Intervals or steady cardio: what head-to-head trials found for VO2max

You have decided to work on your fitness, and the advice splits in two. One camp says short, hard intervals are the efficient way to raise VO2max. The other says long, steady sessions are the foundation and intervals are optional. Both camps quote studies.

The useful question is narrower: when the two were compared directly, in the same trial, how much did VO2max change — and in whom?

This material is for information only and does not replace a consultation with a doctor. Decisions about training load, exercise testing and treatment are made together with a clinician who knows your situation.

What exactly is being compared?

You meet the number as VO2max on a watch or an exercise test, and as VO2peak on a clinic report. VO2max is the highest rate at which your body can take up and use oxygen during hard exercise. It is written in millilitres of oxygen per kilogram of body weight per minute (ml/kg/min). Why the number matters for long-term health is covered in a separate article, and what sets its ceiling — heart, lungs or muscles — in another.

The trials below set two training styles against each other:

• moderate-intensity continuous training — a steady effort, in these trials usually 60–70% of maximal heart rate, held for a longer session;

• high-intensity interval training (HIIT) — repeated bouts at about 85–95% of maximal heart rate with easier recovery in between; the best-known format is four 4-minute bouts, the "4×4".

A third variant, sprint interval training, uses short all-out efforts, typically 30 seconds each.

In healthy adults, how far apart are the two formats?

A pooled comparison in healthy people is a meta-analysis of 28 controlled studies with 723 healthy adults aged 18–45 (Milanović et al., 2015). The average participant was 25 years old and started at about 41 ml/kg/min. Compared with not exercising, continuous endurance training raised VO2max by 4.9 ml/kg/min and interval training by 5.5. Compared directly with each other, intervals came out ahead by 1.2 ml/kg/min — an effect the authors themselves rated as "possibly small".

What this does not say: these were young, healthy people in programmes lasting weeks to a few months. The difference is modest — about 3% of the starting value — and the authors' interval around 1.2 ml/kg/min was about ±0.9. It concerns the number itself, not any health outcome.

Sprint intervals answer a different question — training volume. In a meta-analysis of 16 randomized trials with 318 healthy participants, average age about 24 (Gist et al., 2014), repeated 30-second all-out efforts raised aerobic capacity by about 3.6 ml/kg/min (around 8%). Against continuous endurance training the result was no different, with a smaller volume of exercise. What this does not say: these were young healthy adults in small trials, and a smaller volume is not a measured health outcome.

In heart and metabolic disease, does the early gap last?

Here the early numbers looked dramatic. A meta-analysis of 10 studies with 273 patients with coronary artery disease, heart failure, high blood pressure, metabolic syndrome or obesity found that VO2peak rose by 3.03 ml/kg/min more after interval training than after continuous training (Weston et al., 2014). The best-known single trial randomized 27 patients with stable heart failure after a heart attack, mean age about 75, to intervals, moderate training, or advice only — about 9 in each arm (Wisløff et al., 2007). Over 12 weeks, VO2peak rose 46% with intervals against 14% with moderate training.

Then a larger test was run. SMARTEX-HF randomized 261 patients with heart failure and reduced ejection fraction (35% or lower) across several centres to 12 weeks of intervals, moderate continuous training, or a recommendation to exercise (Ellingsen et al., 2017). Intervals were not better than moderate training for VO2peak or for heart remodelling. For VO2peak, both supervised programmes beat the recommendation alone. For remodelling, only intervals differed from the recommendation; moderate training did not. None of these differences held at the 52-week follow-up. Median age in the trial was 60. Training records showed why the two arms may have converged: 51% of patients in the interval group exercised below their target, and 80% in the moderate group above it.

In coronary artery disease without heart failure, the multicentre SAINTEX-CAD trial randomized 200 patients, 90% of them men, mean age 58, to 12 weeks of supervised cycling three times a week, either intervals at 90–95% of peak heart rate or continuous training at 70–75% (Conraads et al., 2015). VO2peak rose by similar amounts in both groups: 22.7% and 20.3%. The authors noted that this was contrary to earlier, smaller trials.

A trial in 93 people with coronary artery disease, mean age 65, mostly men (FITR Heart; Taylor et al., 2020) shows the same pattern over time. After four weeks, intervals had raised VO2peak by 10% against 4% with moderate training, in the 86 people who were retested. At twelve months from the same baseline, the groups were no longer significantly different: 10% against 7%, in the 69 who were retested. The months after the first four were home-based. It was not twelve months of home training.

What this does not say: the small early trials were not wrong to measure what they measured, but in larger and longer trials the advantage shrank or disappeared. For a person with heart disease, a year of follow-up has been measured, not a year of training kept up. In FITR Heart, adherence to the home programme was 53% (18 of 34) for intervals and 41% (15 of 37) for moderate training. What a 12-week programme achieves in a specialised centre is a different question.

Does a higher number from intervals mean a longer life?

This has been tested once directly. Generation 100 randomized 1,567 people aged 70–77 in Norway to five years of twice-weekly intervals at about 90% of peak heart rate, continuous training at about 70%, or following national activity guidelines (Stensvold et al., 2020). The planned comparison — supervised training of either kind against guidelines — showed no difference in all-cause mortality. Intervals against the guideline group gave a hazard ratio of 0.63 (95% CI 0.33–1.20), and intervals against moderate training 0.51 (0.25–1.02): a trend that did not reach statistical confirmation. The control group, asked to follow national guidelines rather than a set programme, also did a fair amount of high-intensity activity.

So no trial has yet shown that choosing intervals over steady training changes survival.

Why some people barely respond — and why intensity, not intervals, mattered there

In one 24-week trial in 121 sedentary middle-aged adults with abdominal obesity, mean age 53, exercising five times a week (Ross et al., 2015), the share of people whose fitness did not rise beyond the measurement error of the test was 38.5% with a low amount at low intensity, 17.6% with a high amount at low intensity, and 0% with a high amount at higher intensity. At a fixed amount, raising intensity removed non-response in this group.

An important detail: the higher intensity in this trial was continuous exercise at 75% of VO2peak, not intervals. The finding is about effort level, not about the interval format.

Is hard training safe?

Data come from 4,846 patients with coronary heart disease in three Norwegian cardiac rehabilitation centres who did both kinds of training under supervision (Rognmo et al., 2012). Over 175,820 exercise hours there was one fatal cardiac arrest during moderate training (in 129,456 hours) and two non-fatal cardiac arrests during interval training (in 46,364 hours) — one event per 129,456 hours against one per 23,182. The authors described the risk as low in both settings.

What this does not say: these were supervised sessions in rehabilitation centres, and three events are too few to compare the two rates precisely. It is not a measurement of hard intervals done alone at home without a prior exercise test.

Do the guidelines pick a format?

The World Health Organization's 2020 guidelines (Bull et al., 2020) ask adults for 150–300 minutes of moderate-intensity or 75–150 minutes of vigorous-intensity aerobic activity per week, or an equivalent combination, plus muscle-strengthening activity. The vigorous range is half the moderate one in minutes. The guidelines treat the two intensities as alternatives. They do not compare interval formats with continuous training.

What to do with the number

• If you are healthy and want a higher VO2max: both formats raised it in trials. Intervals added on average about 1.2 ml/kg/min in young adults. Sprint formats matched continuous training with a smaller volume of exercise. Which format you can keep up for months is a practical question these short trials do not answer.

• If you have heart disease, high blood pressure or another diagnosis: in the trials above, interval training was started after an exercise test and under supervision. That is a conversation with your doctor or a cardiac rehabilitation team, not a decision to make alone.

• If you do not improve: in one trial of continuous exercise, raising the effort level at the same amount of exercise removed non-response. That trial did not test intervals.

• Measure the same way each time. Use the same method before and after a training block. Two different devices are not a before-and-after comparison. If your watch or a test records VO2max, you can follow the number over time in your Lonevi chart.

Sources

• Milanović Z et al. Effectiveness of HIT and continuous endurance training for VO2max improvements: a systematic review and meta-analysis of controlled trials. Sports Med. 2015. PMID 26243014

• Gist NH et al. Sprint interval training effects on aerobic capacity: a systematic review and meta-analysis. Sports Med. 2014. PMID 24129784

• Weston KS et al. High-intensity interval training in patients with lifestyle-induced cardiometabolic disease: a systematic review and meta-analysis. Br J Sports Med. 2014. PMID 24144531

• Wisløff U et al. Superior cardiovascular effect of aerobic interval training versus moderate continuous training in heart failure patients. Circulation. 2007. PMID 17548726

• Conraads VM et al. Aerobic interval training and continuous training equally improve aerobic exercise capacity in patients with coronary artery disease: the SAINTEX-CAD study. Int J Cardiol. 2015. PMID 25464446

• Ellingsen Ø et al. High-intensity interval training in patients with heart failure with reduced ejection fraction (SMARTEX-HF). Circulation. 2017. PMID 28082387

• Taylor JL et al. Short-term and long-term feasibility, safety, and efficacy of high-intensity interval training in cardiac rehabilitation: the FITR Heart Study. JAMA Cardiol. 2020. PMID 32876655

• Stensvold D et al. Effect of exercise training for five years on all cause mortality in older adults — the Generation 100 study. BMJ. 2020. PMID 33028588

• Ross R et al. Separate effects of intensity and amount of exercise on interindividual cardiorespiratory fitness response. Mayo Clin Proc. 2015. PMID 26455890

• Rognmo Ø et al. Cardiovascular risk of high- versus moderate-intensity aerobic exercise in coronary heart disease patients. Circulation. 2012. PMID 22879367

• Bull FC et al. World Health Organization 2020 guidelines on physical activity and sedentary behaviour. Br J Sports Med. 2020. PMID 33239350