Training & Fitness - Cardio & HIIT
Cardio fitness: easy work, intervals and weekly targets
Last updated 29 September 2026. How we write this .
UK guidance is at least 150 minutes of moderate activity a week, or 75 minutes of vigorous activity, or a mix. Steady work and intervals both build cardio fitness; intervals do it a little faster.
Start here UK guidance is at least 150 minutes of moderate activity a week, or 75 minutes of vigorous activity, or a mix. Steady work and intervals both build cardio fitness; intervals do it a little faster.
By the end, you will understand What is cardio fitness, and why does it matter? How much cardio do I need each week? How do I tell moderate from vigorous? Key words used in this lesson Meta-analysis A statistical method for combining the results of multiple studies into a single estimate. More reliable than any one study, but bounded by the quality of what went into it. Full plain-English glossary
Where this information comes from Claim-cited evidence. Central research claims on this lesson are linked to specific studies or official guidance.
This lesson links the specific studies and guidance used for its central claims.
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Cardio fitness is how well your heart, lungs, blood and muscles take in and use oxygen during sustained effort. The UK Chief Medical Officers advise adults to build up at least 150 minutes of moderate activity a week, such as brisk walking or cycling, or 75 minutes of vigorous activity such as running, or a mix of the two. Most of that can be comfortable. Harder intervals are a useful extra, not a requirement.
What is cardio fitness, and why does it matter?
Scientists usually measure it as VO2max, the most oxygen your body can use per minute during all-out exercise, given in millilitres per kilogram of body weight. Another common unit is the MET. One MET is roughly the energy you use sitting still, so a fitness of 10 METs means your body can work at ten times its resting rate at peak.
Fitness turns out to be one of the stronger health markers we have. In 2009 Satoru Kodama and colleagues pooled 33 studies covering 102,980 healthy men and women for a meta-analysis in JAMA. Each extra MET of fitness, roughly what it takes to jog 1 km/h faster, went with a 13% lower risk of death from any cause and a 15% lower risk of heart disease and cardiovascular events. People with low fitness, below 7.9 METs, had a 70% higher risk of death than people with high fitness.
How much cardio do I need each week?
The UK guideline sets a floor of 150 minutes of moderate activity, 75 minutes of vigorous activity, or even shorter amounts of very vigorous activity such as sprinting or stair climbing, or any mix. The same guidelines add strength work on at least two days a week and less time sitting. The World Health Organization's 2020 guidelines, summarised by Fiona Bull and colleagues in the British Journal of Sports Medicine, set a range rather than a single figure: 150 to 300 minutes of moderate activity or 75 to 150 minutes of vigorous activity a week.
These are not pass-or-fail lines. The UK guidance describes health gains rising steeply from doing nothing up to about 150 minutes, then continuing more slowly. It says the minutes can be built up in bouts of any length, and that people who fit their activity into one or two sessions a week still benefit. If you do nothing now, a ten-minute walk is a real start.
How do I tell moderate from vigorous?
Use the talk test from the UK guidelines. If you can talk but not sing, you are working at moderate intensity. If you find it hard to talk without pausing for breath, you are working vigorously. It sounds crude, but it adjusts itself to you.
That adjustment matters because intensity is relative. The guidance points out that the same activity can be moderate for a fit young person and vigorous for an older adult or someone who has been inactive. A brisk uphill walk can be easy work for one person and a hard session for another. Over months, the same pace should start to feel easier. That is the clearest sign your fitness is improving.
Are intervals better than steady cardio?
For raising VO2max, a little better, and the best-known evidence comes from Norway. In 2007 Jan Helgerud's team at the Norwegian University of Science and Technology split 40 healthy, moderately trained men into four groups for eight weeks, three sessions a week. Every group did the same total amount of work. One ran long and slow at 70% of maximum heart rate. One ran at 85%. Two did intervals at 90 to 95%: either 15 seconds hard and 15 seconds easy, or four bouts of four minutes with three-minute recoveries. The interval groups improved VO2max significantly more than the other two, by 5.5% and 7.2%. The amount of blood the heart pumped with each beat rose by about 10% in the interval groups.
A 2015 meta-analysis by Zoran Milanović and colleagues puts that in perspective. Across 28 studies with 723 healthy adults aged 18 to 45, steady endurance training raised VO2max by 4.9 ml/kg/min compared with no exercise, and interval training by 5.5. Head to head, intervals came out ahead by a small margin of about 1.2. Both produced large gains. So easy, steady work is not wasted time. It is how most people build the bulk of their weekly minutes, and intervals sit on top for those who want them.
What if I'm doing the work and not getting fitter?
Some people seem to respond poorly to exercise, and for years that was put down to genes. A 2017 study by David Montero and Carsten Lundby in Zurich tested the idea. They gave 78 healthy adults the same six-week endurance programme but with one, two, three, four or five hour-long sessions a week. With one session, 69% showed no measurable fitness gain. With two, 40% did not. With four or five, everyone improved. The non-responders then did six more weeks with two extra sessions a week, and every one of them improved. The gains tracked mainly with a rise in the total amount of haemoglobin, the oxygen-carrying protein in red blood cells. For most people, a lack of response means the dose is too low, not that the body cannot respond.
What can't the research tell us yet?
The link between fitness and long life comes mostly from observational studies. Fitter people tend to differ in many ways, so a study like Kodama's shows a strong association, not proof that each extra MET you gain will bring the same benefit. Trials that try to test it directly are hard to run. The Generation 100 study in Trondheim randomised 1,567 people aged 70 to 77 to five years of twice-weekly interval sessions, twice-weekly moderate sessions, or simply following national activity guidelines. Death rates did not differ between the control group and the two exercise groups combined. The interval group showed a trend towards fewer deaths, but the result was not statistically certain. One complication was that the control group was already active and chose to do some hard exercise of its own.
Take that as a reminder that most of the gain comes from being active at all, rather than from any particular format. The UK guidelines say the benefits of activity far outweigh the risks for most people. Even so, stop and call 999 if you get sudden chest pain or pressure that does not go away, pain spreading to your arm, neck, jaw or back, or chest pain with sweating, sickness, light-headedness or breathlessness. The NHS says these can be signs of a heart attack.
This lesson is general education about aerobic fitness, not medical advice or a training plan. If you have a heart condition, another long-term illness or symptoms during exercise, talk to your GP before increasing how hard you train.
Related in this classroom Related in other classrooms The studies behind this page UK Chief Medical Officers' physical activity guidelines - Department of Health and Social CareWorld Health Organization 2020 guidelines on physical activity and sedentary behaviour - British Journal of Sports Medicine, 2020Cardiorespiratory fitness as a quantitative predictor of all-cause mortality and cardiovascular events in healthy men and women: a meta-analysis - JAMA, 2009Aerobic high-intensity intervals improve VO2max more than moderate training - Medicine & Science in Sports & Exercise, 2007Effectiveness of high-intensity interval training (HIT) and continuous endurance training for VO2max improvements: a systematic review and meta-analysis of controlled trials - Sports Medicine, 2015Refuting the myth of non-response to exercise training: 'non-responders' do respond to higher dose of training - Journal of Physiology, 2017Effect of exercise training for five years on all cause mortality in older adults (the Generation 100 study): randomised controlled trial - BMJ, 2020Chest pain - NHSSources for this classroom
Educational content only. Not medical advice, diagnosis or treatment. Always consult a qualified healthcare professional before changing your health regimen.