What the controlled trials show
A 2015 systematic review and meta-analysis by Milanovic, Sporis and Weston pooled 28 controlled trials covering 723 participants, with a mean age of 25.1 years and a mean starting VO2 max of 40.8 ml/kg/min. Interventions ran for at least two weeks.
Both training approaches produced large improvements against a non-exercising control. Endurance training raised VO2 max by 4.9 ml/kg/min (95% confidence limits ±1.4), and high-intensity interval training by 5.5 ml/kg/min (±1.2). The direct comparison between the two favoured intervals by 1.2 ml/kg/min (±0.9) — a real but modest edge.
| Approach | Change in VO2 max vs no-exercise control | Confidence limits |
|---|---|---|
| Continuous endurance training | +4.9 ml/kg/min | ±1.4 |
| High-intensity interval training | +5.5 ml/kg/min | ±1.2 |
| Interval training vs endurance training | +1.2 ml/kg/min | ±0.9 |
Putting five millilitres in context
A gain of about 5 ml/kg/min sounds small until it is read against a classification. On ACSM's percentile classification for men aged 20 to 29, the Poor class runs from 43.6 to 48.9 and Fair from 49.0 to 54.4 — so a five-point gain moves a typical participant a full class, and sometimes two.
It is worth noting what the meta-analysis population was: healthy adults with an average age of 25 and an average starting point of 40.8 ml/kg/min. People who begin lower generally have more room to improve, and people already near the top of their age and sex distribution have less. A published average is a description of a group, not a forecast for one person.
The spread between people dwarfs the spread between methods
The HERITAGE Family Study trained 481 sedentary adults from 98 two-generation families for 20 weeks, measuring VO2 max twice before and twice after. The mean increase was around 400 ml/min — but the striking result was the variation around it. Some participants gained little or nothing; others gained more than 1.0 litre per minute on the same programme.
That variation was not random. The study found roughly 2.5 times more variance between families than within them, with a maximal heritability estimate of 47% for the training response itself. In other words, how much your VO2 max responds to training is itself partly inherited, independently of where you started.
The practical reading is that the choice between intervals and steady endurance work is a small lever compared with individual response — and that a disappointing result after a genuine training block is a well-documented outcome rather than evidence of doing something wrong.
What this does and does not tell you
These are group averages from controlled trials, and this article is educational rather than a training prescription. Programme design, injury history, current fitness, and any medical condition all matter, and none of them are visible in a pooled effect size.
- Both approaches worked in the trials — the question is not which one is valid but which one you will actually keep doing.
- Interval training produced a slightly larger mean gain, at a difference of about 1.2 ml/kg/min.
- Individual response varies from near zero to very large, and part of that is familial.
- Starting point matters: the meta-analysis population averaged 40.8 ml/kg/min at baseline.
- Re-test with the same method under matched conditions, or you will be measuring the weather rather than your training.
- Maximal-effort testing is demanding; anyone with a health condition or a long lay-off should get medical advice before attempting one.
Frequently asked questions
How much does VO2 max improve with training?
In a meta-analysis of 28 controlled trials in healthy young to middle-aged adults, endurance training raised VO2 max by about 4.9 ml/kg/min and high-intensity interval training by about 5.5 ml/kg/min compared with no exercise. Those are group averages over interventions of at least two weeks, and individual results varied widely around them.
Is interval training better than steady endurance training for VO2 max?
In the pooled trials intervals came out ahead by about 1.2 ml/kg/min, with confidence limits of ±0.9 — a real but small advantage. Both approaches produced large improvements against no exercise, and the difference between individuals responding to the same programme is much larger than the difference between the two approaches.
Why did my VO2 max not improve when I trained?
Wide variation in response is a documented finding, not an anomaly. In the HERITAGE Family Study, 481 adults followed the same 20-week programme and some gained almost nothing while others gained more than a litre per minute, with about 2.5 times more variance between families than within them and a maximal heritability estimate of 47% for the response itself.
How long does it take to see a change in VO2 max?
The trials pooled in the 2015 meta-analysis ran for at least two weeks, and improvements were measured over programmes of that length and longer. Because a field-test estimate carries meaningful error, a change smaller than a few ml/kg/min is hard to distinguish from test-to-test noise unless conditions are closely matched.
Does VO2 max decline with age even if I train?
Cardiorespiratory fitness declines with age across the population, which is exactly why every published classification bands its thresholds by age. On ACSM's percentile classification the value needed to reach the Good class falls from 54.5 ml/kg/min for a man in his twenties to 25.9 for a man in his seventies. Training influences where you sit within your age band.
References
- 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 Medicine 2015; 45(10): 1469–1481.
- Bouchard C, An P, Rice T, et al. Familial aggregation of VO2max response to exercise training: results from the HERITAGE Family Study. Journal of Applied Physiology 1999; 87(3): 1003–1008.
- American College of Sports Medicine. ACSM's Guidelines for Exercise Testing and Prescription, 12th edition. Wolters Kluwer, 2024.
- Kaminsky LA, Arena R, Myers J, et al. Updated reference standards for cardiorespiratory fitness measured with cardiopulmonary exercise testing: data from the Fitness Registry and the Importance of Exercise National Database (FRIEND). Mayo Clinic Proceedings 2022; 97(2): 285–293.
- Ross R, Blair SN, Arena R, et al. Importance of assessing cardiorespiratory fitness in clinical practice: a case for fitness as a clinical vital sign. Circulation 2016; 134(24): e653–e699.