Get Fit With HIIT? The Science of High Intensity Interval Training

Have you heard about HIIT (high intensity interval training)? Do you know that it can get you fit in half the time? Sounds too good to be true, right?

Well actually, it’s true!! The science says so. But there is a lot more to it than just that so read on for the scientific lowdown.


HIIT (high intensity interval training) is working out at multiple intervals where a short intense burst of activity is followed by a rest period. It can be undertaken as part of any exercise, but is commonly associated with running or cycling. It requires pushing yourself during these short intense activities which elicits the highest rate of oxygen consumption (VO2 max). An interval will consist of a short intense burst of exercise that can be anything between 10 seconds to 1 minute followed by a longer period of rest. This one interval is then repeated anywhere from 4 or more times.

The reason why it is so efficient and causing such a storm is that it brings a range of benefits (similar to the steady and continuous exercise, and then some!) but only lasting for a short amount of time. Think of a routine where one interval includes 30 seconds of high intense exercise followed by 4 minutes recovery repeated 4 times, giving you a total of 18 minutes. So if you use lack of time as an excuse to not get exercising, sorry, but you can’t use that excuse anymore!


The benefits are pretty extensive. It improves the cardiovascular system, it lowers blood pressure (with more than 12 weeks of HIIT) and improves high density lipoprotein (with more than 8 weeks of HIIT) (The Conversation). One study by Metcalfe et al 2012 measured the sensitivity to glucose and insulin after 6 weeks of doing HIIT (3 times a week at 10 minutes) and found that both glucose and insulin sensitivity had been improved. HIIT increases aerobic capacity (as measured by peak oxygen uptake VO2) and running speed as a 2014 study found out (6 weeks of 3 times of HIIT per week).

The one thing that will appeal to many people is that it reduces fat. Trappe et al 2008 found significant reductions in body fat both in total, abdominal and subcutaneous fat. They conducted their study on women who undertook the HIIT regime for 20 minutes, 3 times a week for 15 weeks. Compared to steady state and continuous (which all of these studies compare with), these women lost 11.2% of mean fat mass. Before I lose some people to this thought and fantasy, I did notice that nearly all the studies I looked at used subjects that stuck out this exercise regime for more than 6 weeks. So there is a considerable amount of dedication involved in doing HIIT. Additionally, it was also found that women who had more body fat to begin with were the ones that lost the most, and the authors do argue against fat spot reduction which is trying to reduce body fat in one area of your body specifically.

One of the most interesting benefits of HIIT I found was it increases the capacity of skeletal muscle function. While doing steady state and continuous exercise can actually cause muscle loss, HIIT instead retains it. It activates a specific type of muscle fibre to promote muscle growth (Wilson et al 2012). Both forms of exercise have similar consequences on a biochemical level that affect the muscles: they both increase mitochondrial enzymes to affect metabolism, specifically fat and carbohydrate oxidation. However two studies found the HIIT technique was more efficient at this than steady and continuous exercise (Burgomaster et al 2008 and Perry et al 2008). This is what metabolic training does: it increases oxidation and muscle mass. HIIT is said is increase mitochondrial capacity and the amount of mitochondria you produce therefore it is changing your metabolism (BioLayne Foundation) even afterwards when you are not working out (Treuth et al 1996).


This is of course the point where I mention limitations and the biggest one is that you will not lose weight insofar that HIIT does reduce fat but muscle capacity may increase as you get stronger. As a consequence, you may drop a size but your weight may stay the same or go up, and this is because muscle weighs more than fat (Science News). Ultimately this is a hard work out and is not suitable for everyone as it can cause injury if not done properly. It can be impractical and intolerable, as it requires strenuous activity (The Conversation; BioLayne Foundation). I verify that from personal experience! One study has actually found that the benefits of HIIT can be offset by a decreasing will-power over a routine that causes discomfort (Foster et al 2015). It is ironic that while HIIT may be more suited to those who are well trained (The Conversation), it can actually be more beneficial for those who are not so fit (Science News).

The important thing is that HIIT might not be suitable for everyone for a variety of reasons (orthopaedic, cardiac or psychological). If you do wish to start doing it, be aware that everyone has different tolerance levels and the strenuous intense activity should be relative to you; only do what you feel comfortable with. The best conclusion based on the literature reviews that I have come across is that while HIIT is a superior form of exercise, there are benefits to the steady and continuous exercise. Both should be incorporated into a routine and don’t do HIIT 4 or more times in a week as it can have a detrimental impact (BioLayne Foundation). Doing steady and continuous exercise can also benefit your brain more than HIIT as it appears that this exercise promotes brain cell growth and has a positive effect on brain structure and function (New York Times). Most studies have been undertaken on participants who have done HIIT only 3 times a week, so that makes it ideal to leave other days for steady and continuous exercise. Anyone for kickboxing?


Ultimately, you need to work out what exercise is good for you and supplement it with a healthy diet. Don’t do anything unless you have spoken to your GP to find out if HIIT is suitable for you, and do speak to your health instructor at your local gym for more information. So stay healthy everyone! But don’t forget to treat yourself occasionally.


Danae Dodge

I received my PhD in Scientific Archaeology from the University of Sheffield in 2011 which specialised in ancient DNA and anthropology. For my profile, see my websites: I started getting involved in Science Brainwaves as a volunteer in 2010. I have volunteered at presentations, events (such as the British Science Festival in 2011) and even participated in the Science is Vital protest march in October 2010. My first blog for Science Brainwaves was "Ancient Humans: Who were they? And who got it on?" which was the written version of a talk I gave for the Natural History Society at the University of Sheffield on 5 December 2011. I also have a public engagement page dedicated to ancient DNA, which I encourage both the public and specialists to join: