Tip · Movement

HIIT 1–2× per week

Q: How much more does HIIT really deliver than easy endurance training?

In healthy adults, the difference is small. In a meta-analysis of 28 controlled studies, VO2max rose by 5.5 ml per kg per minute with interval training and by 4.9 with continuous training, compared with no training. In previously inactive people, a more recent meta-analysis found an advantage of 3.72 ml per kg per minute for HIIT, while blood pressure, blood lipids and body composition did not differ significantly. The often-cited large advantage comes from a study with 27 heart failure patients.

Q: How do I find my interval intensity?

The studies measured peak heart rate in an exercise test and then targeted 90 to 95 percent of it. Age formulas were not used. Anyone taking beta blockers cannot use formulas anyway, because maximum heart rate is lowered by the medication. Without a measured reference value, the percentage remains an estimate with an unknown error.

Q: Is HIIT dangerous with heart disease?

In 175,820 supervised training hours in 4,846 coronary patients, three cardiac events occurred, two of them non-fatal during high-intensity training; there were no heart attacks. Calculated per hour, the risk was higher with interval training than with moderate training, but low overall. These figures come from rehabilitation centers with specialist medical care, not from unsupervised training.

Q: Does HIIT prolong life?

The largest randomized long-term study to date, with 1,567 participants over five years, had all-cause mortality as its primary endpoint and did not reach it: there was no difference between the training groups and the control group. For HIIT alone there was a trend toward lower mortality with a hazard ratio of 0.63, but its confidence interval ranges from 0.33 to 1.20 and thus includes no effect.

Q: Is what classes offer real Tabata?

Mostly not. The original 1996 study trained seven participants five times a week over six weeks at about 170 percent of maximal oxygen uptake to exhaustion on a braked ergometer. The timing pattern of 20 seconds of work and 10 seconds of rest is transferable; the intensity and the frequency are not.

High-intensity interval training raises VO2max substantially.

High-intensity interval training measurably raises maximal oxygen uptake — this has been measured in many randomized studies. This page is about the interval itself: which protocol ran in the original studies, what the percentages refer to and what the event rates from rehabilitation look like.

In short

Compared with no training, the effect is clear: in a meta-analysis of 28 controlled studies with 723 participants, VO2max rose by 5.5 ml·kg⁻¹·min⁻¹, and in previously inactive people, in a more recent analysis of 24 randomized studies with 877 participants, by as much as 7.72 ml/kg/min. Compared with easy continuous training, by contrast, the advantage is small: 5.5 versus 4.9 ml·kg⁻¹·min⁻¹ in healthy people and 3.72 ml/kg/min in inactive people, while blood pressure, blood lipids and body composition did not differ significantly between the two forms. The often-cited large advantage of 46 % versus 14 % comes from a study with 27 heart failure patients. And the intensity figures in the studies always refer to a measured heart rate, never to a calculated one.

What is behind it

At workloads close to or above maximal oxygen uptake, the aerobic and anaerobic systems are taxed at the same time. That is exactly the core finding of the 1996 Tabata study: moderate endurance training raised VO2max but left anaerobic capacity unchanged (p > 0.10), while the interval protocol increased both — VO2max by 7 ml·kg⁻¹·min⁻¹, anaerobic capacity by 28 %.

The VO2max gain arises centrally and peripherally. In heart failure patients, it was accompanied by reverse left ventricular remodeling: end-diastolic volume fell by 18 %, end-systolic volume by 25 %, ejection fraction rose by 35 % and pro-BNP fell by 40 %. In addition, there was better endothelial function and mitochondrial function in the vastus lateralis that increased only in the interval group. Fitness itself is a strong predictor in cohorts: per 1 MET higher maximal aerobic capacity, the pooled risk ratio for all-cause mortality was 0.87 (95 % CI 0.84–0.90).

The percentage needs a reference

In the studies, the reference value was measured beforehand. The well-known 4×4 protocol ran at 95 % of measured peak heart rate, Generation 100 at around 90 % of peak heart rate, a four-week rehab program at around 90 % of maximum heart rate. None of these papers used an age formula.

This is more than a formality. Under beta blockers, maximum heart rate is lower than any age formula predicts — the heart failure patients in the best-known interval study trained under beta blockers and ACE inhibitors, guided by an exercise test carried out beforehand.

How often, and what Tabata really was

There is no hard upper limit from studies. A randomized study in 45 physically active students compared one, two and three sessions of 4 × 4 minutes per week over six weeks: VO2max rose markedly in the two-session and three-session groups and only slightly in the one-session group, and the improvements were largely comparable between two and three sessions. Completion rates were 100 %, 93 % and 80 %. Twice a week is therefore an argument about tolerability, not about safety.

The Tabata protocol itself was trained five times a week over six weeks, by seven participants, with seven to eight sets of 20 seconds at about 170 % of VO2max and 10 seconds of rest, to exhaustion, on a mechanically braked cycle ergometer. The timing pattern is transferable; the intensity is not.

What is well supported

The VO2max gain is the most consistent effect. Compared with control groups without training, interval training in 723 healthy adults with a baseline VO2max of 40.8 ± 7.9 ml·kg⁻¹·min⁻¹ produced an increase of 5.5 ml·kg⁻¹·min⁻¹ (95 % confidence limits ±1.2), continuous training 4.9 ml·kg⁻¹·min⁻¹ (±1.4). With lower baseline fitness, the additional gain was larger (+3.2 ml·kg⁻¹·min⁻¹, ±1.9), as it was with longer interventions (+3.0 ml·kg⁻¹·min⁻¹, ±1.9). In previously inactive people, the analysis of 24 randomized studies adds further effects: body weight −1.60 kg, systolic blood pressure −2.85 mmHg, diastolic blood pressure −6.57 mmHg and LDL cholesterol −0.29 mmol/l.

What the studies show

Wisløff 2007: the 4×4 protocol the number comes from

Randomized study in 27 patients with stable post-infarction heart failure on optimal medication, age 75.5 ± 11.1 years, ejection fraction 29 %, baseline VO2peak 13 ml·kg⁻¹·min⁻¹. Three arms over twelve weeks with three sessions each: interval at 95 % of peak heart rate, continuous training at 70 %, control with standard advice. VO2peak rose by 46 % with interval training and by 14 % with continuous training (p < 0.001); nothing changed in the control group. The sample is small and narrowly selected.

Rognmo 2012: the safety question with real numbers

Not a randomized study but an analysis of the training operations of three Norwegian rehabilitation centers: 4,846 coronary patients, 175,820 training hours, in which every patient completed both intensities. Three events were found — one fatal cardiac arrest in moderate training over 129,456 hours and two non-fatal ones in interval training over 46,364 hours, no heart attack. This corresponds to one event per 23,182 high-intensity hours, counted under supervision.

Generation 100: the cross-check that missed its endpoint

1,567 people born between 1936 and 1942, mean age 72.8 ± 2.1 years, five years, three arms: interval training at around 90 % of peak heart rate (n = 400), moderate continuous training at around 70 % (n = 387), control following the national physical activity recommendations (n = 780). The primary endpoint, all-cause mortality, did not differ between control and the combined training group. For the interval arm alone, there was an absolute risk reduction of 1.7 percentage points compared with the control’s observed 4.7 % (hazard ratio 0.63, 95 % CI 0.33–1.20).

Where the data stop

The hard endpoint was missed. Generation 100 is the only large randomized long-term study with all-cause mortality as the primary endpoint, and it did not reach it; the advantage for the interval arm is a trend with a confidence interval of 0.33 to 1.20, which includes no effect. In addition, 87.5 % of participants reported being in good health and 80 % were already moderately to highly active at baseline.

How to do it

What the studies actually did: four intervals of four minutes each at 95 % of measured peak heart rate, three times a week over twelve weeks, in heart failure patients on medication. In rehabilitation, the same pattern ran in 73 patients with nonspecific musculoskeletal complaints over four weeks, combined with 4 × 4 repetitions of leg press at around 90 % of the one-repetition maximum: VO2peak +12 ± 7 % versus +5 ± 6 % in the standard program (p < 0.001), without adverse events. In the long-term trial it was twice a week at around 90 % of peak heart rate over five years. Training was always guided by a peak heart rate measured in an exercise test, and a warm-up phase preceded the intervals.

Safety

The robust number is: in 175,820 supervised training hours in 4,846 coronary patients, three cardiac events occurred, one per 129,456 hours of moderate and one per 23,182 hours of high-intensity training, no heart attack. The risk is low in both cases, but higher per hour with intervals. The German DVO S3 guideline explicitly names HIIT as safe and effective also in cardiac rehabilitation, but ties this to specialist medical care and qualified supervision. With vertebral fractures and with knee and hip replacements, exercises with impact loading are contraindicated, that is, jumping and running variants of intervals. Under beta blockers, formula-based guidance is unusable. Anyone with known heart disease, high blood pressure treated with medication, or no exercise for a long time should have high-intensity training cleared by a physician beforehand. This is information, not a treatment recommendation.

BK-Score Well supported

Human evidence		8
Mechanism		8
Safety data		8
Hype gap		7
Track record of use		9

The increase in VO2max through interval training has been measured in many randomized studies and meta-analyses. A deduction on safety for the data on people with pre-existing heart conditions and untrained people, who were mostly excluded from the studies.

The score rates the state of knowledge, not the effect. “Safety data 9” means well studied – not harmless.
Subjective assessment by Biohacking Kompakt based on published scoring rules – not a scientific rating and not a medical recommendation. Rules and all ratings (German)

Frequently asked questions about HIIT 1–2× per week

How much more does HIIT really deliver than easy endurance training?

In healthy adults, the difference is small. In a meta-analysis of 28 controlled studies, VO2max rose by 5.5 ml per kg per minute with interval training and by 4.9 with continuous training, compared with no training. In previously inactive people, a more recent meta-analysis found an advantage of 3.72 ml per kg per minute for HIIT, while blood pressure, blood lipids and body composition did not differ significantly. The often-cited large advantage comes from a study with 27 heart failure patients.

Why not more often than twice a week?

There is no hard upper limit from studies. A randomized study in 45 athletically active students found that two and three sessions of 4 times 4 minutes per week over six weeks led to comparable physiological improvements, but the completion rate with three sessions fell to 80 percent compared with 93 percent with two. The Tabata protocol itself was trained five times a week. So twice is a tolerability limit, not a safety limit.

How do I find my interval intensity?