If you aren’t new to the exercise and fitness space, then you’ll undoubtedly have heard of high-intensity interval training, conveniently abbreviated to HIIT. But what does it consist of exactly, and should you consider including it in your exercise regimen?

What is HIIT?

When we talk about HIIT, we’re generally referring to a 30-minute-or-so session involving short bursts of high-intensity exercise (10 seconds to 5 minutes), followed by periods of recovery – either lower-intensity training or rest (Laursen & Jenkins, 2002). These sessions could be aerobic, like running or cycling, or resistance-based, where high-repetition resistance exercises are performed (Kilpatrick et al., 2014).

The idea is to stress the body’s physiological systems enough, within a short space of time, to trigger health improvements that would otherwise require longer sessions of continuous exercise. It’s touted as a time-efficient way of exercising that offers significant health benefits, and so, it seems ideal for the busy individual who ‘doesn’t have time to exercise’.

The Debate

This argument, however, has been questioned. It appears HIIT doesn’t save all that much time in the long run – and really, not having time reveals more about one’s priorities than how busy one actually is. The truth is, we all have enough time to meet the physical activity recommendations if we choose to, but if we don’t prioritise exercise, it can feel as though reaching those targets takes an inordinate amount of time (Biddle & Batterham, 2015).

Another aspect of the HIIT description above might have stood out to you – it certainly did to me! As anyone who’s done exercise knows, 10 seconds of activity feels markedly different from 5 minutes, and herein lies another part of the debate around HIIT: what exactly is it?

There is currently no better or more specific definition than the one I’ve outlined, and this variety in definition fuels debate over whether HIIT is feasible for the general public – i.e. you and me – as opposed to trained athletes (apologies if you are indeed an elite athlete).

This is because exercising at very high intensities (typically for just a few seconds at a time, but in repeated bouts) may prove extremely uncomfortable for most people, and could deter them from returning to exercise in the future. This would, of course, frustrate the very purpose of HIIT: to get more people moving (Biddle & Batterham, 2015).

The efficacy of a training intervention lies, in part, in whether it is enjoyable enough to be repeated. Some argue that adherence to HIIT is higher than to continuous training, as it allows you to complete your session more quickly, and varying the intensity within a single workout may be less boring than maintaining one steady pace. Study participants also seem to enjoy it relative to other forms of training (Biddle & Batterham, 2015; Reljic et al., 2019).

On the other hand, some claim that it can’t be considered suitable for most people. They argue that participants report enjoying it only because they’re asked after the session – by which point the main discomfort has already passed. Their ‘enjoyment’ may be more accurately described as a sense of accomplishment, which doesn’t necessarily indicate a willingness to adopt this training style long-term (Biddle & Batterham, 2015).

Benefits of HIIT

What’s not up for debate, however, is whether HIIT is effective for improving health. The evidence overwhelmingly supports it as a powerful strategy for enhancing both physical and mental health (Edwards et al., 2023; Martland et al., 2021).

It’s been shown that HIIT can significantly improve cardiovascular (including endothelial function) and respiratory fitness. It can also enhance metabolic health markers, such as blood sugar (via improved insulin sensitivity) and cholesterol levels (Edwards et al., 2023). In fact, these benefits are often comparable to, if not greater than, those achieved through moderate-intensity continuous training (Reljic et al., 2019).

Regarding mental health, Martland et al. (2021) compiled data from randomised controlled trials exploring HIIT and mental wellbeing. They found that HIIT appears to reduce depression severity and stress levels, although it’s worth mentioning that it didn’t seem to affect anxiety.

Is it Safe for Beginners?

This brings me to one final question mark I hold: is HIIT a safe modality for beginners or for people who are currently inactive?

A paper I discussed in a previous post, regarding guidelines for starting to exercise, suggests that it’s better to begin with light- to moderate-intensity activity if you’re just starting out. That said, many studies have successfully trialled HIIT in previously sedentary populations (including some of the studies I referenced here), and even in individuals with chronic illness. These suggest it is, indeed, a safe exercise modality.

Kilpatrick et al. (2014), who I believe provide a very balanced view of HIIT, recommend that it only be performed by individuals who are either low-risk, or moderate-risk but have been cleared for vigorous-intensity exercise, or high-risk but under the direct supervision of a medical professional.

Final Thoughts

As always, I would recommend consulting with a healthcare professional before starting any new form of exercise. I’d also suggest reaching out to a trainer if you’re interested in trying HIIT, as they can help design a programme suitable for your fitness level.

If it’s safe for you to try, then there’s nothing stopping you – especially if you haven’t enjoyed continuous exercise at one intensity in the past and are looking for something new. If you enjoy it, HIIT is a brilliant way to train. The benefits it offers are truly impressive, especially given the short duration spent exercising.

If you try it and don’t enjoy it, don’t be disheartened, and certainly don’t let it stop you from exploring other types of physical activity. There’s something out there for everyone. The most important thing is to find what works for you and stick with it, because long-term benefits come from long-term commitment.

References

Biddle, S. J. H., & Batterham, A. M. (2015). High-intensity interval exercise training for public health: a big HIT or shall we HIT it on the head? International Journal of Behavioral Nutrition and Physical Activity, 12(1). https://doi.org/10.1186/s12966-015-0254-9

Edwards, J., Griffiths, M., Algis H P Deenmamode, & O’Driscoll, J. M. (2023). High-Intensity Interval Training and Cardiometabolic Health in the General Population: A Systematic Review and Meta-Analysis of Randomised Controlled Trials. Sports Medicine, 53(9), 1753–1763. https://doi.org/10.1007/s40279-023-01863-8

Kilpatrick, M. W., Jung, M. E., & Little, J. P. (2014). HIGH-INTENSITY INTERVAL TRAINING: A Review of Physiological and Psychological Responses. ACSM’s Health & Fitness Journal, 18(5), 11–16. https://doi.org/10.1249/FIT.0000000000000067

Laursen, P. B., & Jenkins, D. G. (2002). The Scientific Basis for High-Intensity Interval Training. Sports Medicine, 32(1), 53–73. https://doi.org/10.2165/00007256-200232010-00003

Martland, R., Korman, N., Firth, J., Vancampfort, D., Thompson, T., & Stubbs, B. (2021). Can high-intensity interval training improve mental health outcomes in the general population and those with physical illnesses? A systematic review and meta-analysis. British Journal of Sports Medicine, 56(5), bjsports-2021-103984. https://doi.org/10.1136/bjsports-2021-103984

Reljic, D., Lampe, D., Wolf, F., Zopf, Y., Herrmann, H. J., & Fischer, J. (2019). Prevalence and predictors of dropout from high‐intensity interval training in sedentary individuals: A meta‐analysis. Scandinavian Journal of Medicine & Science in Sports, 29(9), 1288–1304. https://doi.org/10.1111/sms.13452