What Is High Intensity
Interval Training (HIIT)
The Definitive Guide

What is High Intensity Interval Training 

High Intensity Interval Training is a specific type of interval training routine. It is mostly used for individuals trying to lose weight. It has several distinct goals and differences which make it unique. 

An HIIT workout is considered to be exercise which comprises activity at high intensity for a period of specific time (from 20 seconds up to 2 minutes), followed by low-intensity recovery period. These ‘sets’ are repeated from three to fifteen times. The whole routine makes up a HIIT workout 

HIIT Training Structure 

HIIT Sessions usually last less than 20 Minutes 

The first difference is in time, excluding warm-ups and warm downs last no longer than 20 minutes. This may seem like a brief space of time but believe me; if you do the workout correctly, it will exhaust you by the end. The goal of HIIT is to hold an anaerobic state for a long cumulative time (click here to see what this means.). It’s designed with rest intervals to allow you to sprint harder for longer. Take this as an example, if you were to sprint for 100 meters, you would go full out for about 15 seconds (depending how fast you were). In an advanced HIIT workout you would probably spend about 7 to 8 minutes going full out in a 15 minute workout. If you want to do a beginner HIIT workout you should start with 4 to 6 minutes total workout time. 

Sprint Intervals 

The high intensity portion is called the Sprint Interval or work interval. We measure sprint intervals either by time or distance. They are intense bursts of energy and can be as short as 15 seconds in activities like HIIT or as long as 2 minutes for aerobic interval workout. 

An example of a Sprint Intervals would be to run at full pace along a stretch of field for 30 seconds, another would be an indoor cyclist spending 15 minutes simulating a climb on the bike. 

Recovery phase 

The periods of recovery are called Rest Intervals. During a rest interval, athletes do not stop the activity but exercise at a low intensity, which allows the body to recover from the sprint interval. (also known as active recovery) The length of these rest intervals is determined primarily by your fitness levels and the type of the sprint interval. 

The intervals are important; the basis of the interval training is to ensure that you do your sprints at an optimal intensity. Without sufficient rest, your interval training will resort back to an aerobic type of activity. 

What Is Intensity With HIIT Training 

The intensity of the sprint intervals is how hard you push yourself during the sprint. For simplicity’s sake, we usually measure the intensity on a scale of 1 to 10, 1 being no effort while 10 represents the most intense exercise (maximum effort possible). 

Now this is a completely personal scale, depending on your own fitness levels and the type of interval training. 

For example, a seasoned athlete may train on improving his speed work, the sprint lasts for 15 seconds, and his ‘10’ may be a flat out sprint with the athlete going as fast as he or she can. 

Another example would be someone who has not exercised in a while does intervals while walking; a 1 minute walk at a brisk pace may leave the person completely out of breath. This would be their 10. 

Now take the original athlete who sprinted for 15 seconds and change the interval to 5 minutes. They couldn’t maintain the same original 15 seconds pace for 5 minutes, but the maximum effort still represents a 10. 

A ‘10’ is merely the maximum amount of effort a person can safely spend for that interval of their HIIT session. 

So why do you need such intense exercise with HIIT? It’s because HIIT intervals get you to exercise in the anaerobic zone. This makes HIIT training different. 

Anaerobic Exercise and HIIT Training 

Anaerobic exercise is where the body is exercising at such a rate that the blood stream cannot supply oxygen to muscles fast enough. 

Anaerobic exercise is essentially exercise without enough oxygen. This happens during high intensity and cannot be held for long periods of time. A product of your muscle exercising in an oxygen deprived environment is large amounts of lactic acid. Lactic acid builds up inside the muscle at a rate faster than it can be cleared, creating muscle failure. 

Anaerobic exercise refers to oxygen consumption of the muscles and not breathing. The muscle still receives oxygen, just an insufficient supply to meet the demands of the activity. 

In simple terms, when you sprint and your muscles burn and feel like they are on fire, you are probably in the anaerobic zone. 

What is anaerobic capacity? 

We refer anaerobic capacity to as anaerobic threshold or lactate threshold. It refers to the point in the anaerobic exercise when the lactic acid builds up at a rate greater than your body can remove it out of the muscle. 

It is perhaps one of the best indicators of performance in athletes. 

Traditionally, we considered an athlete with a higher VO2Max fitter; however, an athlete with a lower VO2Max but higher anaerobic threshold could go further faster without experiencing muscle failure than an athlete with high VO2Max and a low anaerobic threshold. 

Using HIIT to improve your anaerobic capacity 

We can improve your anaerobic threshold with training but this requires an exercise which allows you to spend an extended period above your anaerobic threshold. 

Now remember how we wanted to spend the intense interval of HIIT at an effort of 10. You are pushing yourself into the anaerobic zone and training your body in this anaerobic threshold. 

Although it’s difficult, it provides us with fantastic benefits to our fitness, body shape and all round health. 

Now how can you measure your intensity of your interval with your heart rate. 

Heart Rate HIIT

When you vary your intensity of your HIIT, your heart responds by varying its rate. You have felt your heart beat faster after taking a relaxed jog, and also no doubt felt it try to jump out of your chest after a sprint. Your heart rate correlates directly with the intensity you are putting into your intervals. Because of this, you can now start accurately measuring how intensely you are exercising in your heart rate HIIT routines. This is important, as previously you had to measure your intensity by how you felt, and this is where your mind can play tricks on you and make you believe you are more tired than you actually are. 

Your heart has a maximum heart rate; this varies per person and depends on your genetics, your age and your gender. Knowing your maximum heart rate allows you to do your heart rate HIIT through your heart’s full range. It allows you to measure the intensity of your intervals and giving yourself the correct time to recover. The more accurate your heart rate HIIT is, the more efficiently you will use your workouts and the faster you will reach your goals of being faster, stronger and leaner. 

You can use a formula to calculate your maximum heart rate. This can give you a quick estimation of what your maximum heart rate is. Click here to work out your maximum heart rate using a formula. 

Maximum Heart Rate = 80-90 percent
bpm – beats per minute

Once you know your maximum heart rate, you can calculate your personal heart rate zones. These are classified into groups which will specify what intensity to use for each heart rate HIIT routine. Knowing these zones in a workout can ensure that you are not over or under training. Ensuring this will make your heart rate HIIT routines accurate and efficient and provide you with the fastest improvements without pushing yourself over your own limits. 

For Maximum Benefits, make sure you are using Major Muscle Groups 

There are a range of activities that you can do with HIIT. When choosing an activity, try to pick one that safely uses the largest muscle groups in your body. High Intensity Interval Training will try to tap the energy sources found inside lean muscle. The larger the muscle mass being exercised, the more the benefit. 

The absolute best for HIIT is sprinting. If however you have injuries or other problems which prevent you from sprinting pick something else which you can manage safely. The second best would probably be a stationery spinning bike. 

Do HIIT every Second Day 

You should not do an HIIT workout on consecutive days. The workouts goal is to operate at a high intensity. So when you do your reps, you do them as hard as you can. Working as hard as this produces the benefits of High Intensity Interval Training. The workout burns energy from your muscle systems and it needs time to replenish it. If this doesn’t happen, you cannot workout with as much intensity and you have a much greater chance of injuring yourself. The benefits happen in your body during your rest period and the more rested you are, the more intensity you can put into your workout. 

Alternate Your Interval Training Routines 

One of the key principles around High Intensity Interval Training is preventing any type of plateau. It’s designed around pushing your body to constantly adapt, and as your body is always trying to please, it invariably does. When this happens, your improvements slow down. 

To counter this, always keep your body guessing. After you have done about 4 to 8 weeks on a particular HIIT program, give yourself one week off then start a new one. 

Now lets look at some benefits of HIIT 

The Six Benefits of HIIT 

Burn more calories in a fraction of the time 

A High intensity workout creates different stresses on the body, compared with normal steady state cardio. 

These stressors engage different systems in your body. These would normally go untouched when you do normal cardio exercise. 

So the HIIT you do can be shorter and less often but… as soon as you engage the intensity you burn way more calories and your weight loss moves onto a different level. 

Here is research they did in the University of Western Ontario. They divided men and women into two groups 

After 6 weeks the cardio group lost 5.8% of their fat, the HIIT group lost 12.4% of theirs. That’s 114% more weight loss with a workout that was considerably shorter. 

Increases Your Metabolism by 36%

What if you could your metabolism by 30% all the time? 

In Ontario at the University of Guelph they tested this in 2008. 

Untrained individuals increased the following indicators over 6 weeks. Here is what they said: 

“This study demonstrated that 18 h of repeated high-intensity exercise sessions over 6 weeks (3 d.week-1) is a powerful method to increase whole-body and skeletal muscle capacities to oxidize fat and carbohydrate in previously untrained..” 

Here are the actual specific results (all factors contribute to metabolism) 

 The same university did a two-week experiment with eight women. Exercise whole body fat oxidation increased by 36% after HIIT. 

HIIT increases your metabolism

3. How You Can Lose 9 Times More Body fat 

HIIT loses more weight….and more of it is fat. The study below has probably been quoted more than any other in terms of the benefits of HIIT 

At Laval University in Quebec, 32 men and women took part in an experiment. 

Half of them did normal cardio exercise for 20 weeks, the other half did HIIT for 15 weeks. 

At the end of the program, body fat skin fold measurements were taken. The HIIT group had a 9 times greater body fat reduction than the cardio group 

Ultimately, your goal is to create a workout that you can do less exercise and lose more fat. This workout needs to combine the elements of what we discuss in this article. 

How To Exercise 28% Harder 

Let’s say you started training with a friend. 

Both of you have been jogging around the track for about half an hour and it’s your last lap. You have a bit of a competitive streak in you; you are going to cross the line first. There’s no way you are going to let him win. 

You know you are fitter than him; you are also faster…. So at about three hundred meters to go, you pick up the pace. At first, with your superior speed, you pull you away, but then…. 

Your muscles start to burn and ache… and he catches you…. and passes you. 

His speed is unrelenting and you watch him blast away in front of you as you hit the wall… His speed never wavers and he crosses the line way in front of you…. By the time you finish, he’s standing there watching you, with a big smug grin on his face… 

So what happened? 

You know you are faster than him, and you are sure you are fitter. you decide to carry on training… But even after months. The same thing keeps on happening. He’s there at the line, grinning at you finishing after him. 

The answer is that this has nothing to do with fitness or speed. 

It’s your anaerobic capacity, it’s how long you can keep on running at full pace before you crack. HIIT is one of the few ways to improve this. One of the more famous studies on HIIT is that done by Dr Izumi Tabata. 

The head coach of the Japanese speed skating team, Mr. Irisawa Koichi, had designed a new training protocol. They hired Dr Izumi Tabata to analyze the effects of this protocol, and from this research its popularity exploded. From then on it became known as the Tabata protocol…(although it probably should have been known as the Koichi protocol!) None the less, the HIIT group gained a massive increase of 28% of their anaerobic capacity. (More than massive when you consider these were already Olympic athletes to start with) 

So clearly, if you want to be the one grinning at your mate crawling across the finish line….HIIT! 

5. How To Work Harder For 209% Longer 

Each day we start off with a certain amount of energy. 

And every day we use this energy to get our lives moving….Go to work, go to gym, meet with friends, do projects, play with the kids etc… Some people seem to have enough energy for all of this and more. 

Others are exhausted by evening. 

If you are exhausted, it may be because of your anaerobic threshold being low…. *how long can you go before you get tired?* Or does your energy just last and last? 

In a study published in the American Journal of Cardiology in 2005 they did a little research on this. Once again subjects were split into Cardio and HIIT groups. Anaerobic threshold was measured by “time to exhaustion” in seconds while working at 90% maximal heart rate. At the beginning of the study, the ‘time to exhaustion’ of both groups was about 100 seconds. 

After 16 weeks, both groups had improved. 

The cardio group increased to 230 seconds… a considerable improvement. But the HIIT group had increased their time to exhaustion by a whopping 480 seconds. A phenomenal improvement. 

So you have big goals? Things you want for your life. Picture those dreams you have, everything you want to achieve before this life is over…… 

And you need energy to create these plans, right?

6. Increase Your Personal VO2Max Capacity by 18% 

The Festina affair was a series of doping scandals that broke after the 1998 tour de France. It started after many doping products were found in the Festina team car. By December 2000, all nine Festina riders had confessed to using erythropoietin (EPO) 

In this study, they increased VO2Max by 27% using EPO 

They took EPO because it creates a considerable boost in VO2Max. VO2Max is a measure of the aerobic physical fitness of an individual. The Festina riders were cheating by taking drugs to make themselves fitter than was possible. 

Now we aren’t looking to cheat at cycling, but can we make improvements to our aerobic fitness with anaerobic training?

What happens to VO2Max when you do HIIT?

In a study done in 2015, 55 untrained college aged subjects did HIIT for 8 weeks. There VO2Max increased by 18%, that’s 2/3rds of the effect of taking EPO, achieved in 8 weeks…which is considerable. 

HIIT increases all types of fitness. 


Disadvantages of Interval training are really few and far between, however there are few. 

The first of the disadvantages of interval training affects only elite athletes who are already in great shape. You get three kinds of muscles: 

• Local stabilizers – stabilize joints 

• Global stabilizers – stabilize a range of motion 

• Global mobilizers – power muscles used for action 

If you have any imbalance in any local stabilizer, interval training will stress the use of global mobilizers, while your global stabilizers will brace and stiffen to protect themselves from the imbalance. This will hinder your performance and could cause problems. Once again, this only really affects elite athletes, general people who are not in perfect shape would actually benefit from interval training even if they had an imbalance. If you suspect you do have an imbalance, visit a physiotherapist who specializes in kinetic control. 

The second of the disadvantages affects all of us who aren’t in great shape. Interval training is tough on the body; at some stages you will push your body like a freight train. If it’s not ready for this, it may choose to derail itself occasionally. Strained muscles are much more common in interval training than in low-intensity exercise. 

The third of the disadvantages is not physical but psychological. Interval training can be unpleasant, painful, especially in your first few weeks. Finishing two sprint intervals can leave you gasping for air and light-headed, finishing a whole routine can leave you feeling broken. It’s so tough that most people give it up and as a result don’t gain any benefit from it. Interval training is hard to stick to. 

The last of the disadvantages is doing too much of it. If you have been doing interval training for a while and find your performance dropping off, your body may need some vital rest. Two weeks off is usually all the body needs to recharge but most athletes try and push themselves harder. This may result in you getting extremely tired or worse, pulling muscles. 

What are examples of HIIT workouts?