Training guide
High-intensity interval training (HIIT) means short, hard efforts, where you can say only a few words, with easier recovery between. It improves fitness a little more than steady cardio and takes less time, but it doesn't burn more fat when the work is matched, and its famous afterburn is worth only tens of calories.
It's also safer than its reputation when you build up gradually: it's used in cardiac rehabilitation, and people in their 70s did it for 5 years in a large trial.
This guide sets out what the trials show for fitness, fat loss, enjoyment and safety; how hard, how long and how often to go; who should check with a GP first; and interval moves and follow-along workouts, from a gentle walk to the Norwegian 4x4.
Repeated hard efforts, where you can say only a few words, with easier recovery between. There's no agreed definition; sprint interval training is the all-out version, usually 10 to 30-second sprints. The NHS counts HIIT as very vigorous activity.
In healthy adults aged 18 to 45, VO2 max (the most oxygen your body can use, in millilitres per kilogram of body weight per minute, mL/kg/min) rose about 1.2 mL/kg/min more with HIIT than with steady cardio, and about 3 more in people with heart and metabolic conditions. Longer intervals, of 2 minutes or more, did best.
Interval and steady training lose about the same fat when the work is matched (high-certainty evidence). HIIT's real advantage is time: it took about 40% less in one analysis.
The extra energy you use after a session is about 6 to 15% of the session's own cost: roughly 30 to 70 kcal after HIIT, against roughly 25 to 40 after steady exercise (approximate figures). It isn't a reason to choose HIIT for weight loss.
In supervised cardiac rehabilitation there was about 1 major heart event per 17,000 HIIT sessions, and people aged 70 to 77 did it for 5 years with no heart events in supervised sessions. If you're inactive and have heart, metabolic or kidney disease, or symptoms when active, check with your GP first.
Trials mostly used 2 or 3 sessions a week, and twice improved fitness about as much as three times in one small trial. Even elite endurance athletes keep only about 1 session in 5 hard. Make the rest of your week steady (our suggestion).
Interval training is repeated bouts of hard work with easier recovery between. There's no agreed definition of HIIT, and the US guidelines note there are no accepted lengths for the hard or easy spells, or for how many to do. Leading HIIT researchers propose that, for health, HIIT means bouts above moderate intensity, in the vigorous range, and sprint interval training (SIT) means near-maximal to all-out efforts.
The NHS calls short bursts of maximum effort broken up with rest "very vigorous" activity, also known as HIIT, with examples such as interval running, sprinting up hills, running up stairs and spinning classes. UK guidelines say even short spells of very vigorous activity can meet the weekly target, without setting a number of minutes.
Most trials used one of these:
Vigorous effort is 7 or 8 out of 10, where you can't say more than a few words without pausing for breath. Norway's Olympic scale describes 87 to 92% of maximum heart rate as "a few words or very short sentences" and 92 to 100% as "one or two words at a time while breathing heavily". Heart rate lags behind effort at the start of each bout and is a useful guide only in efforts longer than about 4 minutes; for shorter ones, judge by your breathing.
Breaking hard work into bouts lets you spend more total time near your aerobic limit than one continuous effort would allow. In the 4x4 trial, the gains in VO2 max matched gains in how much blood the heart pumped with each beat, which rose about 10%. Even low-volume HIIT builds mitochondria in muscle, and per hour of exercise, intervals build them faster than steady training.
Healthy young adults, older adults, and people with coronary heart disease, heart failure, high blood pressure, type 2 diabetes and obesity. The US guidelines note that adults with overweight or obesity, and those at higher risk of heart disease and type 2 diabetes, tend to gain more from HIIT than healthy people.
Against steady training, HIIT raised VO2 max about 1.2 mL/kg/min more in healthy adults aged 18 to 45 (28 trials, a possibly small effect, larger in longer programmes). In people with coronary heart disease, heart failure, high blood pressure, metabolic syndrome or obesity, it gave about 3 mL/kg/min (about 9%) more, almost double the gain (10 studies, 273 patients). In cardiac rehabilitation it gave a small extra gain too (17 trials, 953 people).
Across 53 randomised trials, only long intervals (2 minutes or more), at least 15 minutes of hard work a session, and programmes of 4 to 12 weeks or more beat steady training; short, low-volume HIIT still clearly beat no exercise. Another analysis also found the largest gains with longer intervals. The 4x4 raised VO2 max by 7.2% in 8 weeks in moderately trained men, more than steady or threshold training of matched work (one trial, 40 men).
Sprint intervals raised VO2 max by about 8% on average, about as much as steady training in far less time, and fewer sprints per session didn't reduce the gain. But all 28 studies comparing sprints with steady training had an unclear risk of bias, so "equal" is uncertain. Outside the lab, in 31 studies, HIIT beat doing nothing by 6.9 mL/kg/min but beat other exercise by only 1.1.
A high-certainty review of randomised trials found a trivial difference in fat lost between interval and steady training when the work was matched. Body-fat percentage fell by about the same with each (36 studies), though intervals took off a little more fat mass (1.58 against 1.13 kg within the groups). In adults with overweight, HIIT got the same result in about 40% less time.
HIIT also reduces belly and deep (visceral) fat compared with before training, but in short trials neither kind of training produced large fat loss: what you eat and your total activity matter most.
After any session you use extra oxygen for a while (the afterburn, or EPOC: excess post-exercise oxygen consumption). It grows with how hard and how long you exercise, but even after the hardest sessions studied it amounts to about 6 to 15% of the energy cost of the session itself. A long-lasting afterburn needs 50 minutes or more of hard continuous exercise, or 6 minutes or more of work harder than your VO2 max, which most non-athletes can't tolerate. A review called its role in weight loss "generally unfounded".
Across 22 studies, the afterburn averaged roughly 33 kcal after high-intensity intervals against roughly 24 kcal after steady exercise when measured briefly, and roughly 69 against 38 kcal when measured for longer (approximate figures, converted by us from kilojoules). One small study in a whole-room calorimeter found about 190 kcal extra over 14 hours, but that followed 45 minutes of vigorous continuous cycling, not a short HIIT session.
Across 270 trials, interval training lowered resting blood pressure by about 4.1/2.5 mmHg, close to steady aerobic training's 4.5/2.5. In people with overweight or obesity, HIIT also improved fasting blood sugar, waist size and body fat. In older adults it lowered blood pressure by about 7/5 mmHg against no exercise, the same as steady training.
Against steady training, HIIT lowered HbA1c (a measure of average blood sugar) by 0.12 percentage points more in one analysis of 62 trials, and no more in another of 21. In sedentary men, 12 weeks of 10-minute sprint sessions improved insulin sensitivity as much as 50-minute steady rides. Our type 2 diabetes guide covers medicines and safety.
In 25 sedentary men, a 10-minute session with three 20-second all-out sprints, 3 times a week for 12 weeks, improved fitness (by 19%), insulin sensitivity and muscle mitochondria as much as 50-minute steady rides: a fifth of the time. Reviews find low-volume HIIT can match steady training with far less exercise.
In 71,893 adults wearing activity trackers, 15 to 20 minutes of vigorous activity a week was linked with a 16 to 40% lower risk of death. In people who didn't exercise, three 1 to 2-minute bursts of vigorous activity during daily life were linked with a 38 to 40% lower risk. These are observational links, not proof.
HIIT feels less pleasant than moderate exercise while you're doing it, but people often rate it as more enjoyable afterwards (33 studies). Other pooled analyses found a small enjoyment edge for HIIT, though results varied a lot, and in one lab study 11 of 12 people preferred it. In supervised programmes, attendance was about the same for HIIT and steady training: 89% against 93% across 188 studies.
Done alone, it tends to fade. In a 12-month real-world trial, 104 of 250 adults with overweight chose HIIT, and the share of them doing it regularly fell from 61% to 20%. In Generation 100, about half kept to HIIT over 5 years, against about half to two-thirds for steady training.
HIIT is no substitute for strength training: pooled trials found no clear gain in muscle over steady cardio, and strength training built leg strength better (low-certainty evidence). The two combine well: adding HIIT to strength training didn't reduce muscle growth or upper-body strength, though lower-body strength gains were slightly smaller, more so with cycling intervals.
In 44 trials of 1,863 people aged 60 to 81, HIIT improved fitness, resting heart rate, blood pressure, body fat, strength and balance against no exercise, and fitness slightly more than other exercise. Against steady training, effects were mostly similar.
In Generation 100, 400 people aged 70 to 77 were assigned to HIIT twice a week for 5 years: there were no heart events during supervised sessions, and 3 people across the trial (1 in the HIIT group) broke a bone slipping on outdoor surfaces. The NHS guidelines for older adults include 75 minutes of vigorous activity a week as an option for those already active.
In supervised cardiac rehabilitation, HIIT had about 1 major heart event per 17,083 sessions (23 studies, 1,117 patients). In Norway, 4,846 heart patients had 1 cardiac arrest per 23,182 hours of high-intensity training and 1 per 129,456 hours of moderate training, with no heart attacks. In the UK HIIT or MISS UK trial of 382 people with coronary heart disease, ten 1-minute intervals twice a week improved fitness more than steady training, with one serious adverse event possibly linked to it.
It isn't better for everyone. In heart failure, the SMARTEX trial of 261 patients found HIIT no better than moderate training, and half exercised below their target; in another trial, HIIT's head start in fitness was gone by 12 months. UK cardiac rehabilitation has traditionally used moderate training, and guidance on offering HIIT is new. If you have heart disease, do intervals within cardiac rehabilitation or with your cardiac team's agreement.
US emergency departments saw an estimated 4 million injuries linked with HIIT-style exercises and equipment from 2007 to 2016, rising as HIIT grew popular, but with no count of how many people took part, the risk per session is unknown. Rarer is rhabdomyolysis, a breakdown of muscle that can harm the kidneys: 97 cases have been reported after indoor cycling classes, mostly in healthy women at their first class, and 7% developed kidney injury.
Starting points, not prescriptions. Each names its trial or guideline, or says it's our suggestion.
Hard means 7 or 8 out of 10, where you can say only a few words (the World Health Organization, or WHO, and the NHS). In trials of 1 to 4-minute intervals, heart rate reached about 85 to 95% of maximum by the end of each effort. Sprint intervals are all-out. Start firm rather than flat out, so the last effort is as good as the first (our suggestion).
From 10 minutes in all, with 1 minute of all-out sprints (the 12-week sprint trial), to about 40 minutes for the 4x4 with its warm-up (Helgerud 2007; Generation 100). The biggest fitness gains over steady cardio came with at least 15 minutes of hard work a session.
Most trials used 2 or 3 sessions a week. Over 8 weeks, twice a week raised fitness about as much as three times (10.8% against 13.6%), though only three times also reduced body fat and cholesterol (one small trial). More sessions bring bigger gains overall, but in one small study a week of very high interval load impaired mitochondrial function and blood sugar control.
Our editorial suggestion: 1 or 2 HIIT sessions a week for most people, up to 3, with an easier day between, and the rest of your weekly minutes steady.
WHO: start small, and build up how often, how hard and how long gradually. Our editorial approach: get used to regular moderate activity first, start with 2 or 3 short efforts that are firm rather than flat out, and add efforts before you add intensity. Most of the reported cases of rhabdomyolysis after spin classes happened at people's first class.
Trials used warm-ups of 2 to 10 minutes, building from easy to moderate, and easy cool-downs of about 3 minutes. After the last effort, a few easy minutes let your breathing settle; stretches afterwards are optional, for comfort. Our warm-up guide has the evidence.
Adding HIIT to strength training doesn't cost you muscle. If leg strength or power is your priority, keep hard intervals away from heavy leg days, or at least 3 hours apart (the advice in our plans); running intervals may interfere less than cycling ones.
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In pregnancy, the NHS advises against starting a new vigorous activity; if you already did HIIT, our pregnancy guide covers how to adapt it. With high blood pressure, interval training lowers it about as much as steady exercise, but keep to moderate effort until your blood pressure is controlled; our blood pressure guide has the detail. Our menopause and PCOS guides cover HIIT in those situations, and if you leak when you jump, use the no-jump swaps below and see our pelvic floor guide.
Start on a bike, air bike, rower or elliptical: our cardio intervals workout uses 60 seconds hard and 120 seconds easy, building from 4 to 8 rounds over the weeks.
Be able to run comfortably for 30 minutes before you sprint (our suggestion); stair climbs and hill sprints are gentler ways into speed than flat sprints, and shuttle runs add turning.
40 seconds on and 20 off, 2 to 3 rounds, as in our HIIT workout at home: land softly, stop each move when your form slips, and learn the hip hinge before kettlebell swings.
3D demoJumping jackBeginner · No equipment
3D demoJump squatIntermediate · No equipment
3D demoMountain climbersIntermediate · No equipment
3D demoBurpeeAdvanced · No equipment
3D demoSkippingBeginner · No equipment
3D demoBear CrawlIntermediate · No equipment
3D demoKettlebell swingIntermediate · A kettlebellUse these, with the same timings, if you have knee or foot pain, a higher body weight, leak when you jump, or are new to exercise.
New to hard effort? Start with the build-up interval walk or the bike intervals, and keep the jumping workout for when you're used to them. The Norwegian 4x4 is the long-interval session from the trials.
The Norwegian 4x4, a well-studied interval session for raising VO2 max: four 4-minute hard efforts with 3 easy minutes between, on a bike, rower or treadmill.
40 minIntermediate3 movesCardio machine, A wallA 25-minute interval session on a bike or rower: one-minute hard efforts with two minutes easy, building from 4 to 8 rounds.
25 minIntermediate1 movesCardio machineA 20-minute no-equipment HIIT workout: jumping jacks, jump squats, mountain climbers, burpees, skips and bear crawls.
20 minIntermediate10 movesNo equipmentA 25-minute walk with short faster bouts: notice your heart and breathing speed up, then settle. A gentle way to get used to the feelings of effort.
25 minBeginner5 movesCardio machine, A wallA 25-minute standing cardio circuit with no jumping: marching, step jacks, sit-to-stands, punches, step-ups and wall push-ups, with a seated option for most moves.
25 minBeginner9 movesA bench or box, A stepEach has one optional interval day a week, on Saturday, alongside strength training and steady cardio; the Norwegian 4x4 works for that day too (our suggestion).
A full week that mixes intensities: three strength days, two cardio days, a mobility day and an easy day, never seven hard days.
3 training days45 minBeginner3 days a weekA 3-day strength plan for men over 40 with power and bone work, two cardio days and mobility, built on research on ageing muscle.
3 training days45 minBeginner3 days a weekA weekly workout plan for women: three strength days, cardio and pelvic floor training, with the evidence on periods, pregnancy and menopause.
3 training days45 minBeginner4 days a weekA 4-day upper/lower split: two upper body and two lower body gym days, each muscle trained twice a week, with sets, reps and progression.
4 training days50 minIntermediateHard exercise is safe for most people, but the risk of a heart problem during activity is highest when an inactive adult suddenly does vigorous exercise. Talk to your GP before HIIT if:
If you already do moderate activity regularly and have no symptoms, WHO says you can build up gradually to vigorous activity without seeing a doctor first; see one if new symptoms appear. On a beta blocker, go by how hard it feels, not your heart rate.
Stop and call 999 (don't drive yourself to A&E) for:
Get advice the same day for:
The afterburn is about 6 to 15% of the session's own cost: roughly 30 to 70 kcal after HIIT against roughly 25 to 40 after steady exercise (approximate figures). The one study that found a much bigger afterburn followed 45 minutes of vigorous continuous cycling, not a short HIIT session.
About the same when the work is matched (high-certainty evidence). Its advantage is time.
In supervised cardiac rehabilitation, major heart events were rare, about 1 per 17,000 sessions, and 400 people aged 70 to 77 were assigned to HIIT for 5 years with no heart events in supervised sessions. The caveats are real: build up gradually, and with heart disease do it within rehabilitation.
Twice a week improved fitness about as much as three times in one trial, and a very high interval load impaired mitochondrial function and blood sugar control in a small study.
The original was 7 or 8 cycle efforts at about 170% of VO2 max, 5 days a week for 6 weeks, in 7 people, and it raised VO2 max by about 7 mL/kg/min. A 20/10 bodyweight circuit borrows the timing, not the intensity (our reading).
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Pooled trials found no clear gain in muscle over steady cardio, and strength training built strength better (low-certainty evidence). Do both.
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Adding HIIT to strength training didn't reduce muscle growth or upper-body strength; lower-body strength gains were slightly smaller. Keep hard intervals away from heavy leg days if that matters to you.
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WHO says people already doing moderate activity can build up to vigorous activity without consulting a doctor. Check first if you're inactive and have heart, metabolic or kidney disease, symptoms when active, or blood pressure that isn't controlled.
Interval training reduces total, belly and deep (visceral) fat in trials (39 studies). Our menopause guide looks at the cortisol claim in detail.
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High-intensity interval training: short bouts of hard effort, where you can say only a few words, with easier recovery between, repeated several times. There's no single definition, and sessions range from ten 1-minute efforts to four 4-minute efforts; sprint interval training uses short all-out sprints. The NHS counts it as very vigorous activity.
For fitness, slightly: HIIT raised VO2 max about 1.2 mL/kg/min more than steady training in healthy adults, and more in people with heart and metabolic conditions. For fat loss, they're about equal when the work is matched. HIIT saves time and steady cardio is easier to do a lot of, so most people benefit from both.
It works, but no better than steady cardio when the work is matched, and the fat lost in short trials was modest either way. HIIT took about 40% less time in one analysis. What you eat and your total activity matter most.
A little. The afterburn is about 6 to 15% of the energy used in the session: roughly 30 to 70 kcal after HIIT, against roughly 25 to 40 after steady exercise. The energy you burn during the workout matters much more.
Most trials used 2 or 3 sessions a week, and in one trial twice a week improved fitness about as much as three times. More isn't always better: in a small study, a very high interval load impaired blood sugar control. One or two sessions a week, with easier days between, suits most people (our suggestion).
Anything from 10 minutes, including 1 minute of all-out sprints, to about 40 minutes for the 4x4 with its warm-up and cool-down. The biggest fitness gains over steady cardio came with intervals of 2 minutes or longer and at least 15 minutes of hard work a session.
For most people, yes, if you build up gradually: it improves fitness and lowers blood pressure about as much as steady exercise. It's used in cardiac rehabilitation, where serious heart events were rare, about 1 in every 17,000 sessions. If you're inactive and have heart, metabolic or kidney disease, or symptoms when you're active, talk to your GP first.
For most people, yes, built up gradually. In the Generation 100 trial, people aged 70 to 77 were assigned to twice-weekly 4x4 sessions for 5 years with no heart events during supervised sessions, and HIIT improves fitness, blood pressure and balance in adults over 60. The NHS guidelines for older adults include vigorous activity as an option for those already active.
If your blood pressure is controlled, interval training lowers it about as much as steady exercise. If it isn't controlled yet, European guidance says to use high-intensity exercise with caution, so keep to moderate effort until it is. Our blood pressure guide has the detail.
Not much: pooled trials found no clear gain in muscle over steady cardio, and strength training builds strength better. HIIT and lifting combine well: muscle growth wasn't reduced, though leg strength gains were slightly smaller.
Four 4-minute hard efforts, reaching about 90 to 95% of maximum heart rate by the end of each, with 3 minutes of easy recovery between. In a classic trial it raised VO2 max more than steady training of the same total work, and it's the session used in the 5-year Generation 100 trial. Our Norwegian 4x4 workout times it for you.
HIIT means hard intervals, usually 1 to 4 minutes long, above moderate effort. Sprint interval training uses short all-out sprints of 10 to 30 seconds with long recoveries. Tabata was a specific protocol of 20-second cycle efforts at about 170% of VO2 max with 10 seconds' rest; most 20/10 circuits borrow only the timing.
Exercise education, not a personal prescription, and clinical review of these guides is pending. Check with a GP before starting hard training if you have a heart condition, chest pain, dizziness or breathlessness when active, and stop for sharp or increasing pain. Health disclaimer.
