High intensity interval training, more commonly known as HIIT training, is one of the most effective training forms for losing body fat. HIIT training basically involves short bursts of high intensity activity, alternated with longer intervals of low intensity activity. The key to HIIT training is to keep your heart rate between 90 and 150 beats per minute, meaning you should try to keep your heart rate above 90 beats per minute during the recovery phase and not exceed 150 during the working phase (as a rule of thumb). The theory is this, if you increase the heart rate then the physiological response increases promoting fat loss, similar to how you expend more petrol when speeding in a car…but it’s a bit more complicated than this.
The science behind HIIT training
The exact effect and extent of HIIT’s fat burning potential depends heavily on the type of HIIT training performed, the duration of each high intensity bout as well as the rest period. Tipton and Luc van Loon state that numerous HIIT protocols (usually running or cycling) have been seen to enhance the bodies ability to burn fat both with and without the presence of oxygen. Evidence suggests that just 2-3 HIIT sessions a week can significantly increase the level of mitochondrial enzymes (energy breakdown systems), alter substrate metabolism (breakdown) to fat, and improve buffering capacity so that muscles can work for longer. Although the mechanisms of how HIIT training prompts fat loss are not absolutely clear, it is thought that the intermittent nature of HIIT training necessitates the person to have a good balance between aerobic (oxidative) and anaerobic (non-oxidative) energy systems. The main energy substrate during HIIT is glycogen, but Krustrup, Mohr and Steensberg, (2006) noticed that most or all of the muscle glycogen stores had been depleted after a bout of HIIT, meaning the next energy source in line would have to have been fat! The short bursts of activity are again likened to a car being driven at high speed, with sudden bursts of acceleration. These highly intense bouts of activity result in the rapid release of glycogen for energy, as a result your body has to resort to burning fat for energy sooner than if bouts of lower intensity activity were performed.
Should you try HIIT training?
This depends on a number of factors, but chiefly you need to consider your goals. It would make little sense for a marathon runner to perform HIIT training exclusively because they would never reach their anaerobic potential or glycogen limit, meaning appropriate adaptation would be unlikely to occur. A marathon runner is just an example of who might not perform regular HIIT training cycles, but many people may benefit from it, these might include athletes who participate in team sports such as football, rugby or the likes of. The explosive periods in Crossfit, a new training methodology that incorporates high power, explosive moves such as box jumps, kipping pull ups (an faster, adapted form of a regular pull up) and thrusters (front squat with a rapid transition to an overhead press) will also benefit from HIIT training…in fact, the nature of Crossfit is not dissimilar to HIIT training itself!
If you are in need of losing a few pounds then you may find HIIT training effective, not only does it promote fat burning through the rapid rate at which it uses up stored glucose, it’s start/stop nature also makes it good for people who might struggle to sustain long, arduous runs. HIIT training is safe (if performed correctly in otherwise healthy individuals), dynamic, and a useful way to start shifting that stubborn layer of fat…give it a try and see how you get on.
Krustrup, P., Mohr, M., Steensberg, A. et al. (2006). Muscle and blood metabolites during a soccer game: implications for sprint performance.Med Sci Sports Exerc. 38: 1165-1174
Tipton, K, D & Luc van Loon, J. C. (2013). Nutritional coaching strategy to modulate training efficiency. Basel: Karger.