High Intensity Interval Training (HIIT)- What is it?

Written by: Sebastian Mann Bach. Exercise and Sport Science (current)

HIIT – understanding the physiology.

You may be interested incorporating HIIT (High Intensity Interval Training) into your exercise routine but you might not know the science or purpose behind it. Many people wonder what the point of HIIT is and how it differs from traditional endurance training. HIIT- is an abbreviation that refers to High Intensity Interval Training.

HIIT involves short bouts of exercise intensities at a level above what is called Steady State (ST) with the goal to improve the body’s ability to work without oxygen. Steady state exercise is the idea that exercise is being performed at intensity where oxygen consumption is consistently meeting the body’s oxygen demands. Steady state can be maintained at a wide range of intensities from rest up to 70-85% maximum heart rate where oxygen intake through breathing can no longer meet the body’s demands. This maximal ST is also known as the lactate threshold (LT) where lactate removal can no longer keep up with lactate accumulation and we begin to notice a rapid increase in muscle fatigue.

It is from these known physiological states during exercise that HIIT programs where originally designed.  With efforts of training above ST and LT the goal is to improve maximal lactate accumulation (the ability to exercise under accumulated muscular fatigue). The efforts during HIIT which are below LT/ST the goal is to improve how fast the body can recover from accumulated fatigue.

In contrast, when we undertake traditional endurance training the goal is to improve the body’s ability to take up and use oxygen so that a steady state can be maintained for a longer period of time.  With consistent endurance training the goal is to achieve this steady state at higher intensities of exercise before reaching your lactate threshold.  To achieve this you need to work at or just above the LT/ST point.


Examples of HIIT versus Endurance Training

To put this into an example, let’s say if an individual with a max heart rate of 198 bpm had a lactate threshold at 148bpm and running speed of 12km/h. Assuming their HIIT program would involve running, their program would consist of short efforts running at 12km/h whilst maintain a heart rate between 148bpm and 198bpm which would be considered higher than their LT/ST. Each effort would then be alternated with another of lower intensity below 148bpm (known to be their LT/ST) The idea behind this is that you’re working the body past the lactate threshold during these high intensity efforts to improve its maximal lactate accumulation point (total amount of fatigue it can handle) and training it to improve its ability to remove fatiguing by-product of exercise through the periods of work below the LT.

When we train for endurance however. The goals of the training are completely different. If we are to use this same example to look at the differences in an endurance training program the program would involve extended durations of effort at or slightly above the individuals LT/ST heart rate of 148bpm for extended periods of time. The idea of endurance training is to train at intensity just above the individuals lactate threshold, doing this over consistent training sessions will improve/increase the time taken to actually reach LT as well as increase the LT point which in turn delays the onset of fatigue as well as how much fatigue the body can handle during ST exercise.


Is HIIT better for weight loss?

Not only is there a difference in performance goals between HIIT and endurance training, but there is a wide range of research that suggest that HIIT training is actually better for weight / fat loss. Research shows that although endurance training does show the greatest amount of fat loss during the training session, the amount of extra oxygen consumption consumed post HIIT to recover causes an even greater increase in metabolism (calorie consumption) over endurance training which in turn allows for greater weight loss(1) .


Our advice for your training is to include HIIT into your weekly exercise schedule.  Due to the high intensity nature of this type of training it is important that you introduce this gradually into your training routine.  If your goal is to improve performance in a sporting pursuit; or if you’re interested in losing weight to improve your health and wellbeing; we strongly recommend adding HIIT to your training.  We do however recommend being set up correctly by an Exercise Physiologist or Personal Trainer so you introduce the right loads and intensity that’s right for your body.



  1. Townsend J, Stout J, Morton A, Jajtner A, Gonzalez A, Wells A et al. EXCESS POST-EXERCISE OXYGEN CONSUMPTION (EPOC) FOLLOWING MULTIPLE EFFORT SPRINT AND MODERATE AEROBIC EXERCISE [Internet]. Hrcak.srce.hr. 2017 [cited 26 June 2017]. Available from: http://hrcak.srce.hr/index.php?show=clanak&id_clanak_jezik=153667