Norwegian 4x4 intervals

• Raise VO2max, the maximum rate your body can use oxygen.
• Increase how much blood the heart pumps per beat (stroke volume).
• Improve blood vessel function, blood sugar control and other cardiometabolic risk factors.
• Slow the age-related decline in fitness.
• Get a large fitness effect in about 40 minutes, two or three times a week.

The method grew out of work by Jan Helgerud, Jan Hoff, Ulrik Wisløff and colleagues at NTNU's Cardiac Exercise Research Group (CERG) in Trondheim. Their best-known trial compared four ways to train with the same total oxygen cost, 3 days a week for 8 weeks, in 40 moderately trained men [1]:

Stroke volume, the blood pumped per heartbeat, rose about 10% after interval training [1]. VO2max in the 4x4 group went from 55.5 to 60.4 mL/kg/min.

Why 4 minutes? Wisløff has explained that the heart typically needs 2 to 3 minutes to reach its maximal pumping rate, so shorter efforts may not train it fully (see the people section). Bouts of 2 minutes or longer also fare best in meta-analyses of VO2max [5].

NTNU's own guidance adds practical detail: warm up until you sweat, aim for about 90% of maximum heart rate, expect heart rate to take 1 to 2 minutes to reach target in later intervals, and finish feeling you could have done one more.

The case for 4x4 rests on a chain: intervals raise VO2max, and higher VO2max goes with longer life. The second link is strong in observational data.

In 122,007 adults referred for treadmill tests at a US academic medical center, risk-adjusted mortality fell with every fitness category, with no upper limit [2]. The least fit quarter had an adjusted hazard ratio of 5.04 versus the elite group. Being below average rather than above average carried a hazard ratio of 1.41, the same size as smoking (1.41) and diabetes (1.40), and larger than coronary artery disease (1.29) [2].

A meta-analysis of 33 cohorts with 102,980 people found each 1-MET higher fitness level (about 3.5 mL/kg/min of VO2max) was linked to 13% lower all-cause mortality and 15% fewer cardiovascular events [13]. The American Heart Association has argued fitness should be measured routinely as a clinical vital sign [15].

Fitness also falls faster with age. In the Baltimore Longitudinal Study of Aging, peak VO2 dropped 3% to 6% per decade in the 20s and 30s, and more than 20% per decade from the 70s on [16]. That is why 4x4 is often framed as a way to build a reserve for later life.

The caveat: these are associations. Fit people differ in genes, health and habits, and in HERITAGE some people gained little VO2max from the same training while others gained more than 1.0 L/min, with an estimated heritability of 47% [17].

Generation 100 is the largest and longest randomized trial of interval training. Participants did two sessions a week for five years: high-intensity intervals at about 90% of peak heart rate (400 people), moderate continuous training at about 70% (387 people), or advice to follow national activity guidelines (780 controls) [3].

Results [3]:

• All-cause mortality did not differ between the combined exercise groups and controls.
• Versus controls, intervals gave an absolute risk reduction of 1.7 percentage points (hazard ratio 0.63, 95% CI 0.33 to 1.20).
• Versus moderate training, intervals gave 2.9 percentage points lower risk (hazard ratio 0.51, 95% CI 0.25 to 1.02).
• The control group exercised more than expected, partly at high intensity, which shrank the contrast.

A follow-up analysis found that intervals raised VO2peak by 0.76 mL/kg/min more than control after five years, but did not significantly improve a combined cardiovascular risk score [11].

How to read it: the trial shows that older adults who were able to exercise safely could keep up interval training for years, and the direction of the mortality results favored intervals. It does not prove that 4x4 extends life. According to NTNU's summary, overall survival was 95.4%, higher than expected for the age group, which also limited the trial's power. The same summary says the researchers cannot be completely sure exercise prolongs life in older adults, while suggesting high-intensity work as a supplement to other activity.

The early trials were small and positive:

• Coronary artery disease: 21 patients walking uphill intervals raised VO2peak by 17.9% versus 7.9% with moderate walking at the same total load [8].
• Heart failure after a heart attack: in 27 patients with a mean age of 75.5, intervals at 95% of peak heart rate raised VO2peak by 46% versus 14%, reduced heart chamber size and raised ejection fraction by 35% [9].
• Metabolic syndrome: in 32 adults, intervals raised VO2max by 35% versus 16% and removed more risk factors [10].

Then came the test at scale. SMARTEX-HF randomized 261 patients with heart failure with reduced ejection fraction to intervals at 90% to 95% of maximum heart rate, moderate training or advice. Intervals were not superior to moderate training for heart remodeling or VO2peak, gains faded by 52 weeks, and 51% of interval patients trained below their target intensity [4].

On safety, a review of 175,820 hours of supervised rehabilitation in 4,846 coronary patients found 1 cardiac arrest per 129,456 hours of moderate exercise and 1 per 23,182 hours of intervals, with no heart attacks and no deaths during intervals [18]. The authors judged both low risk in a supervised setting. These numbers apply to supervised rehabilitation, not to patients training alone.

If you have a heart condition, this is a decision for your cardiologist or rehabilitation team.

A meta-analysis of 53 randomized trials found that short, low-volume intervals improve VO2max, but only long intervals of 2 minutes or more, at least 15 minutes of hard work per session and programs of 4 to 12 weeks or longer beat moderate continuous training [5]. The 4x4 meets all three: 16 minutes of hard work in 4-minute bouts. Another meta-analysis found studies with longer intervals raised VO2max by about 0.8 to 0.9 L/min, with a marked response in all subjects [6].

The gap should not be overstated. Across 28 trials in healthy adults, intervals added only about 1.2 mL/kg/min of VO2max over continuous endurance training [7]. In fat loss, intervals and moderate training work about equally, so the 4x4 is not a shortcut for weight [14].

One surprise: in a muscle biopsy study, 4 weeks of 4-minute intervals at about 90% of peak power did not raise mitochondrial respiration, while all-out sprints did [19]. The 4x4 mainly trains the heart's pumping capacity, while sprints load the muscle more.

In practice, most longevity and endurance plans pair one or two 4x4 sessions a week with easy zone 2 cardio, or swap in short sprint training when time is tight.

The trial doses:

• Helgerud 2007: 3 sessions a week for 8 weeks in healthy men [1].
• Rognmo 2004, Wisløff 2007, Tjønna 2008: 3 sessions a week for 10 to 16 weeks in patients [8][9][10].
• Generation 100: 2 sessions a week for five years in adults aged 70 to 77 [3].

NTNU's public advice is 2 to 3 sessions a week to build fitness, 1 to maintain it, and a warning that 8 sessions a week caused overreaching in a study. Its free 7-week program builds from one 4-minute effort to full 4x4 sessions, mixing in steady runs, core strength and a long activity.

Answering "can I do 4x4 every day?": the research gives no reason to, and recovery is part of the stimulus. Hard sessions with at least a day between them, with easy aerobic work and strength training around them, is the common pattern. The AumniVIDA Zone 2 base building protocol expects readiness dips in load weeks and suggests a recovery week every three to four weeks, which fits a 4x4 block as well.

Maximum heart rate. Age formulas are rough. 220 minus age underestimates maximum heart rate in older adults; 208 minus 0.7 times age fits better [20]. NTNU's HUNT study found 211 minus 0.64 times age, but with an error of 10.8 beats per minute for an individual [21]. A 60-year-old's predicted maximum is about 173, so 85% to 95% would be roughly 147 to 164 beats per minute, give or take 10.

NTNU's field test is simple: after a full warm-up, do two hard 4-minute intervals with 3-minute walking breaks, then a third where after 2 minutes you go as hard as you can until you cannot continue. Your highest reading is your maximum. Only do this if you are healthy and cleared for maximal effort.

Percentages are a guide. Even with a measured maximum, the same percentage of maximum heart rate sits at different metabolic intensities in different people [22]. So pair the number with feel:

• Breathing hard, speech limited to a few words.
• Legs burning by the end of each interval but able to hold pace.
• Heart rate still rising in the first 1 to 2 minutes; judge the last 2.
• Each interval about as hard as the one before.

Low-impact options. Uphill walking on a steep treadmill was used in NTNU's coronary trial [8] and reaches high heart rates with little joint stress. Cycling and rowing also work.

Heart aging. In a randomized trial of 61 sedentary middle-aged adults, two years of progressive training that included high-intensity sessions raised VO2max by 18% and reduced left ventricular stiffness, while controls did not change [12]. Rhonda Patrick and others describe the program, which included a weekly 4x4, as making hearts look about 20 years younger. The trial tested a whole program over two years, not 4x4 alone.

Fat loss. In adults with overweight, intervals and moderate continuous training reduced fat mass and waist size equally, with intervals taking about 40% less time [14]. A 4x4 session burns calories, but it is not a fat-loss shortcut.

Muscle and testosterone. No trial we found shows that 4x4 builds meaningful muscle or raises testosterone. For muscle, see strength training.

Animal support. In rats, high-intensity training raised VO2max by 71% versus 28% for moderate training and improved heart cell contraction more [23], and rats bred for low aerobic capacity developed metabolic syndrome risk factors [24]. These studies explain mechanisms but do not show effects in people.

For lab markers that track cardiometabolic risk, see HbA1c, fasting insulin, triglycerides and hs-CRP.

• Starting too hard and fading: the first interval should feel controlled, and all four should feel about equally hard.
• Going all out: 4x4 is hard but not a sprint; NTNU notes that pushing to 95% to 100% on the last interval adds little.
• Standing still between intervals instead of an active 3-minute recovery at about 70% of maximum heart rate.
• Using 220 minus age, which underestimates maximum heart rate in older adults, so targets end up too low.
• Doing 4x4 every day: NTNU reports that 8 sessions a week led to overreaching.
• Skipping the easy base: without enough easy volume, hard sessions become harder to recover from and to keep up.

• Chest pain, fainting, unexplained breathlessness or palpitations with exertion: stop and see a clinician before any hard training.
• Known coronary disease, heart failure, valve disease or arrhythmia: do intervals only after clearance, ideally within supervised cardiac rehabilitation.
• Uncontrolled high blood pressure, recent heart attack or stroke, or a recent viral illness with fever.
• Inactive middle-aged or older adults, or anyone with diabetes, smoking or a family history of early heart disease: a check-up and possibly an exercise test first are sensible.
• Beta blockers or other heart rate-lowering drugs: heart rate targets do not apply; use perceived effort under clinician guidance.
• Joint or tendon problems that flare with running: choose uphill walking, cycling or rowing.

• Short-term rise in cardiac risk during hard effort: vigorous exertion raised sudden death risk 16.9-fold for the following 30 minutes, though the absolute risk was about 1 per 1.51 million sessions [@albert2000].
• In supervised cardiac rehabilitation, cardiac arrest occurred once per 23,182 hours of interval exercise versus once per 129,456 hours of moderate exercise, with no heart attacks [@rognmo2012].
• Muscle soreness, fatigue and poor sleep if sessions are too frequent.
• Overuse injuries when running intervals are added too fast.
• Many people find the intervals unpleasant, which can hurt long-term adherence.