hiit explained

HIIT Explained – High Intensity Interval Training – The Complete Overview

Cardio is awesome for fat loss.

But there are many questions surrounding HIIT.

Luckily, I’ve put together this comprehensive guide to answer them all for you.

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

  • What is HIIT?
  • What is the best interval time split to use for fat loss?
  • How long should your high intensity interval training workout be for maximum results?
  • Will you burn muscle if you do HIIT and, if so, how can you prevent this from happening?

These are all questions which are going to be answered in today’s extensive guide to HIIT.


High Intensity Interval Training – The Complete Overview

If you are unsure about any aspect of HIIT, my friend, you are in good hands.

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

It’s been around for over a quarter of a century, and there’s already a stack of research which will teach you the components required to build the perfect HIIT routine to suit your goals in the gym – whether it be weight loss, building lean muscle, or even increasing sporting performance.

hiit explained

Is HIIT Really That Old?


The first references to HIIT date way back to 1985, before it’s “big break” arrived at the 1992 Barcelona Olympic Games where swimmers and sprinters spoke about how their coaches had implemented it into their training routine.

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

However, the fact remains that the foundations of HIIT were laid down long before most people stumbled across it in the gym.


What Is HIIT?

HIIT is based upon manipulating your heart rate.

In a workout, this means you’re constantly switching between a low intensity and a high intensity for the duration of your session, as opposed to working at one continuous pace.


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.

One particular study showed that participants who performed a HIIT routine were able to burn almost 9x more fat as those performing regular cardiovascular workouts, despite the fact that the HIIT group were in the gym for less time overall. (1)

In another trial, it was revealed that while adding regular aerobic work to a weight loss program decreased muscle growth by an astonishing 30% and reduced strength gains by up to 15%, when they looked at short, intense, sprinting-type exercises the negative impact on strength and muscle gains were completely wiped out. (7)

fasted cardio

Where’s The Proof?

You could be forgiven for thinking there was some kind of voodoo magic at work here.

I mean, we live in a world where if something looks to good to be true, it usually is, right?

So how can you spend less time in the gym, have a more interesting workout, and lose way more fat – all at the same time?!

That question alone is why it took HIIT so long 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 at first.

high intensity interval training

Sly Stallone was one of the pioneers of HIIT.

In the early days, the research behind it was sketchy and there were questions raised about whether or not the high intensity would result in huge muscle loss, scaring off most professional bodybuilders and fitness models who relied on their looks for their careers.

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

Researchers directly compared two groups of individuals performing HIIT vs Cardio, they also took biopsies of the participants’ muscles in order to assess the specific adaptations which were happening inside the body.

Over the course of the trial, the HIIT group burned a total of 13,830 calories and the cardiovascular group burned 28,740.

That’s over twice as many calories burned by the cardio group!

This was understandable, too, given that the cardio group were training more frequently and for a longer duration each session.

hiit vs regular cardio

Looking at the difference in calorie burn, it would be considered mighty impressive if the HIIT group were able to stay anywhere near the cardio group in terms of fat lost as a result of training.

Yet, for some reason, the HIIT group had lost 9x more fat for every calorie they burned.


They also discovered, somewhat surprisingly, that regular cardio didn’t increase the body’s production of the fat metabolizing enzyme HADH, whereas HIIT did – despite the fact that cardio mainly uses fat for fuel and HIIT mainly uses carbohydrates.

So once the interesting results had been recorded, the big question which rose from this study was:

If HIIT mainly uses carbohydrates for fuel in the gym, how did this group burn more fat?

The Afterburn Effect

You may have heard of this before.

The reason the HIIT group in the Laval study were able to burn of 9x more fat (despite burning fewer calories during training, and spending less time in the gym overall) came down to EPOCexcess post-exercise oxygen consumption.

EPOC is more commonly known as it’s showbiz name, The Afterburn Effect

After your body has blasted through it’s glycogen (carbohydrate) stores in your HIIT workout it becomes very protective of your few remaining reserves. It will not let you have 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 go on temporary “lockdown”.

While you replenish them your post-workout nutrition, such as a whey protein shake and some fruit or a handful of jellies, your body is in full survival mode and will not allow it’s remaining carb reserves to be used as energy.

However, it has to give you something.

I mean, you’ve just done a tough workout and your body needs fuel.

So it flips things around, protecting your precious carbohydrate reserves and instead shoveling fat reserves into the fire.

A 2011 study published in Medicine and Science in Sports & Exercise confirmed that this post-workout phenomenon of using primarily fat for fuel (EPOC) can last up to 14 hours! (2)

That’s right, despite the fact you didn’t use any fat while you were in the gym, you will now burn fat at an accelerated rate for up to fourteen hours!

Now, hopefully a few things are beginning to fall into place as to why HIIT is so effective.

Which Interval Time Should You Use?

How long should intervals be for maximum results?

If you asked 10 different trainers this question, you would probably get 10 different answers.

But while many people are happy to base their HIIT sessions around other people’s experiences, i.e. “well, it worked for me”, I encourage you to be different.

Fortunately, there are several key studies which have looked into different interval times and their effects on the human body.

high intensity interval training

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

They discovered that participants following a program consisting of six rounds of 30 second intervals and four minutes of moderate recovery (27 mins total workout) were able to burn over twice as much fat as regular cardio.

That discovery came despite the HIIT group working out for less than half the time of the cardio group! (3)

This study 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 interval, it will not be at maximum intensity.

This is why the common approach of, “I’ll do a burst every 30 seconds”, doesn’t lead to the best possible results, because while the workout itself may feel hard, by not allowing for adequate recovery between bursts we are defeating the entire point, at least from a fat loss perspective.

Two minutes simply isn’t enough for the majority of people, whereas a four minute split was very effective.

Of course, this is subjective to the individual.

If you need four minutes to fully recover from a flat-out thirty second burst, then take four minutes. But if you felt you had fully recovered within less time, then take less time.

As I mentioned above, proper HIIT is about manipulating your heart rate, so it must be based around your own capabilities. And now that you know the importance of adjusting the recovery period between intervals, you’ll be able to make your HIIT workouts much more progressive as you get fitter and stronger.

This flexibility is one of the reasons I’ve always believed HIIT is better for fat loss than home workout programs like Insanity.


Then there’s the much publicized Tabata protocol.

First dating back to 1996 in Japan, Professor Tabata worked with a group of highly trained elite athletes using a simple eight minute workout that operated on a 20 seconds high, 10 seconds low split.

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 group in this study were already elite athletes.

Consider how hard it is for a top bodybuilder to pack on an extra inch to his already bulging biceps, or for a great sprinter to shave an extra tenth of a second from his or her 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 astonishing, and seems to suggest that shortened recovery periods are great for highly trained aerobic individuals. (4)

should you do cardio before or after weights

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 that study earlier in this article.

Interestingly, those findings also back up Dr. Lemon’s initial study of a 30/4 split for fat loss.

Now let’s travel to Australia, where researchers used eight three minute high intensity bursts followed by a recovery period which was simply as long as it needed to be for the individual’s pulse to return to 70% of their maximum heart rate (probably around two minutes in most people), giving a total workout time of about 40 minutes on average.

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

So the studies above indicate to us that for those who are performing HIIT with specific goals in mind, the following protocols are optimal:

  • Those with fat loss goals should go with a 30 second high intensity burst followed by a 3-4 minutes of recovery depending upon the fitness levels of the individual.
  • Those with goals of improving aerobic sporting performance would suit the Tabata protocol.
  • Finally, the Australian study with the group of highly trained rowers indicates that those who are aiming to improve explosive power and speed should utilize a longer high intensity burst followed by a recovery period of roughly the same length.

high intensity interval training

Should You Do HIIT Before Or After Weights?

Prepared to be surprised here.

I’d guess that around 80% of gym members and/or trainers would tell you the best time to fit cardiovascular work into your routine is after you’ve finished your lifting.

Heck, I used to believe this myself, too.

The theory is that you would tire out your muscles if you did cardio before weights. And for many years this theory made perfect sense – despite having no actual proof behind it.

When the subject was looked into by researchers at the University of Memphis, however, the findings were surprising to say the very least!

Let’s cut straight to the point here – you should do your cardio before you lift weights!

That’s right, I said it!

In a 2013 study published in the Journal of Strength and Conditioning Research, researchers tested the old belief that pre workout cardio would blast your muscles and leave you too tired to hit the weights.

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

Notably, the pre workout cardio only affected the performance in the squats.

The bench press was completely unaffected by the pre workout cardiovascular exercise.

The researchers concluded that cardiovascular exercise only affected the performance of the muscles which were primarily used during the cardio itself. (6)

Given that most cardio work is legs-based, my advice would be to simply avoid doing any cardio on leg day.

cardio before or after weights

The real reason you shouldn’t be doing cardio after weights is discovered when you look at what goes on inside the body during a weights workout.

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

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

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

You might have seen that word on several muscle building supplements. It’s the enzyme which “turns on” the muscle building process after a workout.

Given that we want to build lean muscle and torch body fat, ideally we want to maximize m-TOR, right?

That just makes perfect sense!

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

Therefore, it makes absolutely no sense to finish a tough resistance workout and then jump on an exercise bike and do an hour of cardio!

This research should literally change the way most bodybuilders train from now on, 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 six 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 our workout into two separate sessions.

The take-away lessons here are:

  • Do your HIIT before your resistance training to maximize m-TOR release and minimize the effects of AMPK.
  • The commonly-used approach of doing cardio in the morning and weights in the evening is also fine.
  • Weights in the morning and cardio in the evening would also work, particularly if there was at least six hours between sessions, as this would leave more than enough time to maximize the heightened m-TOR release from your weights session.

post workout carbs

Should I Do HIIT On An Empty Stomach?

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

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

And while this is perfectly fine in most cases, it isn’t optimal pre workout nutrition for high intensity interval training.

Given that we are blasting through our carbohydrate reserves during HIIT in order to reach that golden “Afterburn Effect” when our workout ends, it doesn’t make sense to eat a bunch of carbohydrates 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.

In a recent post here on the blog, I showed you how consuming whey protein before early morning cardio allowed you to increase the calorie burn normally associated with fasted cardio to a higher level.

This same principle can be used before performing HIIT to give you even more bang for your buck.

Research has shown us that consuming protein before any workout will enhance your results further.

Having a serving of protein before you hit the gym has been proven to protect your lean muscle tissue and also increase your metabolism far more than training on an empty stomach – helping you to burn more fat in your upcoming workout. (9)

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


What Supplements Should I Use For HIIT?

Most people have a rough idea that they should be using whey protein supplements, but beyond that they get a little bit lost.

That’s because every supplement hypes itself up as “the one you need to succeed” and you can spend an awful lot of cash in the supplement industry if you don’t know exactly what you require.

To compliment a fat loss HIIT program there are only three or four major supplements that I would recommend.

  • Whey Protein

The benefits of whey protein are well documented.

Not only will it help you to build more lean muscle tissue, it will also aid with encouraging fat loss and a wealth of other benefits which we won’t go into here. If your goals with HIIT are fat loss orientated, and you’re looking for a whey protein supplement which you can fit in before and after your workouts, you should go with a product that offers you around 20 grams of protein per serving alongside low carbohydrates and low fat (under 5g each).

You’ll be able to see which one I use here.

  • HIIT Pre Workout

Instead of using a pre workout your body adapts to within two weeks, and leaves you wishing you’d never wasted your money, I encourage my clients to make their own pre workout by picking up some caffeine, beta-alanine and citrulline.

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

In combining these three ingredients we create a supplement which boosts our ability to perform intense exercise, with zero carbohydrate content.

Here’s a bit of information on each one.

  • Caffeine

One of the oldest supplements in the book, caffeine is still king when it comes to pre workout energy boosting supplements. As well as increasing energy, it also has a mild fat burning effect. 100-200mg should be enough, depending upon your caffeine tolerance.

  • Beta-alanine

Beta-alanine has been shown to boost training output and it remains one of the most useful supplements to use before a HIIT session.

Most of us know this as the ingredient in most pre workouts that gives us that tingly, skin-crawling sensation. A full clinical dose for training purposes is 3.2g, but you may want to start at 1.5g and work your way up, as those jitters can hit quite hard if you’re not used to it.

In 2012, UK researchers reported that beta-alanine supplementation in amateur boxers increased their average punching power in the last ten seconds of a round by 2000%. (11)

  • Citrulline

A powerful member of the amino acid family, citrulline’s primary function is to boost energy.

And it’s no slouch.

In fact, citrulline has been shown to significantly boost performance in bouts of explosive exercise, making it very useful for those performing heavy weights sessions or, you guessed it, HIIT workouts. A full clinical dose of citrulline malate is 6 grams. Most supplements come in well under this. (12)

Of course, for those of you who’d simply prefer a ready-made pre workout for convenience purposes, I recommend something which is clinically dosed, like this.

best supplements for hiit

HIIT – In Summary

I believe one of the major problems with the fitness industry is that people often just don’t have a trainer who is prepared to take the time to sit down with them and explain things properly.

Everyone is in such a rush to find “the next big thing”.

Now you have that – me.

Congratulations, you now know more about HIIT than just about anybody in your local gym and probably more than about 85% of trainers, too! Let’s summarize the main points now.

The majority of people who are looking to incorporate high intensity interval training into their workout routine have fat loss goals.

If that’s you, then the following protocols will enable you to get immediately on the right track:

  • Use the 30/4 split. As your fitness improves you will notice that your recovery time can be lowered. It is crucial that you are fully recovered before you tackle another high intensity burst, so don’t rush into lowering your recovery times before you are ready for it. Three-to-four minutes was adequate recovery for most people. The majority of people who could handle less than two minutes or recovery were athletes.
  • Perform your HIIT as part of a resistance training program, you can optimize it further by doing your cardio before you hit the weights. Skip cardio on leg day.
  • Consume protein before your HIIT workout to increase fat burning even further. Try to avoid carbohydrates in the final one hour building up to your workout, as this will enable you to deplete your carb stores faster in the gym and get the full benefits from high intensity interval training. If you train first thing in the morning, simply avoid carbs and have a protein rich breakfast.
  • Supplement your HIIT routine with whey protein, caffeine, beta-alanine and citrulline if you wish to boost performance further.

Good luck with your new routine and you can ask me any questions in the comments section below.

If today’s post helped you, share the link to a gym buddy and help them too. That’s what it’s all about.

Get on my free email list – bottom of this article – to receive more useful training tips like this. I send stuff out weekly.



  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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