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The Ultimate Guide to High-Intensity Interval Training

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High-intensity interval training, aka HIIT. It’s the cool kid of the exercise universe. Yet this trendy, most modern of workouts, is anything but. More than a hundred years old, elite athletes have been using some iteration of HIIT to improve their performance since before Fleming accidentally discovered penicillin.1

The only thing new about HIIT is the scientific interest it has garnered in the last two decades, which has led to a towering pile of research into this unique form of training. So the secret’s out. HIIT is no longer the preserve of elite athletes. Today, you’re as likely to find it in a chic, boutique studio as you are in a barebones, stripped-back CrossFit gym. From high-tech group classes with inspiring instructors to at-home circuit workouts, HIIT is everywhere. But what is HIIT, and why exactly should you do it? Here’s what you need to know.

What is HIIT?

HIIT is a form of interval training that involves alternating between short bouts of high-intensity exercise and recovery periods of less intense exercise or rest. This on-off cycle is repeated several times and a workout can last anywhere from just 4 minutes to 30 minutes, or more.234

How HIIT works: During the high intensity intervals your heart rate shoots up, as does the production of lactic acid (the stuff associated with muscle burn and fatigue). The basic premise of HIIT is that the brief rests allow your heart rate to slow and your lactic acid levels to fall, so you are able to recover enough to complete the next high-intensity work interval.56

Why it works: The intermittent pattern means you are able to complete more time exercising at high intensity, compared to exercising continuously. For example, if you performed eight 30-second sprints, that would amount to a total sprint time of 4 minutes. That’s a lot longer than you could ever sprint continuously.This is important because higher intensity exercise is thought to elicit greater responses at the cellular and molecular levels, which lead to large improvements in fitness and accompanying health enhancing benefits.7

How high is high-intensity? The bouts of high-intensity exercise are performed at 80% of maximum heart rate (MHR) or above.8 On the Ratings of Perceived Exertion (RPE), a scale of 1 – 10 of how difficult exercise feels to you, high-intensity intervals are performed at RPE 7 or higher.9

Types of HIIT

Based on the level of exercise intensity, HIIT can be sub-divided into two categories:1011

  1. High Intensity Performed at RPE 7-9 or approximately 80-95% MHR, this is the kind of workout most people associate with HIIT and what you’ll find most often at the gym. The intensity is near, but not maximal effort.
  2. Maximum Intensity The most intense iteration of HIIT, during which you perform exercise at an all-out effort (RPE 10). Picture trying to outrun a very hungry lion. This type of HIIT is also called sprint interval training (SIT).12

Read more: The essential guide to high-intensity exercise

Health & Fitness

The Benefits of HIIT

Woman taking break from HIIT workout

A celebrated wunderkind with more acolytes than you can shake a stick at, does HIIT live up to the prodigious praise or is it the hyperbolic, fanciful stuff of snake oil salesmen?

It’s rare, but HIIT is one of the few trends that really does live up to the razzmatazz:

  • It’s quick. HIIT’s unique selling point is it’s brevity. Short and sweet, this workout fits into any schedule. While your bog standard, run-of-the-mill cardio workout will probably run close to an hour, HIIT sessions tend to last about 30 minutes or less. If lack of time is an issue, HIIT might just be the solution.
  • Fun. Short, sharp. Easy, hard. You have to stay on the ball. The one thing HIIT is not, is monotonous. While steady-state cardio allows you to zone out (thoughts flowing, feeling zen), HIIT allows you to let it rip, getting you pumped and your adrenalin flowing. Indeed, research suggests that HIIT may be more enjoyable.131415
  • Efficient. HIIT offers similar and sometimes better results than endurance training (exercising at moderate intensity continuously for 20 minutes or more – aka normal cardio) in less time.1617 The variety that makes HIIT so interesting, is also what makes it so effective.
  • Weight loss. HIIT achieves similar weight loss as moderate-intensity exercise, but takes about half the time to burn the same amount of calories.1819 Furthermore, HIIT is at least as effective at reducing body fat, including belly fat.202122232425

    Research shows that HIIT improves the body’s ability to burn fat, increases metabolism post-workout so you continue to burn more calories (the “afterburn effect”), decreases appetite, lowers insulin resistance, and improves blood sugar control. 2627282930313233
  • Muscle gain. HIIT doesn’t just do the same in less time. Whereas regular endurance-based exercise doesn’t build muscle, HIIT can increase muscle mass while simultaneously reducing body fat.343536

    Regular cardio works the kind of muscle fibers that have little capacity to grow in strength or size. On the other hand, HIIT targets the type of muscle fibres (called fast-twitch) that can lead to increases in muscle mass, strength, and power – much like lifting weights would do (but not quite as much).37383940
  • Boosts fitness. The go-to-method for increasing cardiovascular fitness has always been endurance training. But research shows that HIIT can boost cardiorespiratory fitness (VO2max) faster and at least as much as endurance exercise.41424344 Plus, unlike endurance training HIIT increases anaerobic fitness (speed and power) too.45

    The improvements in endurance can be quite staggering. In one study, participants doubled the amount of time they could exercise continuously at high-intensity after just two weeks and six HIIT workouts.46
  • Better cardiovascular health. HIIT improves heart structure and function, as well as the health of blood vessels. It reduces blood pressure, resting heart rate, and blood cholesterol.474849 Research suggests that HIIT may be better than steady-state, moderate-intensity workouts at improving cardiovascular health.50

    HIIT is particularly beneficial for people at higher risk of cardiovascular disease and type-2 diabetes, who tend to reap even greater cardiovascular benefits than healthy people.51525354
  • Metabolic health. HIIT improves type-2 diabetes, and also reduces the risk of suffering with diabetes. It lowers insulin resistance, which is a precursor to type-2 diabetes, and improves blood sugar control.555657

    HIIT also significantly improves metabolic syndrome (a constellation of cardiovascular risk factors that include high blood pressure, high blood sugar, excess belly fat, and high cholesterol) – more so than traditional cardio.58
  • Other health benefits. Research shows that HIIT can reduce pain, has a positive impact on cognitive function and mental health, boosts the function of mitochondria (our body’s energy factory), and increases longevity.5960616263646566

The HIIT Formula

How to Do HIIT

Woman doing HIIT Running Workout

Switch up your workout to suit your mood or increase the challenge as you get stronger. Take your workout from the street to the pool in the heat of summer, or onto a treadmill in the winter. On the road? Keep things practical with jump rope and burpees. Achy joints? Jump on the elliptical machine. With HIIT you can adapt your workout to any occasion.

The genius of HIIT is that because it is infinitely variable you can always keep tweaking and changing it. You can also change the length, intensity, and frequency of the intervals. The combinations are limitless.

To make things easier, here’s a formula that breaks it down and makes sense of the whole thing. So, whether you’re just following someone else’s exercise plan, want to adapt a workout, or you want the flexibility of making your own, here’s how to do an HIIT workout.


Choose your exercise

The first step is to determine whether you’re doing a cardio-based workout or a circuit workout.

Cardio HIIT

Best for: Improving cardiorespiratory fitness (VO2 max).

HIIT can be applied to just about any type of cardiovascular activity that allows you to safely perform intense bursts of exercise. Activities include:

Lower impact exercises can feel less uncomfortable than high-impact activities and tend to have a lower risk of injury. For example, swimming, stair climbing or cycling are far less jarring on the joints than jump rope or running, but can be just as intense.

Read more: High-intensity exercises that are low-impact

Next up, HIIT’s even cooler cousin. Combine circuit training with HIIT and you’ve got a level of efficiency even a German would envy. Though less well researched, this rising star of the fitness firmament is an all-in-one, time-efficient strategy that builds muscular strength, power, and endurance, as well as aerobic and anaerobic fitness. Plus, it’s fun.6768

Circuit HIIT

Best for: Increasing muscle strength, power, and muscular endurance.

Also known as high-intensity circuit training (HICT), these workouts may consist solely of whole body, multi-joint exercises or can be combined with cardio. Exercises include:

  • Bodyweight exercises (e.g. push-ups, lunges, squats)
  • Pylometric exercises (e.g. box jumps, burpees, mountain climbers, ball slams, battle ropes)
  • Strength exercises using weights, resistance bands, Kettlebells, TRX (e.g. Kettlebell swings, deadlifts)
  • Punch bag

Given the inherent high-intensity nature of HIIT, it’s easy to rush and fudge exercises. Therefore, avoid high-risk exercises and only choose moves that you can nail perfectly. Do not sacrifice form for intensity. If you’re not familiar with the exercise, practice it first.


Choose the length of the work intervals

Generally, high-intensity intervals are between 4 seconds to 8 minutes long.69 

  • Shorter intervals (less than 30 sec) are more anaerobic and favor building power, strength, and speed.
  • Longer intervals (1 – 8 min) are more aerobic and build greater endurance. This is favored by endurance athletes. Intervals 3–5 minutes in duration are thought to be most effective at increasing in cardiorespiratory fitness (VO2max).70

HIIT Circuit: Keep intervals relatively short (typically, 1 minute or less). If you’re including cardiovascular intervals in your circuit, these may be longer.


Choose the intensity

As a rule of thumb, the shorter the work interval, the higher the intensity and vice versa.

Switch up the intensity by:

  • Changing pace
  • Changing resistance on cardio machines (e.g. stationary bike, elliptical machine)
  • Adding weights to strength exercises (HICT)
  • Choosing more/less challenging HICT exercises

A couple of points about intensity:

It’s subjective. RPE and heart rate are commonly used to determine intensity. This means the pace of the work interval depends on your level of fitness or tolerance. So a grandma walking briskly, an out-of-shape person jogging, or Usain Bolt, well, going super fast might all be exercising at the same RPE. In other words, everyone’s “high intensity” will look a bit different.

A little consistency. Run up a flight of stairs five times and the first time will feel easy, the last one super challenging. Generally, for HIIT workouts that do not require an all-out effort the goal is to perform at a consistent level for every work interval. What you did for the first interval should be roughly the same as for the last. That may mean keeping roughly the same heart rate, RPE, or pace.


Choose your recovery intensity

Recovery intervals may seem like the boring part, but this is where the magic happens. There are two types of recovery:

  • Active recovery involves low-intensity movement, such as walking. Staying active during recovery is thought to aid the recovery process by facilitating better blood flow, which helps clear lactate from the muscles and replenishes oxygen.7172
  • Passive recovery, which is less common, involves complete rest. This tends to be employed during intervals where the intensity is very high and the recovery periods are short, such as the famous Tabata training protocol. Stopping exercising completely allows the heart rate to decelerate more quickly, requires less energy and allows greater recovery of the muscle.7374


Choose the length of the recovery intervals

There are two types of recovery intervals:

  • Almost complete recovery. For most HIIT workouts, the recovery period is long enough for your heart rate to slow down to a much lower level (about 60-70% MHR) by the end of the recovery interval. This allows adequate recovery so you can complete the next work interval at the same effort and quality as the previous one. Typically, the recovery periods are about the same length or longer than the work intervals.
  • Incomplete recovery. Some types of HIIT workouts (e.g. Tabata) have really short recovery periods. The goal isn’t to completely recover, but for heart rate and other physiological responses to remain elevated. Minimizing rest causes metabolic rate to remain high throughout the workout, and increases the “afterburn”.75 However, shorter recoveries also make HIIT feel harder.76 Generally, the recovery period are shorter than the work interval. 

The relative length of the work and recovery periods is often expressed as a ratio. For example, a work to rest ratio of 1:2 would mean the recovery period is twice as long as the work interval. That might be 30 seconds work to 1 minutes rest.


Choose the number of repetitions

The number of times you repeat the cycle of work/rest intervals varies. Usually work intervals that are quite long (e.g. 4 minutes) or very intense (all-out effort) are repeated fewer times.

Research shows that fewer intervals are more effective for all-out efforts. This is because it’s not feasible to perform all-out sprints many times without dropping the intensity.7778

HIIT Circuit: Determine how many exercises you want to include in your workout and whether you want to cycle through it once or more. For example, you might have 10 exercises you perform once, or 5 exercises you cycle through twice.

HIIT for Beginners

How to Get Started

Woman doing HIIT Stair climbing Workout

HIIT is an incredibly effective strategy to increase fitness and health, and lose weight. It’s about the closest thing to a magic bullet. However, to reap the immense rewards you have to exercise at a very high level, pushing your body near its limit. And that takes a little prep:7980

Step 1 | Regular Cardio

Before you start HIIT you should have a solid base of at least 6 weeks of regular moderate-intensity exercise under your belt (20-60 minute workouts, 3-5 times a week). This step strengthens your muscles, joints, and heart, and lays the groundwork of your fitness. It is crucial in preparing your body to handle the stress and strain of HIIT, and to reduce the risk of musculoskeletal injury. It’s also key to making sure you’ll actually enjoy HIIT.

Go to: Moderate-intensity exercise

Step 2 | Intervals

If you’re regularly doing moderate intensity exercise, start adding a few short, slightly faster paced stints into your workout.

For example, if you walk briskly for exercise, incorporate 30–60 second bouts of jogging every 5 minutes.

Go to: Beginners run-walk plan

Step 3 | HIIT

Ease into HIIT with easier workouts that have shorter and less intense work intervals. Don’t jump straight into all-out sprints.

Start with an intensity of RPE 7 and adapt any workout you’re following to suit your fitness level. 

Go to: HIIT Workouts

As you get more fit, you can slowly and steadily increase interval duration, intensity, or number. Only modify one of these at a time, such as increasing duration or intensity, but not both simultaneously.

Exercise is key to better health. However, jumping in at the deep end with extreme workouts your body is completely unprepared for can lead to injury, overtraining, or in rare cases, rhabdomyolysis (a serious condition caused by the breakdown of muscle).81 So, don’t do it. Take things step by step.

Beginner’s luck. If you’re just starting out and super keen know this – research shows that moderate-intensity workouts are just as effective as HIIT at increasing fitness in people who have been inactive.82 In other words, there really is no need to jump the gun.

How Often Should You Do HIIT? 

HIIT is tougher and taxes the body more than moderate-intensity workouts, which means it takes longer to fully recover. Therefore, aim for 1-3 HIIT workouts per week and allow 48 hours between high-intensity sessions, avoiding back-to-back days of HIIT. 

HIIT is potent, precise, and highly effective. While moderate-intensity exercise is more of a low-key, easy do-every-day sort of workout, HIIT is only needed in small doses. Indeed, research suggests that if you do too much HIIT you start losing some of its benefits.83

On your “off” days you can perform less intense workouts such as moderate-intensity cardio (e.g. brisk walking, jogging, tennis) or strength training. This creates a well rounded exercise program and reduces the risk of overtraining and injury.

Quick Tips

  • What’s up Doc? Before starting a new exercise program check in with your doctor to get the all clear, especially if you have a medical condition.
  • Form before Speed. Focus on practicing proper form and technique during the work interval. If in doubt slow down the pace.
  • Weighty Issues. If you’re using weights for a circuit HIIT workout, keep the weight lower than what you might normally lift, so you are able to complete the moves safely and with proper form.
  • Can You Feel it? Hit the right intensity during your HIIT workout by using RPE. While heart rate works very well for other workouts, it doesn’t work brilliantly for HIIT. When you exercise at high intensity, it takes time for the heart to reflect how hard you’re actually exercising. This lag time means heart rate doesn’t work well when doing short, intense bouts of exercise.84 If you have a heart rate monitor, you can use it in conjunction with RPE.
  • Steady does it. Keep a steady, but challenging pace throughout the interval.
  • Workout sandwich. Always warm-up before starting exercise and cool down afterward.

Read more: 5 Science-Backed HIIT Workout Plans

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