HIIT Explained: High Intensity Interval Training – The Complete Overview

HIIT cardio is awesome for fat loss.

But there are many questions surrounding HIIT that you need answered before you can truly get the most out of it.

So I’ve put together this comprehensive guide to answer them all for you in one place!

Today, I’ll cover various topics which generally hold people back when setting up a solid HIIT workout program for fat loss, including:

  1. What is HIIT?
  2. What is the best HIIT interval time split to use for fat loss?
  3. How long should a high intensity interval training workout be for maximum results?
  4. Will you burn muscle if you do HIIT? And, if so, how can you prevent this from occurring?

These are all questions which will be featured below, alongside various others, in my complete guide to HIIT! Are you ready?


High Intensity Interval Training Explained – The Complete Overview

If you are unsure about any aspect of HIIT, you are in the right place my friend.

Because despite the fact that it has become very popular in the last few years, it’s not a new technique.

HIIT has been around for over a quarter of a century, and there’s already a stockpile of detailed research which can teach you the components required to build the perfect HIIT plan to suit your gym goals, whether it be weight loss, muscle building, or even increasing sporting performance.

HIIT is really an “all rounder”.

HIIT explained

Is HIIT Really That Old?


The first references to HIIT date way back to 1985, before its “big break” arrived at the 1992 Olympic Games in Barcelona after swimmers and sprinters spoke about their coaches implementing HIIT into their training routine.

Unfortunately, it went largely unnoticed at the time.

It was overshadowed by the discovery of creatine monohydrate (again, used by sprinters at these Games), which went on to become the best-selling bodybuilding supplement of all time.

However, the fact remains that the foundations of HIIT were laid long before most of us stumbled across it on the gym floor or in a magazine.

high intensity interval training

What Is HIIT?

HIIT is based on manipulating your heart rate while training.

This means you’re frequently switching between periods of low intensity and periods of high intensity, as opposed to working at a continuous pace (steady state cardio).


Is HIIT More Effective Than Regular Cardio For Fat Loss?


It took a while to be acknowledged, but HIIT is now accepted by most of the world’s top coaches to be a far more effective way to burn fat while maintaining lean muscle tissue.

The study which really got the ball moving for HIIT’s fat loss benefits was published back in 1994. It showed that participants incorporating interval training into their cardio routine were able to burn almost 9x more fat versus regular cardio, despite the fact that the HIIT group were in the gym for less time! (1)

hiit vs cardio calorie burn

In another trial, it was shown that adding regular cardio alongside a resistance training program had a significant impact on the amount of muscle which could be built, reducing it by almost 30% and reducing strength gains by 15%. When they looked at short, intense, sprint-type training alongside a resistance training program, those negative impacts on strength and hypertrophy were completely wiped out! (7)


Where’s The Proof?

You could be forgiven for thinking there’s some kind of magic happening here.

If something looks too good to be true, it usually is!

So how can you spend less time working out, have a more interesting training session, and lose way more fat – all at the same time?!

That question alone is why it took so long for HIIT to become widely accepted.

Even though Hollywood stars like Sylvester Stallone had adopted the technique way back in the mid-1980’s, the mainstream didn’t recognize it as a legitimate training protocol until many years later.

high intensity interval training
Sly was one of the pioneers of HIIT.

In the early days, the research behind it was sketchy and there were more questions than answers.

Would training at a high intensity cause muscle loss? Would you feel too tired to lift weights?

These questions (and their lack of answers at the time) were enough to scare most professional bodybuilders and fitness models away. After all, their careers depend entirely on their physiques.

And if they weren’t doing it, chances are the public weren’t going to either.

That all changed in 1994, when a study from Laval University, Quebec, presented some startling facts. (1)

Researchers directly compared the fat loss results two groups of trainees performing either a high intensity interval training routine or a steady state cardio routine. Essentially, this was HIIT vs Cardio for the first time ever.

They took muscle biopsies to assess the specific adaptations the subjects made during the workout program.

Over the course of the full program, the HIIT group burned 13,830 calories in the gym and the cardio group burned 28,740.

hiit explained

At first glance, it looks like the cardio group came out as clear winners. After all, they burned through over twice as many calories…

This was understandable, and somewhat expected, because the cardio group were training more frequently and for a longer duration than the HIIT group.

But this is where this study got interesting, and blew the lid off fat loss training forever…

high intensity interval training guide

For some reason, the HIIT participants lost 9x more fat for every calorie they burned in the gym! Furthermore, regular cardio did not increase production of the fat metabolizing enzyme HADH, whereas HIIT actually did, despite the fact that HIIT relies on carbohydrates for fuel and cardio relies on fat.

The big questions which arose from this study were:

  1. How did the HIIT group lose more fat, despite burning less calories during training?
  2. Given that HIIT relies on carbohydrates for fuel during training, how did they burn more fat?

The Afterburn Effect

The key reason for the difference in results is EPOC (excess post-exercise oxygen consumption).

This is often referred to as “the afterburn effect” (rolls easier off the tongue), and it explains exactly how the HIIT group in the Laval study lost more fat overall.

After your body has blasted through its glycogen (carbohydrate) reserves in your HIIT workout, it becomes very protective of your few remaining reserves. It won’t let you use them, and it lays down the law; “You’re not getting any more of these, until I’ve got enough of them in my system!”

So your carb stores are placed on temporary “lock down.”

When you replenish them via your post-workout protein shake, or a handful of Haribo jellies, your body is in full survival mode and begins the process of recovering from the previous workout, so those carbs are greatly appreciated.

However, it also needs fuel now. After all, you’ve just completed a hard workout, and your body needs energy.

This is where EPOC comes in. The body’s fat burning mechanisms are temporarily flipped upside down, protecting your precious carbohydrate reserves and shoveling fat into the furnace instead.

A 2011 study published in Medicine and Science in Sports & Exercise confirmed that this post-workout phenomenon can last as long as 14 hours! (2)

So despite the fact that they didn’t use too much fat during training, our HIIT group will now burn primarily fat for the next 14 hours.

Now hopefully a few things are beginning to slide into place, regarding why HIIT is so effective at fat loss!

what is the afterburn effect

Which HIIT Interval Time Should You Use?

If you ask this question in a gym, you’ll usually get the following answer:

“I do an hour of HIIT; 30 seconds hard, 30 seconds easy.”

– Gym bro

I call this approach “HIIT & Hope.”

Don’t do this, because this approach presents numerous problems which will hold back your results. They are:

  1. If you can do an hour of HIIT, you’re not working hard enough.
  2. Interval times can be structured in a way to match your goals. Thirty seconds might not be the right choice (more on this below).
  3. Your rest period must be consistent with your recovery speed. Otherwise, you’re not getting maximum results from your HIIT session.

Fortunately, there are several key studies which break down different interval times and how they’ll impact your HIIT results.

what is the best hiit workout for fat loss

The majority of people using HIIT are doing so for fat loss, so let’s start there.

In my opinion, the best study on this topic was performed at the University of Ontario, Canada, by Dr. Peter Lemon and his team back in 2011.

They discovered that the optimal interval split for fat loss consisted of six rounds of 30 seconds and four minute recovery periods, lasting a total of 36 minutes.

Using this style of training, participants were able to double their calorie burn versus regular cardio. (3)


The researchers arrived at the conclusion that an adequate recovery period is absolutely vital to results during high intensity interval training. Meaning, if you are unable to recover before your next burst, it will not be at maximum intensity.

This is why the common “I’ll do a burst every 30 seconds” approach doesn’t yield maximum results, because while the workout itself may feel difficult, by not allowing ourselves to recover between intervals we are defeating the entire point, at least from a fat loss perspective.

30 seconds simply isn’t enough for the majority of people to recover to the stage where they can do another burst at 100% effort. This study found that 4 minutes was very effective.

Of course, this is subjective to the individual’s fitness levels.

If you need four minutes to fully recover from a flat-out 30 second burst, then take four minutes. But if you feel fully recovered in less time, then take less time.

As I mentioned earlier, when HIIT is structured correctly it’s about manipulating your heart rate, so it must be based around your own capabilities. And now that you know the importance those recovery phases have on overall results, you will be able to make your HIIT workouts much more progressive as you become fitter, faster, and stronger.

The flexibility and progressive nature here are some of the reasons I stated in a recent article that HIIT is better than home workout programs like Insanity for sustainable fat loss results.


Then there’s the much publicized Tabata protocol.

Dating back to 1996, Professor Tabata worked with a group of elite athletes following a simple eight minute workout based around 20 seconds high intensity, 10 seconds low intensity. (4)

During this study, he was able to increase their VO2 Max (the best measure of cardiovascular fitness) by a massive 28%!

To make these results even more impressive, you must bear in mind that the study group featured only elite athletes. Consider how difficult it is for a top bodybuilder to pack on an extra inch to his or her already bulging biceps, or a sprinter to shave an extra 1/10 second from their personal best time…

Suddenly, the results of the Tabata study really hit home!

A 28% improvement in the aerobic ability of athletes who were already considered to be at the top of their game is really quite something, and suggests that shortened recovery periods are great for highly trained aerobic individuals (endurance athletes).

Going back a couple of years before the Tabata study, researchers from Canada showed that high intensity bouts of 15-30 seconds interspersed with much longer recovery periods caused 9x more fat loss per calorie burned. (1)

I mentioned this study earlier in the article.

Those findings back up Dr. Lemon’s 2011 study which concluded that a 30/4 split is optimal for fat loss results.


But what about for athletic performance?

Let’s travel to Australia, where researchers tried a completely different style of HIIT…

In this study, they used eight 3 minute intervals at high intensity (that’s a crazy long time to be going hard!) followed by a recovery period which simply lasted as long as necessary for the participant’s pulse to return to 70% of their max heart rate (around 2 minutes for most people), giving a total workout time of about 40 minutes.

They found that when this approach was performed by highly trained rowers, their performance was increased significantly, as well as their muscular power output and VO2 Max. (5)

So the studies above indicate to us that the following forms of HIIT would be most suitable to individuals with these goals:

  1. Those with fat loss goals should begin with a 30 second high intensity burst followed by a 3-4 minute recovery phase. This can be lowered based on the fitness levels of the participant.
  2. Those with goals of improving aerobic sporting performance would suit the Tabata protocol.
  3. Finally, the Australian rowing study indicates that those aiming to improve explosive power and speed should opt for longer intervals, followed by a recovery phase of roughly the same length.

Pick the one which best suits your current goals, and start from there!


Should You Do HIIT Before Or After Weights?

Prepare to be surprised.

I’d guess around 90% of gym members and/or personal trainers would say the best time to fit cardiovascular work into your routine is after you’ve finished lifting weights.

Heck, I used to believe this, too.

The theory is that you’ll tire out your muscles if you did cardio before weights, and for many years it made sense, despite having no proof behind it.

When the subject was finally looked into by researchers at the University of Memphis, the findings were definitely surprising!

But let’s cut straight to the chase here; you should do your cardio before you lift weights!

I know… I know…

It goes against everything you’ve ever been told. But I’ll explain.

In a 2013 study published in the Journal of Strength and Conditioning Research, scientists tested the old belief that pre workout cardio would tire out your muscles before lifting weights. (6)

They put two groups of trainees through an intense 45 minute workout on a rowing machine, while two other groups of trainees simply rested, then all groups hit the gym for six sets to failure on either heavy barbell squats or heavy bench press.

Interestingly, the cardio only affected the group who performed squats.


The bench press group were completely unaffected by the cardio beforehand, and the researchers concluded that cardiovascular exercise only impacts the performance of the primary muscles which were used during the cardio itself, i.e. don’t do cardio before leg day!

But while this research shows that doing cardio before weights is no worse, and won’t leave you fatigued, it doesn’t show that it’s better either.

For that, we must go a little deeper…

should you do cardio before or after weights

The real reason you should do cardio before weights is explained when we look at what’s going on inside the body during a workout.

Two enzymes will explain everything here, and this should completely change the way you train forever.

During your cardio workout, an enzyme known as AMPK is released, and this helps us to make several adaptations to endurance exercise by encouraging the body to build more mitochondria, in order to process more energy for you to use.

After a weights session, an enzyme known as m-TOR is released.

You may have seen this word before on muscle building supplements. It is the enzyme which “flips the switch” for the muscle building process to begin after a workout.

Given that we want to create lean muscle growth and torch body fat, ideally we want to maximize m-TOR after a workout, right?

But what most people don’t know is that AMPK kills off m-TOR. (8)

Therefore, it doesn’t make sense to finish a hard resistance training session then jump on a bike and do an hour of cardio!

This research should literally change the way most bodybuilders train forever, but the study in which it was discovered was terribly under-reported in the media at the time.

Production of m-TOR lasts for up to 6 hours, and we really want to get the most from this prolonged muscle building period, so ideally we should be doing our cardio before we start lifting or splitting the workout into two separate sessions.

The take away lessons here are:

  1. Do your HIIT before your resistance training workout to maximize m-TOR and minimize the effects of AMPK.
  2. The commonly used approach of doing cardio in the morning and weights in the evening is also fine.
  3. Weights in the morning and cardio in the evening would also work, particularly if there was at least 6 hours break between the two, as this would leave ample time to maximize heightened m-TOR release from your weights session.

Should You Do HIIT On An Empty Stomach?

Getting your pre workout nutrition in check is another way to take your results to a higher level.

Lots of people fuel up before hitting the gym by eating a banana or other fruit.

And while this is perfectly fine in most cases, it isn’t optimal for high intensity interval training if your goal is fat loss.

Given that we are blasting through our carbohydrate reserves during HIIT, in order to reach that golden ‘afterburn’ when our workout ends, it doesn’t make sense to eat a bunch of carbohydrates right before we train. This would merely buffer our carb reserves and leave us with more work to do in the gym to reach our desired target.

Research shows that consuming protein before a workout can enhance results. It’ll protect lean muscle tissue, provide a slight boost to metabolism, and increase the calorie burn of your upcoming workout. (9)

When you are trying to blast body fat, protein is your best friend!

best supplements for hiit

Which Supplements Should You Use For HIIT?

The supplement industry can be your ally, or your worst enemy…

Products like whey protein make it incredibly useful to top up our protein intake without too much hassle, which is great. But stray too far into the “dark side” and you’ll be spending more on pills and powders than on food!

Worse still, they all claim to be crucial to results!

So here’s a quick guide to the supplements my clients use when performing high intensity interval training programs.

optimum nutrition gold standard whey
  • Whey Protein

The benefits of whey protein are well documented.

You’ll build more muscle, you’ll burn more fat, and a wealth of smaller benefits which we’ll save for another post. If your goals are fat loss orientated, and you are looking for a whey protein supplement which you can use before or after training, you should go with a product that offers you around 20 grams of protein per serving, alongside low carbs and low fat (under 5g each).

See which one I use here.

best pre workout for HIIT
  • A Pre Workout Designed For HIIT

If you’re a regular reader, you’ll know that sometimes I make my own pre workout.

That being said, there are some easy-to-grab pre workouts on the marketplace which suit HIIT workouts just fine, like this one.

When I create my own, I do so using three key ingredients; caffeine, beta-alanine and citrulline malate.

The links above will show you the ones I personally use, too.

Caffeine is still king when it comes to pre workout stimulants. It also has a mild fat burning effect. 200mg should be enough, depending upon your tolerance levels.

Citrulline malate is a powerful member of the amino acid family, and can significantly improve performance during explosive exercise because it boosts recovery speed between sets, as well as delivery of nutrients to working muscles. (12)

A clinical dose of CitMal is 6g. Most supplements come in well under this mark, or use cheap substitutes like arginine in its place.

Beta-alanine is useful for buffering your ability to push past the build-up of lactic acid during training (“the burn”). This was shown in a startling 2012 study, where amateur boxers increased their punching power in the closing stages of three minute rounds by a whopping 2000% (no typo). (11)A

A full dose is 3.2 grams, but this is too much for many people to handle, so start with half and work yourself up over time.

HIIT explained
Team GB athlete Amanda Lightfoot in the gym for a HIIT session.

HIIT – In Summary

Congratulations, you now know more about HIIT than just about anybody at your local gym, and probably more than about 80% of personal trainers.

The majority of people looking to perform HIIT have fat loss goals, so let’s summarize the main points below from that perspective…

  1. Use the 30/4 split, and reduce your rest as your fitness begins to improve. It is crucial that you are fully recovered before you tackle your next interval, so don’t rush this process thinking “more is better”, because research actually shows it’s not.
  2. Perform your HIIT alongside a resistance training program. You can optimize further by doing HIIT before lifting weights, but skip the cardio before leg day if your HIIT is primarily leg-based work.
  3. Consume protein before your HIIT workout to increase fat burning slightly further. Try to avoid carbohydrates in the final hour before training, as this will enable you to deplete your carb reserves faster in the gym and reap the full benefits HIIT has to offer.
  4. Supplement your HIIT program with wwhey protein, caffeine, beta-alanine, and citrulline malate to boost training performance further.

Good luck with your new HIIT adventures! You can ask me questions in the comments section below.

If you’ve enjoyed reading this comprehensive breakdown of high intensity interval training for fat loss, get on my email list below for more training tips, and if you’re looking for a comprehensive HIIT training plan which gives you a proven structure to get insane results with, here you go!

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  1. Trembalay, A., et al. Impact Of Exercise Intensity On Body Fatness And Skeletal Muscle Metabolism. Metabolism. (1994)
  2. Knab, A. M., et al. A 45 Minute Vigorous Exercise Bout Increases Metabolic Rate For 14 Hours. Med Sci Sports Exerc. (2011)
  3. Lemon, P. W., et al. Run Sprint Interval Training Improves Aerobic Performance But Not Max Cardio Output. Med Sci Sports Exerc. (2011)
  4. Tabata, I., et al. Effects Of Moderate Intensity Endurance And High Intensity Intermittent Training On Anaerobic Capacity And VO2 Max. Med Sci Sports Exerc. (1996)
  5. Driller, M. W., et al. The Effects Of High Intensity Interval Training In Well Trained Rowers. Int J Sports Physiol Perform. (2009)
  6. Schilling, B. K., et al. Acute Neuromuscular And Metabolic Responses To Concurrent Endurance And Resistance Exercise. J Strength Cond Res. (2012)
  7. Wilson, J. M., et al. Concurrent Training: A Meta Analysis Examining Interference Of Aerobic And Resistance Exercise. J Strength Cond Res. (2011)
  8. Hardie, D. G. AMP-Activated/SNF1 Protein Kinases: Conserved Guardians Of Cellular Energy. Nat Rev Mol Cell Biol. (2007)
  9. Hackney, K. J., et al. Timing Protein Intake Increases Energy Expenditure 24 H After Resistance Training. J Am Col Sports Exerc. (2010)
  10. Paoli, A., et al. Exercising Fasting Or Fed To Enhance Fat Loss? Influence Food Intake On Respiratory ratio And Excess Post Exercise Oxygen Consumption After A Bout Of Endurance Training. Int J Sport Nutr Exerc Metab. (2011)
  11. Donovan, T., et al. Beta-alanine improves punch force and frequency in amateur boxers during a simulated contest. Int J Sport Nutr Exerc Metab. (2012)
  12. Pérez-Guisado, J., et al. Citrulline malate enhances athletic anaerobic performance and relieves muscle soreness. J Strength Cond Res. (2010)

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