HIIT Training: The Complete Guide to High-Intensity Intervals

Hardly any training concept has been hyped as much over the past ten years as HIIT. Twenty minutes instead of an hour, fat burning into the next day, athlete-level fitness in a fraction of the time – that is roughly what is promised. And in fact: high-intensity interval training is one of the most efficient forms of training there is. But only if it really is high-intensity, sensibly dosed and part of a well-designed overall programme.

The reality in many gyms and fitness videos looks different. Under the "HIIT" label you find 45-minute circuits of burpees, jumping jacks and mountain climbers in which nobody really works hard but everyone is constantly out of breath. That is tiring, but it is not HIIT in the true sense – and it delivers less than it could.

This guide explains what real HIIT is, which protocols work, how to control intensity, how often you should do it and how it fits with strength training. It is the foundation of our HIIT & Functional topic.

What actually is HIIT?

HIIT stands for high-intensity interval training: short to medium-length efforts at very high intensity, interrupted by recovery periods. The key word is "high". In research, HIIT typically means efforts at at least 80 to 90 per cent of maximum heart rate or well above the power at the anaerobic threshold.

Two related forms should be distinguished from it:

All three have their place. But if you want the specific benefits of HIIT – above all a rapidly rising VO2max – you have to genuinely reach the intensity and choose rest periods long enough for every effort to be hard.

What HIIT does in the body

VO2max rises quickly

Maximal oxygen uptake (VO2max) describes how much oxygen your body can use at maximal effort – it is the most important indicator of endurance capacity and a strong predictor of life expectancy. Intervals in which you work for a few minutes close to your VO2max are the strongest stimulus for improving it. The heart has to pump as much blood as possible in each effort; as an adaptation, stroke volume increases. How to raise your VO2max deliberately is covered in more depth in Improving your VO2max.

Time efficiency

Studies show that HIIT achieves similar or sometimes greater improvements in endurance than long, moderate training in considerably less time. For anyone with a full calendar, that is a real advantage. The catch: the efficiency per minute is paid for with a higher need for recovery.

Metabolism and insulin sensitivity

Intense intervals quickly empty the muscles' glycogen stores and improve the muscle's ability to take up glucose. That has a positive effect on insulin sensitivity – a health benefit offered by both HIIT and regular zone 2 training.

The afterburn effect: real, but overrated

After intense training, the body uses more oxygen and therefore more energy than at rest for a while – the so-called EPOC (excess post-exercise oxygen consumption), colloquially the afterburn effect. It exists, but it is much smaller than often claimed: depending on the session, roughly a few dozen to a few hundred extra kilocalories, not a "fat-burning mode for 48 hours".

HIIT vs. zone 2: not either-or

In the fitness world, HIIT and easy endurance training are often pitted against each other. That is a misunderstanding. The two provide different stimuli and complement each other ideally.

Many endurance athletes roughly follow an 80/20 split: around 80 per cent of training volume easy, around 20 per cent hard. For strength athletes who train endurance mainly for health and work capacity, a simple version makes sense: two easy sessions and one HIIT session per week. The detailed comparison is in HIIT vs. zone 2: which is better for fat loss and health?.

The most important HIIT protocols

There are countless interval formats. The following have proven themselves in research and practice.

1. Norwegian 4×4

4 × 4 minutes at 85–95 % HRmax, 3 minutes active recovery. The protocol comes from research at the Norwegian University of Science and Technology and is one of the best-studied interval formats for VO2max – including in heart patients in medically supervised programmes. The long intervals keep you in the range of high oxygen uptake for a long time. Ideal for the bike, rowing machine or uphill running.

2. Short intervals: 30/30 or 40/20

10–20 × 30 seconds hard, 30 seconds easy. Because the breaks are short, your heart rate stays high throughout the series without the individual intervals becoming extreme. Good for beginners because the load is easier to control.

3. Minute intervals

8–10 × 1 minute hard, 1 minute easy. A good middle ground between short and long intervals, easy to remember and doable on almost any machine.

4. Sprint intervals (SIT)

4–8 × 20–30 seconds maximal, 2–4 minutes rest. Truly maximal effort – there is no more to give. Because of the high load, best done on a bike trainer or assault bike rather than sprinting on a track, where the injury risk for the untrained is higher. How to learn to sprint safely is explained in Sprint training.

5. Tabata

8 × 20 seconds extremely hard, 10 seconds rest = 4 minutes. In the original, a brutal effort on a bike trainer at an intensity well above VO2max. Most "Tabata" workouts with squats or burpees do not reach this intensity. If you want to do it properly: bike trainer, maximal power, only once per session. More in Tabata: 4 minutes that pack a punch.

• Tabata4 min
• 30/3010–16 min
• 1/112–20 min
• SIT15–30 min
• 4×425 min

›Sýna gögn sem töflu

Flokkur	Total time
Tabata	4 min
30/30	10–16 min
1/1	12–20 min
SIT	15–30 min
4×4	25 min

Your first HIIT session (Norwegian 4×4)

1. 1

   Skref 1

   Choose your equipment

   Use a bike trainer, rowing machine, assault bike or a running route with a slight incline. What matters is that you can raise and lower the intensity quickly.

2. 2

   Skref 2

   Warm up for 10 minutes

   Start easy in zone 1–2 and, in the last three minutes, add two or three short, brisk surges of 15 seconds.

3. 3

   Skref 3

   Interval 1: 4 minutes hard

   Raise the intensity so that after about a minute you are at 85–95 % of your maximum heart rate. On the effort scale that is an 8 out of 10: you can only speak in single words.

4. 4

   Skref 4

   3 minutes of active recovery

   Keep moving easily, at about 60–70 % of your maximum heart rate. Do not stop – active recovery helps clear lactate.

5. 5

   Skref 5

   4 rounds in total

   Repeat effort and recovery four times. The last interval should be hard but still controlled. If you fall apart, the pace was too high.

6. 6

   Skref 6

   Cool down for 10 minutes

   Finish the session with easy movement until your heart rate has dropped significantly. Then drink and plan a meal with carbohydrates and protein.

Controlling intensity properly

The biggest problem with HIIT is controlling the intensity. Too easy – and it is just strenuous cardio. Too hard at the start – and you collapse after the second interval. Three tools help:

1. Heart rate. For longer intervals such as 4×4, heart rate works well. But note: heart rate reacts with a delay. In the first 30 to 60 seconds of an interval it is still rising. For short 20- to 30-second intervals it is therefore a poor guide.

2. Rating of perceived exertion (RPE). Subjective effort on a scale of 1 to 10 is often the best tool for short intervals. HIIT sits at 8 to 9 out of 10; sprint intervals at 10.

3. Power or pace. On a bike trainer or rowing machine you can use watts or pace. This is the most objective method: power in the last interval should be no more than about five to ten per cent below the first. If it drops more, the start was too hard.

A wearable with heart rate tracking helps you follow intensity over weeks. With rapid changes in intensity, however, optical wrist sensors often deliver delayed or inaccurate values; for precise interval control, a chest strap is the better choice. How to use heart rate data sensibly in interval training is explained in Controlling HIIT with heart rate data.

The right equipment

In principle, HIIT works with almost any movement where you can raise the intensity quickly. But some machines are much better suited than others.

For strength athletes, the bike trainer, assault bike and rowing machine are the first choice. They allow high intensity with low impact and joint stress, and your legs are less wrecked the next day than after sprints. We present air bike workouts in Assault bike & co., and rowing is explained in Rowing machine training.

How often HIIT? Less is more

The most common question – and the most common source of mistakes. HIIT is a strong stimulus that needs a correspondingly large amount of recovery. If you do hard intervals three or four times a week and also lift weights, you quickly accumulate more fatigue than you can clear. Performance drops, sleep gets worse, motivation fades.

A good indicator of whether you are recovering enough is your resting heart rate and heart rate variability in the morning. If your resting heart rate rises over several days and your HRV falls, that is a signal to reduce intensity. How to read these values is covered in Understanding HRV. More on dosing in How often should you do HIIT per week?.

Combining HIIT and strength training

High-intensity intervals compete with strength training for recovery more than easy endurance training does. With a few rules, the combination still works well:

1. Separate days, where possible. Ideally: HIIT on a day with upper-body training or on a day of its own.
2. Not right before heavy leg exercises. If you sprint in the morning and squat in the evening, you lose strength.
3. Bike and rowing instead of running, to spare your legs and joints.
4. HIIT after strength training, if both happen on the same day – but keep it short (e.g. 6–8 × 30/30).
5. Watch your total load. If you are new to HIIT, reduce your strength-training volume slightly for the first few weeks.

A sample week for a strength athlete who wants to build conditioning:

You will find the matching strength plan in Upper/lower split. If you want to combine both at competition level, read Becoming a hybrid athlete.

HIIT for weight loss

HIIT is often sold as the ultimate fat-burning method. The sober truth: the calorie balance decides fat loss, not the type of training. Studies comparing HIIT and moderate endurance training with similar energy expenditure usually find only small differences in fat loss.

HIIT still has advantages while dieting: it saves time, improves fitness even with reduced calorie intake and can briefly suppress appetite. The downside: in a calorie deficit your ability to recover is limited, and too much HIIT competes with the strength training that is supposed to protect your muscle while dieting. Our recommendation: one HIIT session per week while dieting, complemented by easy training and plenty of steps. The full strategy is in Lose fat, keep muscle.

HIIT at home without equipment

Not everyone has a bike trainer in the living room. You can train intensely without equipment too – with limitations. The challenge with bodyweight exercises: intensity drops quickly as you tire, because technique suffers and you slow down. So choose simple exercises that you can still perform cleanly when exhausted.

A proven format for home:

• Warm-up: 5 minutes of easy running on the spot, jumping jacks, mobility.
• Main part: 10 rounds of 30 seconds hard / 30 seconds rest, alternating: high knees, squat jumps, burpees without the push-up, mountain climbers, stair sprints (if available).
• Cool-down: 5 minutes of easy walking.

Complete bodyweight plans are in Home workout without equipment and Circuit training at home, and the most popular burpee variations in Burpees: curse or blessing?.

Measuring progress: how you know HIIT is working

Because HIIT always feels hard – no matter how fit you are – the subjective feeling is a poor indicator of progress. Objective values help more:

• Output in the same protocol: if after six weeks you ride the 4×4 at a higher average wattage or row the 30/30 intervals at a faster pace, your engine is getting stronger.
• Faster heart rate recovery: how quickly does your heart rate drop in the first minute after an interval? A faster drop is a sign of improved fitness.
• Lower heart rate at a standard load: if you ride ten minutes at a fixed wattage once a month, your heart rate should fall over the weeks.
• Better performance in everyday life: stairs, football with the kids, a spontaneous sprint for the train – fitness often shows up in real life first.

After each session, note the protocol, average power or pace, maximum heart rate and how hard it felt. After four to six weeks a clear picture emerges. How to evaluate training data sensibly in general is explained in Analysing training data: the 5 metrics that matter.

HIIT after 40: fewer reps, same effect

For people over 40 in particular, intense endurance training is valuable – without training, VO2max falls with every decade, and interval training is one of the strongest levers against that. At the same time, recovery takes longer and tendons and joints are less forgiving.

Three adjustments make HIIT safe and effective for older trainees: joint-friendly equipment such as a bike or rowing machine instead of sprints on hard ground, a longer warm-up of 12 to 15 minutes with a gradual build, and one session per week instead of two, complemented by easy training. If you have high blood pressure or other risk factors, discuss getting started with your doctor first. How endurance training becomes a health factor as you age is covered in Building endurance at 40+.

The 7 most common HIIT mistakes

1. Too long. 45 minutes of "HIIT" are not real intervals. Real intensity can only be sustained briefly.
2. Rest periods too short. If you only rest 10 seconds between sprint intervals, you will no longer reach the necessary intensity from the third interval onwards.
3. Too often. Three, four, five HIIT sessions a week overwhelm recovery.
4. Complex exercises under fatigue. Olympic lifts and heavy jumps have no place in exhausting intervals.
5. No warm-up. Going straight to maximal intensity significantly increases the risk of injury.
6. Everything maximal. Not every interval has to be all-out. Most protocols work better at 85–95 % than at 100 %.
7. No base. Without an aerobic foundation, HIIT delivers less and costs more. Zone 2 first, then intensity.

Safety: who should not just jump into HIIT

HIIT puts heavy strain on the heart and circulation. For healthy people, that is a desirable training stimulus. But anyone with pre-existing cardiovascular conditions, high blood pressure, chest pain during exertion, dizziness or a long period of inactivity should have a doctor clarify which intensities make sense before starting. Many rehab programmes use interval training deliberately today – but dosed and supervised. Advice on exercising with high blood pressure is in Exercise with high blood pressure.

Nutrition around HIIT

Because high-intensity intervals mainly use carbohydrates as fuel, you should not do them on empty stores. A meal with carbohydrates two to three hours beforehand, or a small snack such as a banana 30 to 60 minutes before, noticeably improves performance. Fasted HIIT early in the morning is possible but usually costs intensity. After the session, a meal with protein and carbohydrates aids recovery – especially if strength training is on the schedule the next day.

Conclusion: short, hard, rare

HIIT is an extremely effective tool – if you treat it as one. Short, genuinely hard intervals, enough rest, one to two sessions per week, embedded in a foundation of easy endurance and strength training. That way you get the benefits for VO2max, heart health and conditioning without sabotaging your training and recovery.

If you do not yet have an aerobic base, start with the zone 2 guide. If you do: pick a protocol from this article, get on a bike trainer and find out how long four minutes can be.