Key takeaways
Meta-analyses find intervals raise VO2max somewhat more than steady training, with gains of several ml/kg/min in 6–12 weeks
Low-volume HIIT can match much longer steady sessions for many health markers when time is short
The best plan for most people combines 1–2 interval sessions with mostly easy aerobic work
Few training debates are as lively as this one. On one side, high-intensity interval training (HIIT) promises more fitness in less time. On the other, moderate continuous training (MCT) — steady running, cycling or swimming at a conversational pace — is the traditional backbone of endurance sport. The research gives a clear, if nuanced, answer.
What counts as HIIT
HIIT is an umbrella term:
| Type | Example | Intensity |
|---|---|---|
| Long intervals | 4×4 minutes, 3 min recovery | 90–95% max heart rate |
| Short intervals | 15/15 or 30/30 seconds, repeated for 10–20 min | Near VO2max pace |
| Sprint interval training (SIT) | 4–6 × 30 s all-out, long rests | Maximal |
| Low-volume HIIT | 10 × 1 min hard, 1 min easy | About 85–90% max heart rate |
Moderate continuous training usually means 30–60 minutes at around 60–75% of maximum heart rate, similar to zone 2 training.
What the research shows for VO2max
- Helgerud et al., 2007: in moderately trained men, interval training improved VO2max substantially, while steady training matched for total work did not change it significantly.
- Milanović et al., 2015: this meta-analysis of controlled trials found that both HIIT and continuous training improve VO2max in healthy young and middle-aged adults, with a moderately larger improvement after HIIT. People with a lower starting fitness improved most.
- Weston et al., 2014: in patients with lifestyle-induced cardiometabolic disease (such as heart disease, obesity or type 2 diabetes), HIIT raised cardiorespiratory fitness roughly twice as much as moderate continuous training.
The direction is consistent: intensity is the stronger stimulus for VO2max, because it pushes stroke volume and oxygen delivery close to their limits (see what limits VO2max). But steady training is not useless — it still raises VO2max, especially in beginners, and builds the capillaries, mitochondria and fat metabolism that support everything else.
Low-volume HIIT: the time-efficient option
Martin Gibala's group at McMaster University showed that very small doses of hard exercise can produce surprising adaptations. In one study by Gillen and colleagues, 12 weeks of three 10-minute sessions a week, containing just three 20-second all-out sprints, improved VO2max and insulin sensitivity about as much as 50 minutes of steady cycling — for roughly a fifth of the time commitment.
This matters for health. Lack of time is the most common reason people give for not exercising, and short HIIT sessions are a legitimate way to meet part of the WHO activity guidelines, which count vigorous minutes double.
Why not do only HIIT?
- Recovery cost: hard sessions need 48–72 hours before the next one. Too many, and quality drops and injury risk rises.
- Adherence: all-out efforts feel unpleasant for many people, and drop-out outside supervised studies can be high.
- Plateau: interval gains are fast but tend to level off within a few months unless total volume grows.
- Endurance performance depends on more than VO2max: threshold, economy and durability come largely from volume.
Elite endurance athletes almost all train in a polarised, roughly 80/20 pattern: mostly easy, with a small dose of high intensity. Recreational athletes do well with the same structure.
A practical week
| Goal | Hard sessions | Easy sessions |
|---|---|---|
| Health, short on time | 2 × low-volume HIIT (20 min) | Daily walking, 1 longer easy session |
| General fitness | 1 × 4×4 or 30/30s | 2–3 × 30–60 min easy |
| Endurance racing | 1–2 interval sessions | 3–6 easy sessions incl. one long |
Progress by adding a rep, extending intervals, or raising speed slightly once a session feels controlled. Expect measurable VO2max changes in 6–8 weeks; see how to improve VO2max for a full plan and VO2max trainability for why responses vary between people. Check your progress with the VO2max calculator.
The format works across sports: on the bike for cycling, in the pool for swimming, or on the erg for rowing.
Safety: if you have heart disease, high blood pressure, or have been inactive for a long time, get medical advice before starting hard intervals, and build up from moderate exercise first.
Frequently asked questions
Is HIIT better than steady cardio?
For raising VO2max in a short time, intervals have a modest edge, but steady training builds the aerobic base and is easier to recover from, so most people benefit from both.
How many HIIT sessions a week should I do?
One or two is enough for most people, and three is the upper limit for all but well-trained athletes.
Is HIIT safe for heart patients?
Supervised interval training is used in cardiac rehabilitation, but anyone with heart disease should only start it with medical approval.
Sources
- Helgerud J, et al. Aerobic high-intensity intervals improve VO2max more than moderate training. Med Sci Sports Exerc, 2007.
- Milanović Z, Sporiš G, Weston M. Effectiveness of high-intensity interval training (HIT) and continuous endurance training for VO2max improvements: a systematic review and meta-analysis of controlled trials. Sports Med, 2015.
- Weston KS, Wisløff U, Coombes JS. High-intensity interval training in patients with lifestyle-induced cardiometabolic disease: a systematic review and meta-analysis. Br J Sports Med, 2014.
- Gibala MJ, et al. Physiological adaptations to low-volume, high-intensity interval training in health and disease. J Physiol, 2012.
- Gillen JB, et al. Twelve weeks of sprint interval training improves indices of cardiometabolic health similar to traditional endurance training despite a five-fold lower exercise volume and time commitment. PLoS One, 2016.
- Bull FC, et al. World Health Organization 2020 guidelines on physical activity and sedentary behaviour. Br J Sports Med, 2020.