It may have achieved unprecedented popularity in the past few years but whatever you’ve heard, high-intensity interval training (HIIT) isn’t new. In 1912, Finnish athlete Hannes Kolehmainen used interval-style training in his preparation for the Olympics, and came away with three golds in the 5,000m, 10,000m and cross-country events. In 1952 Czech Emil Zátopek, one of the most celebrated distance athletes of all time, won the Olympic marathon on a regime that included 400m interval sprints. And in the 1970s, Briton Sebastian Coe’s father Peter used HIIT principles to create sessions of repeated 200m sprints that shaped his son into the world’s best middle distance runner.

It’s important to understand that while HIIT is an effective fat burner, it has many other benefits: upping your VO2 max (the amount of oxygen your body can use and an indicator of cardio fitness), reducing lactate accumulation (so you can train harder, for longer), and increasing enzyme activity to reduce fatigue. When you start, almost any format will work, but as you get better adapted to the workout method, tweaking your routine will help you focus on what you need to improve.

It starts with your body’s energy pathways. There are three: the ATP-PC, which fuels high-power, short-lived activities like explosive weightlifting or sprints; the glycolytic, which takes over for moderate-duration activities; and the oxidative, which is in control for anything beyond that. The first two are anaerobic, which means they don’t use oxygen, and the last is aerobic because it does. HIIT works both your aerobic and anaerobic systems, but how it works the different energy pathways depends on the work/rest ratios you’re using.

Interval training has been around since 1912 when it was used by the world 5,000m and 10,000m track champion.

In a 2001 study,  for instance, researchers found the aerobic system’s contribution to energy rockets from 6% after ten seconds of exercise to 45% after 60 seconds. But the same happens during repeated sprints: in one university study, the anaerobic systems provided all of a test subject’s energy for the first of ten six-second sprints (with a 30-second rest), but by the end they were supplying around 35%, with the rest coming from aerobic fitness.

That means 30 seconds’ rest isn’t enough to improve power, but the main takeaway should be that your workout doesn’t have to leave you in a pool of sweat. A 2011 study published in the Journal Of Strength And Conditioning Research found test subjects doing a “descending” sprint protocol, which was rated easier than an “ascending” protocol that used the
same distances, experienced a higher rise in growth hormone and testosterone.

What’s changed since then is the science. You’ve probably heard of the Tabata regime — eight sets of 20 seconds’ high-intensity work with ten seconds of rest, based on a 1996 study by Professor Izumi Tabata — but that research is just the tip of the iceberg. In recent decades, there’s been a huge amount of research into exactly how different work-rest intervals, levels of intensity, and movements affect the results you get from HIIT.

If you’re feeling worn down in the first place, of course, HIIT isn’t the session to go for. “A common mistake with HIIT is the assumption that it trumps steady-state cardio at all times, which isn’t true,” says trainer David Jordan. “HIIT is highly effective because it requires less time and burns calories during recovery. However, to reap the benefits of HIIT you need to attack it with a lot of energy. On days when you’re feeling less than 100% or, more importantly, you’re sore from your previous workout and are at risk of pulling a muscle, then steady-state cardio is probably more effective – and safer.”

Finally, it’s important to consider how often you can do “real” HIIT. “It’s true that HIIT can trigger protein synthesis but it also causes protein breakdown,” says Jordan. “Doing several HIIT sessions a week would be catabolic so while you’d lose weight overall, some of that loss would be muscle mass. If building muscle is a goal, proper weight training still needs to be your primary focus with HIIT as a supplement. A training split of two weights sessions and two HIIT a sessions a week would keep you lean, while making sure you aren’t overtrained.”

Remember: it’s supposed to be short, intense and infrequent, not an everyday effort. Read on to find out how to structure your HIIT workouts.

Know the Variables of HIIT

Work duration You can measure this in time under tension or reps. Either way, it’s dependent on your goals: shorter/harder is better for power; longer/more builds endurance.

Rest duration More rest builds power, less builds cardio. Minimal rest is best for fat loss, but you’ll compromise on intensity.

Work intensity “You need to know your target heart rate or understand the rating of perceived exertion (RPE),” says personal trainer Philippe Ndongmo. Rate the latter out of ten and try to keep the effort constant across every interval.

Recovery intensity Are you going to stop completely, or do “active” recovery like pedalling slowly on the bike? Sometimes, the latter can help to flush away lactate, which is something to think about when you’re choosing exercises.

Volume: It’s easy to do too much, which is when intensity drops. As  a rule, start with low volume and go as hard as possible. When it feels easy, add a round or two. You’ll have to drop the RPE slightly.

CHOOSE YOUR WEAPONS

Some tools are better than other

Kettlebells: For cardio and fat loss

A Louisiana University study that compared kettlebell swings, cleans and deadlifts with a more traditional sprint training program found that maximum heart rate was only slightly higher in the sprints, while calorie expenditure was bigger with the bells.

HIIT Training

Battle Ropes: For an all-day burn

In a College of New Jersey study, battle ropes beat 13 other exercises for energy expenditure, including burpees — and produced the highest average heart rate. The protocol: 15 seconds of single-arm waves, then 15 of double-arm waves, 60 seconds’ rest, repeated eight times.

Bike: For all-out intensity

There’s a reason lots of  studies use exercise bikes: going all-out on the pedals isn’t too technical, injury risk is low, and you can ruin yourself. For “supramaximal” efforts, which stimulate every available muscle fibre, the bike is the perfect choice.

Burpees: For improved endurance

 In the same New Jersey study, burpees beat four other bodyweight moves and every free weights exercise for VO2 response. If you’re short on time and space, use the Wingate protocol: 30 seconds all-out, then four minutes of rest, done four
to six times.

Workout

The most famous HIIT protocol is ideal for increasing V02 max — 
as long as you do it right. Twenty seconds of all-out work, followed by ten seconds of rest, repeated eight times, improved endurance as much as 30 minutes of steady-state cardio in a Canadian study. The key is keeping intensity high — if you can talk during the session, you’re getting
it wrong.