

The 4x4 works by pushing hard and then recovering fully, over and over. The same pattern applies to leadership, and it is exactly where most founders struggle: the pushing is automatic, while the recovery has to be learned.

Learning to shift out of high alert, deliberately and quickly, is a trainable skill. It is much of what meditation practice does, and why we think of it as training rather than relaxation. Our guides to meditation for founder anxiety and to what a meditation coaching session actually looks like go deeper.

Train the recovery, too

At Cinder, we work 1:1 with founders and executives on stress regulation, attention and recovery, so that the intensity you bring to your work and your training is something you can actually come back from.

Apply for private coaching