The Science Behind High Intensity Interval Training | HIIT Cardio Benefits


By Scott Sowerby  | Personal Trainer

BSc Sports Coaching Science

High Intensity interval training (HIIT) or Sprint interval training (SIT) is a training method that has garnered alot of attention within sports science and has become a recent favourite among fitness professionals (Driver 2012).

However, a lot of articles write about how it should be implemented, but very few seem to explain the science behind it. The aim of this article is to do just that!

What Is High Intensity Interval Training?

Everybody from professional athletes to the average gym goer wants to make physiological changes and adaptations to their bodies in as short a time as possible. The addition of SIT to a training program allows physiological adaptations and improvements in performance to be made using a lower training volume (Kubukeli, Noakes, & Dennis 2002).

SIT and HIT, consists of numerous repeated maximal effort sprints. The volume of this training is usually four to six sprints per session, with four minutes of recovery in between (Hazell, Olver, Hamilton & Lemon 2012).

So How Does HIIT Work?

This type of training aims to induce Overload. By undergoing strenuous training to significantly fatigue the body in the hope for super-compensation.

Supercompensation occurs when the training overload is supported by significant recovery, in the hope that physiological adaptations will take place, leading to an increase in performance above the baseline (Smith 2003).

What Does The Research Say?

Early studies noted that SIT programs have helped to improve 10km performance time (Acevedo & Goldfarb 1989) and 40km cycle time has also noted to be improved when supplementing SIT training into a training regime (Hawley, Myburgh, Noakes & Dennis 1997).

Participants in both of these studies however reported no change in VO2 peak.

sprint training

Sprint Interval Training for Improved Performance 

More recently, studies have shown that a smaller amount of time performing SIT has helped to improve performance. Burgomaster, Hughes, Heigenhauser, Bradwell, Gibala (2005) performed a study comparing an SIT group performing with a control group.

 They found when utilising a changing amount of wingates ( an anaerobic test often performed on a cycle ergometer) during six training sessions across two weeks, endurance capacity increased during an aerobic cycle test – however VO2 peak did not. Burgomaster, Heigenhauser & Gibala (2006) furthered their research by using the same protocol using both males and females, however adding a cycling time trial. They found that the use of SIT improved cycling time trial performance!

High Intensity Interval Training for Improved Performance 

A study comparing effects of several short bursts of High Intensity Exercise, compared to continuous endurance training, whist both using the same total training time, found that after completing the training both managed to finish the half marathon (Hottenrott, Ludyga & Schulze 2012).

 Positive effects were also shown by reducing visceral fat and increasing lactate threshold. Unlike the previously mentioned studies, both training regimes proved effective in improving participants VO2 peak.

Is HIIT The Best Fat Burner?

HIIT doesn’t only have a positive effect on overall fitness and performance, it has shown to be an excellent method of fat-burning (Borsheim & Bahr 2003).

The impact of HIIT on fat loss and body composition has been researched for over 20 years. Tremblay, Simoneau & Bouchard (1994) analysed the effect different training intensities have on participants body fat and skeletal muscle.

hiit workout

After the given programs, both programs helped to reduce both visceral and subcutaneous fat. However, the HIIT group received much greater adaptations and fat loss than the endurance training (ET) group with the skinfold reductions being 9-fold larger than the ET.

One of the main reasons for this was the enhancing effect of the HIIT on enzyme activity on muscle 3-hydroxyacyl coenzyme A dehydrogenase (HADH), which plays an important role in Beta-oxidation, responsible for lipid oxidation.

Perry, Heigenhauser, Bonen and Spriet (2008) also found that HIIT training programs enhances the bodies abilities to oxidise both carbohydrates and fats during aerobic exercise.

Excess Post-Exercise Oxygen Consumption

One of the contributory factors leading to this rise in fat burning potential is Excess post-exercise oxygen consumption (EPOC) introduced by Gaesser and Brookes (1984). After exercise, an increase in oxygen uptake is experienced to support the energy needs of the participant.


As research has continued, several other mechanisms have been found to contribute to the increased metabolic effect induced by HIIT training. These include:

✓ Replenishment of adenosine triphosphate/creatine phosphate resynthesis.
Lactate removal.
Increased body temperature.
Increased metabolism.
Increased muscle protein turnover, lipolsis and circulatory ventilation (Borsheim, et al. 2003).

Total energy expenditure is the underlying factor for inducing fat loss (Slentz, Duscha, Johnson., Ketchum, Aiken, Samsaet 2004). Higher intensities have been shown to lead to a more sustained increase in circulation, ventilation and body temperature.

One of, if not the most significant factor of a prolonged EPOC effect is the increased rate of triglyceride cycling, meaning the body shifts from carbohydrates to fat as the main substrate post exercise (Borsheim et al. 2003).

…What does all this mean?

The more sustained EPOC experienced during HIIT allows for more calories to be burnt at rest and whilst recovering, leading to an increased Resting Metabolic Rate (RER). This enhanced fat burning effect induced by HIIT has resulted in participants reporting, lowest wasit/hip ratios, lowest waist circumferences and lowest body fat percentage (Macpherson, Hazell, Olver, Paterson, Lemon 2011).

This research shows that HIIT is an extremely efficient method of fat burning. One of the many benefits of burning fat, specifically visceral fat, is the reduced chance of cardiovascular reated disease Sironi, Petz, De Marchi, Buzzigoli, Ciociaro & Positano 2012).

A study by Wisløff, Nilsen, Drøyvold, Mørkved, Slørdahl & Vatten (2006) directly examined this and found that a single, weekly 22.5 minute bout of high intensity exercise can to help increase general health and to help achieve exercise induced protection against premature cardiovascular mortality.

Further HIIT Studies 

As can be seen during the analysis of several different experiments, the use of HIIT has a considerable amount of benefits that can be utilised with in a training regime. One of the main reasons why HIIT is so effective is the fact that similar adaptations are witnessed, by using a much lower duration of training!

hiit training

One example of this is through the study by Hazell, Olver & Craig (2012) when it was showed that just two minutes of sprint interval exercise elicits similar oxygen consumption post exercise as 30-minutes continuous endurance exercise!

This is a benefit as post-exercise metabolism is increased to the same amount when performing HIIT exercise for less than 10% of the time of endurance training. Exercising less and burning more calories whilst resting! Isnt that what everyone wants? That’s correct and its the reason why people lose more body fat and are able to control their weight more effectively.

Its not necessarily about how many calories you burn in your 1 hour training session, its the intensity of the session that determines how many you burn during the other 23 hours of the day that truly matters the most.

The studies analysed throughout this paper have clearly shown there is a huge benefit to the inclusion of HIIT in a training program, it can help to improve performance, has a positive effect on body composition, mainly by increasing the bodies ability to utilise fat as an energy store and by sufficiently inducing EPOC.

Take Home Message 

Although all of these adaptations have been induced more quickly by HIIT, continuous and endurance training, can still play an important role within a training program for recreational exercisers and professional athletes.

Hoffman Reed, Leiting, Chiang & Sone (2014) recommend that to achieve the best possible physiological adaptations and improve fitness levels, then both HIIT and CT should be programmed together.


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