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. No longer the preserve of athletes, you’re as likely to find HIIT in a chic, boutique studio as you are in a barebones, stripped-back CrossFit gym. 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 about 30 minutes.234
The bouts of high-intensity exercise are performed at 80% of maximum heart rate (MHR) or above.5 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.6
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.78
The Benefits of HIIT
The intermittent pattern of HIIT 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.9
Types of HIIIT
There are an endless number of different HIIT workouts. However, HIIT can be broadly broken down into two main categories:
- Cardio HIIT. HIIT can be applied to just about any kind of cardiovascular activity that allows you to safely perform intense bursts of exercise. Activities include running, cycling, swimming, and rowing. This is the original HIIT. Cardio HIIT is best for improving cardiorespiratory fitness (VO2 max).
- Circuit HIIT. Also known as high-intensity circuit training (HICT), it consists of whole body, multi-joint exercises (e.g. push-ups, burpees). Though less well researched, this rising star of the fitness firmament is an all-in-one, time-efficient strategy that builds aerobic and anaerobic fitness, including muscular strength, power, and endurance. It’s a level of efficiency even a German would envy. Plus, it’s fun.1516
Tip! HIIT is often associated with high impact exercises, such as running, jump rope, or burpees. However, lower impact activities such as swimming and cycling are far less jarring on the joints, but can be just as intense.
Read more: High-intensity exercises that are low-impact
How to Do HIIT.
The genius of HIIT is that it is infinitely variable. This allows you to easily change your HIIT workout to suit your needs – make it easier or harder, exercise indoors or out, adapt it for the seasons, or take your workout on the road. You can change the length, intensity, or 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, need to adapt a workout, or want the flexibility of making your own, here’s how to do an HIIT workout.
1 Choose your exercise
The first step is to determine whether you’re doing a cardio-based workout or a circuit workout.
Cardio HIIT: running, cycling, swimming, jump rope, stair climbing, hill walking, rowing, elliptical machine.
- 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
Or try a mixture of cardio and circuit exercises.
Tip! 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. This is particularly true for circuit HIIT.
2 Choose the length of the work intervals
Generally, high-intensity intervals are between 4 seconds to 8 minutes long.17
- 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).18
HIIT Circuit. Keep intervals relatively short (typically, 1 minute or less). If you’re including cardiovascular exercise intervals (e.g. jump rope) in your circuit, these may be longer.
3 Choose the intensity
HIIT can be done at high-intensity or maximum intensity:1920
- High intensity, aka regular HIIT. 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.
- Maximum intensity, which is also called sprint interval training (SIT). Performed at maximum intensity, it’s 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.21
As a rule of thumb, the shorter the work interval, the higher the intensity and vice versa.
You can switch up the intensity by changing the pace, the resistance on cardio machines, adding weight to strength exercises, or simply choosing more/ less challenging strength exercises.
4 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.2223
- 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.2425
5 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”.26 However, shorter recoveries also make HIIT feel harder.27 Generally, the recovery period are shorter than the work interval.
The 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.
6 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.2829
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.
How to Get Started.
HIIT is an effective strategy to increase fitness and health. It’s also often seen as intimidating. But it doesn’t have to be. That’s because everyone’s “high intensity” will look a bit different. RPE (a scale of 1-10 of how difficult exercise feels to you) and heart rate are commonly used to determine intensity. This means when exercising at high-intensity your pace will depend on your level of fitness. So a beginner jogging or Usain Bolt going super fast, might both be exercising at RPE 8, for example.
Still, whatever your high-intensity looks like, you are still pushing your body near its limit. And to do that takes a little prep:3031
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).32 So, don’t do it. Take things step by step.
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.33 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.34
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.
- 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.35 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. 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. That may mean keeping roughly the same heart rate, RPE, or pace.
- Workout sandwich. Always warm-up before starting exercise and cool down afterward.