If you’ve been to a fitness class lately there’s a strong chance that you did HIIT, even if you didn’t realise it at the time. Growing fitness studios like Orange Theory and F45 are essentially 45-min HIIT workouts.

“The best way to explain it is repeated bouts of high intensity followed by a bout of recovery,”

Tom Cowan, Exercise physiologist at the Centre for Human Health and Performance (CHHP).

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What is HIIT and what does it do?

According to the ACSM, high-intensity intervals are exercises that you typically perform at 80 to 95% of your maximum heart rate, for anywhere from five seconds to eight minutes. Generally, the shorter the interval, the higher the intensity and vice versa. The work intervals are alternated with periods of complete rest or active recovery performed at 40 to 50 per cent of your maximum heart rate, lasting for the same duration (although they can be longer or even shorter depending on your fitness).

HIIT is highly adaptable for varying fitness levels and goals, which partly explains its popularity among everybody from elite athletes to cardiac rehab patients. It can be performed on gym equipment such as static bikes, treadmills and rowing machines (cardio HIIT), or via exercises such as press-ups (bodyweight HIIT).

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HIIT workout example

10-Minute Cardio Blast

Start with a light jog in place for 30 seconds. Then:

‍Jumping jacks 30 seconds

Rest 30 seconds

Burpees 30 seconds

Rest 30 seconds

High knees 30 seconds

Rest 30 seconds

Squat jumps 30 seconds

Rest 30 seconds

Push-ups 30 seconds

Take a 1 minute rest and repeat the circuit one more time.

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HIIT Health Benefits

Studies show that HIIT workouts can burn more calories in less time than other types of workouts, specifically steady-state exercise such as jogging. In fact, one study suggests HIIT can produce the same health benefits as moderate-intensity continuous exercise in half the time. Other research proves HIIT to be a helpful tool for reducing resting blood pressure, increasing VO2 max, losing body fat and other benefits. HIIT also reduces fat – both abdominal and the deep, visceral kind that engulfs your inner organs – while maintaining muscle mass or, in less active individuals, increasing it.

According to a small study in Cell Metabolism in 2017, HIIT effectively halts ageing at the cellular level, by increasing the production of proteins for the mitochondria, your cells’ energy-releasing powerhouses, which otherwise deteriorates over the years. Other forms of exercise such as strength training do likewise, but HIIT is more effective. The study also stated that muscle cells, like those in the brain and heart, wear out and aren’t easily replaced, so if exercise prevents deterioration of mitochondria in muscle cells, or even restores them, then it likely does so in other tissues too.

Similarly, HIIT beats continuous moderate-intensity exercise when it comes to releasing brain-derived neurotrophic factor (BDNF), a protein that protects nerve cells. This promotes plasticity (the forming of new connections, which aids learning and memory) and may even help regulate eating, drinking and body weight.

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How long should a HIIT workout last?

Perhaps HIIT’s biggest selling point is its efficiency: a workout can last up to an hour, but it can also be completed in 20 minutes or less.

In a famous 1996 study by the Tabata workout’s namesake, Dr Izumi Tabata, participants performed one of the most well-known HIIT protocols today. The Tabata workout,one of the most well-known HIIT protocols, consists of 20 seconds of work followed by 10 seconds of rest, repeated eight times for a total of just four minutes, not counting the warm-up and cool-down. In the study, the participants performed the four-minute protocol on an exercise bike five times a week. They improved their VO2max– the uppermost rate at which your body can utilize oxygen for energy during exercise – by 15% after six weeks. People who exercised for 60 minutes at moderate intensity five times a week only improved their VO2max by a mere 10%. Moreover, the Tabata group boosted their anaerobic capacity – the body’s ability to produce energy without oxygen, used for short bursts of hard effort – by 28%.  The continuous exercise group’s stayed the same.

Dr Izumi Tabata (in the lab coat) has lent his name to a HIIT protocol that produces greater improvements in fitness than longer bouts of moderate exercise

A short workout is better than no workout

Clearly, the answer is yes! Quick, short workouts are great when you travel, have only 10 minutes for a workout before you have to take a shower, or just want to gain more focus throughout the day.

If you live a sedentary lifestyle and are out of shape, you can improve your general fitness with 10-minute workouts. Focus on short bouts of vigorous exercise (over 90% of your HRmax) and stay consistent for at least a month. 3-5 times a week is enough.

For achieving specific fitness goals, it’s still better to have a training plan, such as this 12-week bodyweight training plan that you can do at home.

Speaking of a quick workout...Next time a commercial break is on TV, you could test yourself to see how many of these Burpee variations you can do with good form without stopping. ;)

HIITing the spot

HIIT is legit – It’s not a fad and there are more and more studies coming out in support of it.

HIIT can be a powerful weapon in the fight against unhealthy lifestyle-related diseases such as obesity and type 2 diabetes. And if nothing else, it’s convenient: one of the most-cited obstacles to physical activity is lack of time.

This variety is what makes HIIT so impactful. Because you get the rest between every work interval, you can spend more time doing each one at a higher intensity, which you couldn’t sustain otherwise.

Ramping up the intensity forces your body to tap into its anaerobic system for energy, because it can’t supply the oxygen required to work aerobically quickly enough; in the recovery intervals, your body reverts to its aerobic system. As the session goes on, your body relies less on the anaerobic system, because quick-release energy sources of phosphocreatine and glycogen (glucose stored in your muscles) become depleted. Your body will therefore start to rely more on the aerobic system, which releases energy more sustainably but slowly from fat. You won’t be able to achieve quite as high an intensity as you could at the start, but the upshot is a double whammy. You’re essentially using a mixture of the anaerobic and aerobic systems, so you get an improvement in both.

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"Respiring anaerobically has knock-on effects. It’s like when you’re puffing and panting after running for a bus, you’re trying to repay the oxygen deficit. This excess post-exercise oxygen consumption, which is more pronounced with HIIT than continuous exercise, burns a further 6 to 15 per cent more calories as your body replenishes itself.

Tom Cowan, Exercise Physiologist at CHHP

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Lactic acid and hydrogen ions produced during anaerobic respiration have to be cleared, as do hormones such as adrenaline; your body temperature and heart rate also need bringing back down. All of those things increase the workload following the exercise. Workload is one of the reasons why we’re not all doing HIIT all the time: you have to recover adequately between sessions. Three sessions a week is probably okay but it’s not something that you’re recommended to do every day.

If you don’t sufficiently replenish your glycogen after HIIT with quality carbohydrate sources, you won’t be able to achieve a high-enough intensity in subsequent sessions. “And, actually, diet in combination with exercise is what’s really going to help with fat loss,” says Cowan. “It’s no good training like this and eating rubbish.” Bodyweight HIIT meanwhile will, like other forms of resistance training, cause micro-tears in your muscle that need time and protein intake to repair. Research recommends 60g of carbs with between 10g and 20g of protein post-exercise to optimise glycogen synthesis.

Don’t overdo it:

You can have too much of a good thing though, especially if you’re a beginner and don’t have a qualified professional on hand to ensure that the intensity isn’t too high for you. HIIT workouts are more exhaustive then steady state endurance workouts. Therefore, a longer recovery period is often needed. A study in the Federation Of American Societies For Experimental Biology Journal showed that interval training can actually halve the function of mitochondria in newcomers.

HIIT is considered safe for most if correctly prescribed, although it may raise coronary risk for sedentary people. You should consult a doctor if you’re new to exercise or have any kind of clinical diagnosis

Another risk with HIIT is that it’s perceived as an easy option when it’s not. That said, HIIT has been rated in some studies as more enjoyable than continuous vigorous and even moderate-intensity exercise. That might be because it’s less mind-numbing.

Finally, while short workouts sound enticing, and remove the excuse of lack of time, they may give the impression that only a little exercise is necessary when most people should be getting more, not less. As the ACSM admits: “Meeting the goal of 75 minutes of vigorous activity per week may prove challenging through HIIT alone.” Alternatively, the ACSM recommends 150 minutes of moderate-intensity exercise per week spread over five days, or a combination of the two. HIIT gets the arduous exercise out of the way quickly, not so you can put your feet up, but so you can enjoy more activities that feel less like hard work.

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When not to HIIT it

Before you begin a HIIT exercise program, think over the following questions. This physical activity readiness questionnaire (PAR-Q) from ACSM will help determine if you’re ready to begin an exercise routine or program.

• Has your doctor ever said that you have a heart condition or that you should participate in physical activity only as recommended by a doctor?
• Do you feel pain in your chest during physical activity?
• In the past month, have you had chest pain when you were not doing physical activity?
• Do you lose your balance from dizziness? Do you ever lose consciousness?
• Do you have a bone or joint problem that could be made worse by a change in your physical activity?
• Is your doctor currently prescribing drugs for your blood pressure or a heart condition?
• Do you know of any reason you should not participate in physical activity?

If you answered yes to one or more questions, if you are over 40 years of age and have recently been inactive, or if you are concerned about your health, consult a physician before starting a HIIT workout or substantially increasing your physical activity.

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Most Popular HIIT Workouts of YouTube in 2021 (by length)

1. 8 MIN No Equipment HIIT

It's only 8 minutes long, but don't let that fool you – it's still a hard HIIT workout. From squats to plank jumps you'll get a properly good sweat sesh in a short burst.

Level: Beginner
Equipment required: None

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2. 10 MIN No Equipment HIIT

If you're looking for another quick and easy to follow HIIT sesh, look no further. 12 exercises in a no repeat format, with 30 seconds on, 10 seconds rest. 10 minutes doesn't sound long, but it'll burn. As a bonus, Chloe Ting offers both low impact and higher intensity options for each exercise.

Level: Beginner + Intermediate
Equipment required: None

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3. 12 MIN Happy Cardio | Pamela Reif

This workout is a mix of cardio exercises and choreography. The mix of a short workout and having a fun time while doing it made this the most popular YouTube HIIT workout of 2021. If you can't keep up, just slow down to a rhythm you can maintain.

Level: Intermediate
Equipment required: None    

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4. 15 MIN HIIT

This 15-minute HIIT workout is a classic. You'll be moving through 32 exercises in just 15 minutes, in a 20-30 seconds on, 10-20 seconds rest format. This is another workout with both low and high impact options for people of different fitness and mobility levels. Challenging, but doable – make sure to give it everything you've got left in the tank!

Level: Intermediate
Equipment required: None

5. 20 MIN Full Body Cardio HIIT (No Repeat)

Get ready to burn some serious cals in this short HIIT workout. You'll be working through 20 exercises in 20 minutes with no repeat. This one is a challenging HIIT with 50 seconds of work to 10 seconds of rest.

Level: Intermediate
Equipment required: None

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Try out your own HIIT workout

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Whether you have 5 or 45 mins available to workout on a given day, Basis will automatically generate workouts, according to what your body needs and how much time you have in your day.  
Few forms of exercise have been as highly and intensely researched – or publicized – in recent years as high-intensity interval training (HIIT). In 2018, HIIT headed up the yearly survey of worldwide fitness trends by the American College of Sports Medicine; it has remained in the top five since.