HIIT Broken Down

Lets talk HIIT 


It is the buzzword in fitness. It’s a training protocol that has been used by the worlds leading athletes for decades, loved by Hollywood actors to get fitter, faster and stronger in a fraction of the time. High Intensity Interval Training is employed by the worlds best sports coaches to rapidly increase VO2 fitness levels and help furnace torch fat from the body’s of the athletes they train.

Now, like any training methodology HIIT for those of you starting out or new to this type of hyper training, can find it daunting or confusing. So let me, a leading expert of HIIT clear up a few common mistakes being made by many of you, whilst giving you a few key pointers to help you train more efficiently.

What is HIIT?

HIIT can be performed on most indoor cardiovascular equipment, i.e. the Wattbike, Concept 2 Rower, Ski Erg, Cross Trainer, Treadmill or outdoor sprints, Boxing or Skiing! 

An exercise places the body into a HIIT training protocol by electing the correct “training intensity”. A common mistake I see is people confusing HIIT with cardio. HIIT is NOT cardio. This is a key point you must remember.

The importance of training intensity

Training intensity governs how the body elects and manages energy, which is essentially a chemical reaction from within the body, which is either aerobic, (steady state or FatMax cardio) occurring in the presence of oxygen or anaerobic (HIIT or weight training) without oxygen. We must continually produce energy or the tissues, cells and organs within our body will die. Now a specific amount of effort requires a given amount of energy, and the body must have an appropriate level of energy available to meet the demands placed upon it.

Now lower intensity, one interval cardio training takes place below the “ Anaerobic  Threshold” or AT  meaning there will be a regular supply of oxygen coming into the body, and as this continues the aerobic engine fires up enabling it to provide the fuel (energy) to support the interval. This energy comes from Adipocytes (fat cells) that release more and more fat from the oxidation process, which in turn is the preferred fuel source for training at aerobic intensity.

However, as we decrease the volume of training, and increase its intensity the body creates more mitochondria. They are a crucial element in energy metabolic pathways and play a big role in fatty acid oxidization. The greater the fat oxidization ability the greater the fat loss results. To achieve fat loss, a fatty acid has to be freed from the fat cell store and driven to the mitochondria where it can then be used as energy and destroyed.

So to release more fat from stored fat deep within stubborn fat cells, the tempo needs to step up to 85 % MHR or above for short sprint intervals. This increase in training intensity will shut off the oxygen supply to the cells of the body, thus the ability to produce energy aerobically will not be able to cope with the higher energy demands.

This shut of point typically takes place at 70/75 % of VO2 max, where the aerobic system cannot produce energy fast enough, hence the switch to the anaerobic system so that energy demands can be met.

Therefore HIIT demands the same chemical adaption from the body as weight training. Both are anaerobic.

Now a key feature with HIIT is that it gives you the ability to exercise for the shortest possible time (7-20 min) with the greatest benefit to both fat loss and increases in baseline fitness levels.

You essentially achieve this by training at the highest possible intensity you can place your body under, in short “ Sprints” of typically 20/30/40 or 60 seconds, known as “ Push Intervals”. These push intervals are set against “Fitness markers”, the most common and frequently used training marker sets training  intensity at a pace of 85% or above of your MHR (Maximum Heart Rate).


· If you don’t know your MHR then you will have noted Your CardioGuru provided an estimate. The formula used was set by Sally Edwards, CEO of Heart Zones who proposes a set of gender specific formula for predicting Maximum Heart Rate. This is considered one of the most accurate estimation methods:


For males: 210 - 1/2 your age - 5% of total body weight (in pounds) + 4 = HRmax
For females: 210 - 1/2 your age - 5% of total body weight (in pounds) + 0 = HRmax


Example... Dave is 39 and weighs 220 lbs

210- 19.5 (that's 1/2 half his age) - 11 (that's 220*.05) + 4 gives a predicted MHR 183.5 * 85% = 155 bpm ( beats per min) is the targeted HIIT marker in the push intervals.

** The software used in Your CardioGuru will also monitor your training performance and update you MHR if you train at a higher intensity than recorded.

Now after each “ Push” Interval, the body will have been placed under stress at cellular level, and this is when you then enter what’s known as the “recovery” or “ rest interval”.  This interval is used to recover and prepare for the next “push” interval where you place the body back into battle. By lowering your heart rate to approx. 60% of its max and halting the exercise you are then able to use this 30/40/60/120 sec interval as an effective recovery period.


“The high intensity is where all the work is done and where if performed correctly, all of the “benefits” are made, while the lower intensity is there to stimulate recovery and have you catch your breath”


Now HIIT has evolved as a leader among mainstream training protocols for increasing fitness and decreasing body fat only very recently, despite being adopted by many of the worlds best athletes since the 1970’s.


As demonstrated by the graph below, it was not until 2007 that research carried out by Gibala (http://link.springer.com/article/10.1007%2Fs11932-007-0033-8?LI=true) changed the way scholars viewed exercise intensity and how much volume is needed to burn fat and increase cardio fitness.

Pre 2007 it was suggested that exercise should be performed in the small corner section of the graph below at training intensity levels of 40-60 %. It was considered that this intensity range would lead to optimal fat loss and increases in cardio capacity. Training in the higher intensity zone was thought to gain power, speed and drive metabolism.

Gibala proved that by cranking up the training intensity into 6 sessions, spread over 2 weeks, consisting of 4 to 6 x 30second sprints (4 minute recovery between sprints) 630 kj total work , the body can achieve the same results with 10 times less the volume.

The Gibala study used a training protocol design that prescribed the training sessions in kilojoules rather than time or distance. Kilojoules (kj) are a unit of energy and are useful for measuring energy demands in exercise. Kj provide an objective measure of the ‘work’ done in a session, which can also be used to estimate how much energy an athlete has expended. Using Kj, rather than time or distance, ensures that all the subjects in the study did the same amount of physical work. For example, 2 people may ride for 1 hour, but depending on the intensity they ride at, energy expenditure could be very different. Setting a Kj target for the participants is more consistent and repeatable which improves the reliability and validity of the study’s results.

The 2006 Gibala study required the conventional group to expend 6500 kj and the HIIT group to expend 630 kj over the course of the 2 week training programme

At the end of the study, Gibala and his team tested both groups in two time-trial efforts – a short 50kj TT and a longer 750kj TT effort. Gibala also assessed molecular and cellular adaptations in the subject’s skeletal muscle: markers of adaptation to endurance training.

Incredibly, there was no difference in the time-trial performance between the groups, despite the fact that the total volume of training amongst the HIIT group was more than 10 times smaller that in the conventionally endurance trained group (6500kj vs 630kj over 2 weeks of training).

Muscle biopsies also revealed similar adaptations in both groups, notably:


Increases in the muscle’s capacity to use oxygen.
Increases in buffering capacity – the muscle’s ability to neutralize acid and delay the onset of fatigue.
Similar resting muscle glycogen levels in both groups.


The study demonstrated that short, high intensity efforts could be as effective at improving aerobic performance as conventional endurance training involving moderate intensity cycling.

How does HIIT help you achieve fat loss? 

I’ve highlighted how training intensity governs decreasing training volume yet increasing Vo2 Fitness. Indeed this is further strengthened by another paper Sloth M, et al (2013) who conducted a systematic review of 19 studies which evaluated the effects of HIIT studies. Sloth and the team concluded that


“Strong evidence support improvement of aerobic exercise performance and VO2max following HIIT”


What is happening here is an adaptive response to the immense demanded stress that is placed upon the body in the “ push intervals”.

We must accept that our bodies are stuck in an evolutionary black hole.


Physiologically, we're still Neanderthal (extinct species of human) and the body does not know you are exercising on a Wattbike or lifting a barbell. The workout itself does not actually cause any improvements directly. The body as a negative event perceives the workout; it’s an extreme stress to the entire supporting metabolism. The workout in fact acts a messenger, it’s a stimulus and a way to tell your body that that the environment is placing a demand upon it that it must respond and adapt to. The body only knows to hunt or be hunted.

Exercise makes us fit and the time in-between exercise is when we become fit. HIIT performed correctly in my opinion places such a huge demand upon the body it creates an adaptive responsive like no other training protocol.

The body wants to repair that damage and heal stronger so that the trauma is reduced on the next hunt. This is how we begin to get fitter, faster, stronger and leaner.

The body wants to become the ultimate hunter and be equipped with superior adaptive responsive abilities to avoid being hunted and killed.

This now leads me to how HIIT can turn your body into a fat burning furnace. It achieves this in two ways ;


EPOC (excess post-exercise oxygen consumption) or the after burn refers to how HIIT training will force your body to use its sugar stores in muscle (known as glycogen) to supply you with energy much faster than with low/medium intensity exercise. This is however achieved through anaerobic metabolism (energy production without oxygen). This creates an "oxygen debt”, typically enabling you to burn 20 - 30% of the body’s sugar stores in 20 seconds vs 30 minutes or more of lower intensity exercise. 

In a nutshell you have borrowed energy to fuel the HIIT session and you must repay this energy debt post workout.  Indeed studies have shown over a 24 hour period the body can burn up to 9 times as much fat post HIIT exercise in order to repay its debt.

Now the theory behind EPOC is far from an exact science, yet I am of the opinion its a key element in fat loss (its also where a lot of people go wrong as they neglect to factor in the calories expended from EPOC thus ending up with too large a calorie deficit!) the harder and more intense the workout the greater the energy debt, and this places a heightened demand upon the body as it attempts to return the body back to its resting state. The body depleted of glycogen/carb stores from the HIIT session will fuel this recovery process from all available and stored fats. As the body begins to normalize body temperature, resting heart rate, hormones, stress to the liver, it will be forced to work up to 36 hours post workout, all the time electing fat its primary energy source, turning your body into a fat torching furnace.


Fat loss has everything to do with insulin, which is indeed the master hormone that regulates the flow of fat (and glucose) into and out of a fat cell. Many  “experts” fail to acknowledge how HIIT effectively lowers insulin levels. 

Insulin is a "sugar transporter" and "fat blocker", the more insulin you have in your body, the less likely you are to be able to oxidize fat. Less insulin means our bodies burn fat more efficiently, rather than storing it in the body as stubborn fat.

Research carried out by Timmons/Vollaard 2009 you will notice on the graph to the right that after only 2 weeks of HIIT training, the subjects required less insulin levels enabling them to clear more rapidly post workout glucose. This is what we are trying to achieve for fat loss and recovery. Aerobic training will not achieve this size effect. This demonstrates how HIIT greatly assists fat loss.

Top HIIT Pointers


HIIT has everything to do with training intensity and guess work and luck never lead to success. Get an HRM, and get to grips with your training zones and MHR (maximum heart rate). Make sure your HRM is Bluetooth compatible to use with this app!

* Prepare for Battle

HIIT is hard. Even I fear getting on my Wattbike most days knowing I need to push my body to the very brink of total failure. I therefore condition my mindset to the pain, focus on the glory and go into my personal battle knowing a “win” will get me that inch closer to my ambitions….

* Never Stand Still

Employ Progressive Challenge Markers. Our bodies are built to survive. They adapt very quickly. HIIT is effective because of the demand placed upon the body to respond and adapt. The moment you slack off the intensity level, you give the body an inch to adapt and relax, meaning you will slip backwards..

* Fasted or Fed ?

I get asked this a lot. For most of you I would recommend training HIIT in the fed state twice a week and building up to a max of three sessions a week fused with weight training and fasted FatMax training days.

Remember HIIT works the body like a weights session so allow recovery days in-between HIIT and weights sessions.

When you have obtained mastery of HIIT and know how to listen to your body, and then explore the advanced protocol of fasted AMPK HIIT Intervals or training at an intensity of Pmax, fasted.

* Finally

In a nutshell the longer the “Push” interval and the shorter the “ Rest” interval the harder you tax your fitness engine. By targeting to decrease rest intervals and maintaining performance in the push intervals, you will increase VO2 max, thus is how you progressively get fitter, stronger and quicker whilst burning more fat in less time.

Remember the “ Rest” interval is always used as a set up to ensure you smash the “Push Interval” which is the priority of HIIT  training. If you you need to take a longer “rest” interval that’s fine ... but as I mention above, aim to decrease those rest intervals to really start benefitting from HIIT! 

#Focus On Performance, Aesthetics Will Follow