HIIT has been one of the most popular and, ironically, long-lasting fitness trends of the past few years, underpinning pretty much every overpriced boutique studio class and home workout programme promising an six-pack in an improbable time span. (“Listen to this: Seven. Minute. Abs.”) But what exactly is HIIT? Should you be doing it? And if so, how?

What is HIIT?

For the uninitiated, HIIT stands for High-Intensity Interval Training. What that means in practice can vary wildly, but according to the American College of Sports Medicine, which really knows its HIIT (and publishes an annual survey of worldwide fitness trends), “high” corresponds to 80 to 95 per cent of your maximum heart rate. 

“In other words, taking your heart rate to an elevated level, getting sweaty and generally working close to your limit,” says Oliver Lee, a Tread instructor for Peloton, the cult Netflix-style streaming workout service known for its two-grand, touchscreen-equipped bikes used by the likes of David Beckham and Hugh Jackman, currently only available in the US. (And you don’t need to fork out to access workouts via the Peloton Digital app – just a £20 monthly subscription, which is the same as one IRL class at many of those boutique studios.)

An “interval” meanwhile can last anywhere from five seconds to eight minutes. Intensity and interval duration tend though to be inversely proportional: the shorter the bouts, the sharper, and vice versa. And crucially, they’re interspersed with periods of complete rest or active recovery (40 to 50 per cent of your maximum heart rate). These can be longer than the work intervals, the same duration or, if you’re feeling particularly masochistic, shorter.

HIIT Workouts

Why do HIIT?

How does “results in less time” sound? Like infomercial charlatanism, to put it politely. But that’s precisely what HIIT is scientifically proven to do. 

In a famous study by a certain Dr Izumi Tabata, subjects cycled hard for 20 seconds, rested for 10, then repeated that eight times for a total workout time of four minutes (not including warm-up and cool-down, but still). After performing this protocol five times a week for six weeks, they improved their VO2 max – the uppermost rate at which your body can use oxygen for energy during exercise, and a measure of fitness – more than subjects who cycled with the same frequency at moderate intensity for an hour a go.

Wait, there’s more. The group who did Tabata, as it’s now known, also significantly increased their anaerobic capacity – the ability to produce energy without oxygen for short, sharp bursts – whereas the steady-state group’s didn’t get any better at all.

As well as recruiting both energy systems, HIIT creates a greater oxygen deficit than continuous exercise, which your body continues to pay off after your workout is over. This Excess Post-exercise Oxygen Consumption can amount to a further six to 15 per cent – an EPOC win if your goal is fat loss.

“You could potentially burn up to 200 extra calories compared to a regular workout,” corroborates Peloton’s Lee. “This then leads to potentially faster results – if your diet is in check too of course.”

HIIT Workouts

HIIT burns both visible fat and the visceral kind that constricts your organs, while also preserving muscle mass, which can be cannibalised by constant, continuous endurance exercise. (NB you should still do some – just not all the time.)

HIIT also improves fitness, cardiovascular health, cholesterol profile and insulin sensitivity. And it’s more efficient than other forms of exercise at effectively stopping ageing at the cellular level by maintaining production of proteins for your energy-releasing mitochondria, and releasing brain-derived neurotrophic factor, which protects nerve cells and promotes new connections or plasticity – important for learning and memory. Brain gains, if you will.

Research is currently underway into whether HIIT can deliver a perfect-consistency Brexit. But joking aside, a single session has been shown to immediately reduce cancer cell growth, and HIIT elicits a greater response than its continuous moderate-intensity counterpart. Not quite curing cancer, we hasten with high intensity to add, but certainly not hurting.

HIIT Workouts

Who is HIIT for?

Everyone, if the quantity of boutique fitness studios offering a variation on the HIIT theme is anything to go by.

“I believe anyone can do HIIT,” says Peloton’s Lee. “There are always ways to modify or progress it to suit every level from beginner to advanced.”

HIIT is considered safe for all, but it’s best to start slow and build up. In newcomers, overenthusiastic HIIT can halve mitochondrial function or, in extreme cases, precipitate rhabdomyolysis, where muscle fibres break down and leak into the bloodstream. And if you’re completely new to exercise or have a pre-existing medical condition then consult a doctor first.

Even if you’re fit, you can easily overdo HIIT. Penn State University advises spending a total of no more than 30 to 40 minutes a week above 90 per cent of your maximum heart rate to avoid overtraining. Also consider your overall “allostatic load” or stress level, which intense exercise adds to. If you’re feeling the strain professionally or personally, maybe give HIIT a miss for the time being.

Indeed, while a HIIT workout is often shorter, it’s definitely sharper. Pushing yourself close to your limit will hurt, whatever your fitness level. Some of the subjects in the Tabata study couldn’t even last the full four minutes – and they were Olympic speed skaters.

On the plus side, HIIT releases more painkilling endorphins – the source of the runner’s high – than CMIT, as we’re going to have to refer to Continuous Moderate-Intensity Training. Which, results aside, may explain why something so otherwise unappealing is proving so popular. Studies have also found that subjects rate HIIT as more enjoyable than continuous moderate- or vigorous-intensity training, perhaps because it’s psychologically easier when you know that you’re getting a rest soon. Not to mention less monotonous.

30-minute HIIT Treadmill Workout

This power half-hour from Peloton’s Lee is, he assures us, “intermediate” in difficulty, but if your definition of that differs from his then you can modify the speeds at any time. There’s a four-minute warm-up built in but he also recommends doing some dynamic stretches such as leg swings, heel flicks and striders before firing up the treadmill.

HIIT Workouts

Warm-up

Walk at a speed of 2-4mph and incline of 5-6 per cent for 1 minute. Then take the incline back to 0 per cent and quicken your pace to a jog at 5-7mph for three minutes. Gradually speed up for the rest of the warm-up: “You should aim to be between 0.5-0.8mph above where you started.”

Intervals

Your speed range for the high-intensity parts is 7-9mph. During the blessed recovery periods, you can either walk (2-4mph), jog (5-6mph) or do a Peter Kay-style “dad run”.

Round 1
1-minute run followed by 2-minute recovery x 3

Round 2
45-second run followed by 90-second recovery x 3

Round 3
1-minute run followed by 1-minute recovery x 2 

Round 4
45-second run followed by 45-second recovery x 4

Cool down

Reduce your speed to 2-4mph and walk for 1 minute to bring your heart rate back down from the ceiling. “Make sure you end with a minimum of 2 minutes of static stretching,” says Lee. Then pick yourself up off the floor, not forgetting to wipe up the puddle of sweat.

10-minute Bodyweight HIIT Workout

You might associate HIIT with cardio workouts on treadmills, bikes and those weird elliptical things. But you can also perform HIIT with bodyweight exercises – or weights, for that matter, but that self-evidently brings a heavier risk factor when you’re going as fast as you can while simultaneously getting fatigued.

This workout, again from Peloton’s Lee, requires nothing except a mat and some intestinal fortitude, plus takes only 10 minutes to complete, not including the (essential) warm-up and cool down. That might not sound like enough, but it’s called “high-intensity” for a reason. And you can always add rounds.

HIIT Workouts

Round 1

Work for 20 seconds then rest for five seconds between moves, unless otherwise stated. After you perform the first lot of jumping jacks, rest for 20 seconds then go back to the start. After your second lap, rest for 30 seconds before moving onto round two:

Walkouts
From a standing position, bend forward until your hands touch the floor in front of you, then ‘walk’ them out until you find yourself in a push-up position. Aim for 4-6 reps.

Thrusters
From a push-up position, jump your legs forward so that you’re in crouched position with your hands in front of you. Then jump back.

Mountain climbers
From a push-up position, drive one knee forward beneath you until it reaches the opposite elbow. Return to the start and repeat on the other side.

Rest for 20 seconds

Burpees
Starting in a chest-to-the-floor push-up position, push yourself up and tuck your feet underneath your body. Immediately jump up with your hands above you then, on landing, return to the start position.

Jumping jacks
Aka, a star jump. From a standing start, jump on the spot, bringing your legs out wide and your arms above your head. Return and repeat.

Squat jumps
Squat until your thighs are parallel with the floor, then jump up, swinging your arms up as you do so.

Round 2

 
Work for 20 seconds then rest for 5 seconds between moves. Once you’ve completed the left-leg hops, rest for 35 seconds before moving on to the finisher.

In-and-out jumping jacks
Like a regular jumping jack but instead of putting your hands above your head, put them out to your sides and clap them together in front of your chest.

Hop on right leg
Drive your right leg up as you hop.

Hop on the left leg
Drive your left leg up as you hop.

1-Minute Finisher

Fast feet x 3
Do fast feet on the spot for 5 seconds followed by 5 seconds of two steps or skips to recover. Repeat thrice.

Skaters x 15 seconds

Hop from side to side for 15 seconds. Each time your foot lands, sweep the other leg behind you and tap the floor to the side with your toes.

Fast feet + burpee
Do fast feet for 3 seconds on the spot then jump straight into a burpee for good measure. Repeat four times.