HIIT is often put on a pedestal in the fitness community as being a superior way to keep fit. However like any type of exercise, it's not magical, and in fact there are problems and mistakes you might be making with this type of training which it's important to be aware of if you want to get the most out of your workout routine.
What is HIIT?
HIIT stands for high-intensity interval training. This involves alternating periods of high-intensity exercise with rest periods (Kravitz, 2014). Versions of HIIT training have been used for over 100 years, and can be traced back to the Finnish Olympic Gold medallist Hannes Kolehmainen in 1910, who supplemented his middle-distance running with 150m sprints at 100% effort (Magness, 2016). However, HIIT workouts have become increasingly popular in the past few years thanks to the likes of Joe Wicks and other influencers who offer follow- along workouts on Youtube and Instagram. I can see the appeal of HIIT – you can complete a vigorous cardiovascular workout in 15-30 minutes with minimal equipment or just bodyweight. This also explains why HIIT workouts have been so popular over the past year and a half of lockdowns, a time when the majority of people have not had access to high-tech gym equipment, and why HIIT is predicted to be the fifth most popular fitness trend worldwide in 2021, according to the American College of Sports Medicine (Thompson, 2020). So what’s not to love?
Most people aren’t doing it right
There are a number of different HIIT protocols that have been used by exercise scientists, but they tend to involve between 20-60 seconds of work at 85%-95% maximum heart rate, typically on a stationary bike or treadmill, and 10-75 seconds of rest between each interval, with the duration of rest intervals generally increasing in line with the work intervals (Tabata et al., 1996; Little et al., 2010; Zuniga et al., 2011).
However, in my experience a lot of HIIT workouts and classes set their work periods towards the higher end of this spectrum (e.g. 45 seconds), yet keep their rest periods towards the lower end (e.g. 15 seconds). This is not optimal, because the longer you’re working at a high intensity the longer you’re going to need to recover from it. I think this may partly be explained by the intensity at which people are completing their intervals; if you were really working at 90% of your maximum heart rate for 45 seconds you’d need at least the same time again to recover, if not longer. This suggests to me that people are generally not working at a high-enough intensity when they do HIIT. I think the exercise selection has a big part to play here; I don’t know about you but jumping jacks are never going to make me gasp for breath as much as sprinting does. Most lab-based HIIT research has been carried out using cycling and running sprints, whereas popular HIIT workouts in the media tend to favour bodyweight-exercises such as jumping jacks, squats and crunches. There’s nothing wrong with bodyweight exercises, but often the ones that are chosen are not particularly high-intensity. I think sometimes workouts that claim to be HIIT workouts are actually more like circuit training (which is certainly not a bad way of training, we just need to stop calling it HIIT!).
It’s not meant to be done every day
Another common mistake I see being made with HIIT is that many people do it every day and rely on it as their only form of exercise. However, HIIT was not designed to be done daily - the American College of Sports Medicine recommends doing no more than two HIIT sessions per week (Kravitz, 2014). HIIT is taxing on the body if you’re doing it properly, and therefore requires sufficient rest days to recover from. It puts a lot of stress on your nervous system, muscles and joints, and during high-intensity exercise the stress hormone cortisol is released. If cortisol is released too often for too long it can cause adverse effects such as fatigue, impaired performance, low mood and increased risk of injury and illness. HIIT is especially hard on your body if you’re not used to exercising regularly, you’re injured or you’re pregnant. If any of those apply to you, then I'd strongly advise against doing HIIT – you’d be far safer opting for a lower-intensity form of cardio, such as swimming or walking.
It's not the best exercise for skeletal health
As we age we naturally lose bone density and muscle mass, and the rate at which this occurs becomes accelerated in women around the time of the menopause. This is damaging because if our bones are weak we’re more prone to developing osteoporosis and getting fractures if we fall. This is why it’s so important to do resistance training (i.e. working against resistance to strengthen muscles and bones) to minimise the risk of these age-related problems. Although HIIT sometimes incorporates exercises using light resistance such as bodyweight, HIIT (when done correctly) is not considered resistance training, it is cardiovascular training, because it primarily works the heart and lungs rather than the major muscle groups. Cardiovascular exercise is essential for overall health, but it shouldn’t be the only form of exercise you do. It’s important to combine it with some form of resistance training, ideally twice per week – this could be lifting weights in a gym but it could also be gardening or yoga. Try not to do HIIT at the expense of resistance training, otherwise you’ll risk losing strength and becoming injured as you age.
It’s ‘afterburn’ effects are overstated
Many people are drawn to HIIT workouts because they’re led to believe that they’ll burn loads of extra calories once they’ve finished exercising, thanks to HIIT’s excess post-oxygen consumption (EPOC) effect, which is the idea that your metabolism gets revved for a couple of hours after doing high-intensity exercise. But in fact this effect is negligible. Research shows that although there is a slight afterburn from HIIT compared to doing lower-intensity cardio, such as walking, it only equates to an extra 6-15% of overall energy expenditure (Laforgia et al., 1997). So if you burn 300 calories during your HIIT workout, that’s between 18 and 45 extra calories over the next few hours (a couple of bites of banana). Doesn’t sound so enticing now, right?
My final thoughts
If you enjoy doing HIIT workouts then there's no need to stop - I'm all for doing exercise you enjoy! Just make sure that you’re doing it correctly and not overdoing it - you need to work hard and rest hard, and limit these types of training sessions to no more than two per week. But if you don’t love HIIT then you absolutely don't need to be doing it – you can get the same cardiovascular health benefits from a lower intensity bike ride, walk, run, swim, or however else you like to move your body. And either way, make sure that you’re supplementing your cardiovascular exercise with some resistance training at least twice per week to support skeletal health and prevent injuries.
Kravitz, L. (2014). High-intensity interval training. ACSM. Retrieved from: https://www.acsm.org/docs/default-source/files-for-resource-library/high-intensity-interval-training.pdf?sfvrsn=b0f72be6_2
Laforgia, J., Withers, R.T., Shipp, N.J., & Gore, C.J. (1997). Comparison of energy expenditure elevations after submaximal and supramaximal running. Journal of Applied Physiology, 82(2), 661-6. doi: 10.1152/jappl.1918.104.22.1681. PMID: 9049750.
Little, J.P., Safdar, A., Wilkin, G.P., Tarnopolsky, M.A., & Gibala, M.J. (2010). A practical model of low-volume high-intensity interval training induces mitochondrial biogenesis in human skeletal muscle: Potential mechanisms. The Journal of Physiology, 588(6), 1011–1022. https://doi.org/10.1113/jphysiol.2009.181743
Magness, S. (2016). A brief history of interval training: The 1800’s to now. Science of Running. Retrieved from: https://www.scienceofrunning.com/2016/08/a-brief-history-of-interval-training-the-1800s-to-now.html?v=47e5dceea252
Tabata, I., Nishimura, K., Kouzaki, M., Hirai, Y., Ogita, F., Miyachi, M., & Yamamoto, K. (1996). Effects of moderate-intensity endurance and high-intensity intermittent training on anaerobic capacity and VO2max. Medicine and Science in Sports and Exercise, 28(10), 1327-30. doi: 10.1097/00005768-199610000-00018. PMID: 8897392.
Thompson, W.R. (2020). Worldwide survey of fitness trends for 2021. ACSM’s Health and Fitness Journal, 25(1):10–19.
Zuniga, J.M., Berg, K., Noble, J., Harder, J., Chaffin, M.E., & Hanumanthu, V.S. (2011). Physiological responses during interval training with different intensities and duration of exercise. Journal of Strength and Conditioning Research, 25(5), 1279-84. doi: 10.1519/JSC.0b013e3181d681b6. PMID: 21522072.