There is currently a big trend in the fitness industry towards “high-intensity interval training” (HIIT) as the favourite form of cardiovascular exercise. I must say, it makes sense for many people – you can get a lot done in little time, and you can do so many things with it that it’s pretty difficult to get bored. Whether you are using it in a traditional cardiovascular activity such as cycling or running, or applying it in the gym with circuit training, it makes things interesting.
However, I want to discuss a few points about the claims and comparisons made about HIIT. While HIIT is incredibly effective in its own way, and holds some benefits that “steady-state” cardio does not, there are some misconceptions about other forms of cardio, the rest-to-work ratios that people are capable of physiologically, and the importance of people’s personal preferences. But fear not – I am also going to discuss why HIIT is also a great way of exercising, so hang on.
1) What do people mean by “steady-state” cardio?
One problem with comparing HIIT to other forms of cardio is that people tend to talk about cardio as if it’s a dichotomy, of HIIT versus “steady-state”. This is a problem, because “steady-state” could be used to describe a gentle jog, or a sustained 30-minute run done at lactate threshold/tempo pace, both of which are very different. It isn’t a case of just doing HIIT OR gently jogging. There are other methods in between that include sustained efforts, but will make you feel on the verge of passing out as if you have just done some HIIT.
For example, I do tempo runs of 5k, which take me 17 minutes, hopefully under in a good race. My heart rate is constantly high throughout, and I’m pushing hard from the start. The same happens in a 10k race, which is double the distance, over 30 minutes. I am running right on the brink of my lactate threshold, and it takes speed to be able to run at that pace.
Now don’t get me wrong, you do need a very good endurance base to be able to run at near max heart rate for a sustained period of time. However, everyone is capable of running at a high level of effort for a sustained period. So just because something isn’t split into high-intensity intervals doesn’t mean it isn’t going to have significant effects on your fitness or metabolism.
I feel that when people refer to “steady-state” cardio, they are really referring to easy running, or gentle jogging, maybe fast walking. Indeed, these lower intensities have different physiological effects on the body as they are entirely aerobic, and build endurance instead of anything else.
2) Work-to-rest ratios – pacing yourself
HIIT consists of set work intervals e.g. 30 seconds, followed by rest intervals. If arranged correctly in the right ratios with the right types of exercise, you can achieve very high cardiovascular intensities while activating all of your muscles for a strength-building effect. For circuits, you generally want to be able to do a movement for long enough to seriously tire yourself out, but not so long that your form deteriorates and you can no longer do it properly. It’s a fine balance. If you are doing different exercises one after the other that involve different muscle combinations, you can afford to have very little rest, if not no rest, and do them all-out.
However, I often see recommendations of work-to-rest intervals that simply do not work physiologically for some activities. For example, I have seen people recommending HIIT intervals involving running that involve 10 rounds of flat-out sprinting for 30-40 seconds, followed by the same time in rest.
Now, I know that some people are not aiming for maximum speed and are more looking to completely knacker themselves. But it is not possible to sprint flat-out 10 times for 30/40 seconds with that little rest. You can certainly try and give it your all, but you will very quickly slow down, lose all form and not achieve the same muscle activation, if that’s what you are looking for. The people making the recommendations need to be clear about what they mean by “sprinting”.
The whole point in doing something several times over (i.e. 10 times) is that you get progressively more tired over the duration of the session. In addition, the whole point in rests is that you can recover just enough to perform the same thing again. If you are doing that kind of work-to-rest ratio, you need to pace yourself just a little. Using HIIT in running is very different to circuit exercises such as jump squats, press-ups and so on. You can only do squat jumps at a certain intensity, whereas flat-out sprinting is completely different. To be able to replicate a flat-out 200m sprint time, you need 8 minutes or more to replenish energy stores such as ATP and creatine-phosphate, whereas the same cannot be said for jump squats.
Where that session COULD work is with something slightly less intense that sprinting. But hold up, that doesn’t mean you won’t reach an incredibly high intensity. Doing something like 10 x one minute on, one minute off would work very well if you ran at 8/10 effort (10 being flat-out and 1 walking). This is something I often do at 1500m pace, and after that I am not prepared to do any more exercise!
3) Calorie burning and excess post-exercise oxygen consumption (EPOC)
One claim made about HIIT is that the intense nature of it causes your metabolic rate to be significantly raised for 24-48 hours afterwards, and therefore it is more effective for weight loss than other cardio. The first part of that sentence is true – when you exercise at an intense level, you put your body into “oxygen debt”, which means that after you have finished, you will be consuming significantly more oxygen for a while afterwards as your body returns to its normal resting state.
However, does that mean it’s necessarily the best way to lose weight?
It depends. Research will vouch for the fact that exercising at high intensities results in higher metabolic activity for a period of time afterwards, meaning you burn more calories than you might after doing something less intense. But to leave it at that would be to dismiss a lot of contingencies.
The amount of calories you “burn” each day amounts from several things: your resting metabolic rate, exercise activity, non-exercise activity (e.g. day-to-day tasks/movements) and the thermic effect of eating (how much energy you use digesting the food you eat). While you can burn hundreds of calories through exercise, the post-exercise thermic effect of exercise is relatively quite small in comparison to your total daily energy expenditure, and also to the total calories you burn doing the exercise (1). So while it is true that high-intensity exercise causes greater metabolic activity afterwards, it isn’t as significant as people make it out to be.
The main point is that you burn more calories in total by doing 20 minutes of HIIT compared to 20 minutes of running non-stop at a less intense pace, due to work intensity and EPOC. In other words, relative energy expenditure is higher per minute overall. HOWEVER, you will likely burn more calories in TOTAL by exercising for longer a slightly lower intensity, e.g. a 30-minute run at a reasonable pace. NB: that’s just a guess, I haven’t looked into research about exact cross-over points yet, but you get the idea.
However, this is not to say you can amble along and expect to achieve the same fitness gains – you still have to be working hard. The magnitude of EPOC depends on both duration and intensity (2), and the relationships are different.
One point that makes the EPOC aspect potentially irrelevant is personal preferences. A lot of the time, people will be more inclined to do exercise that they find more enjoyable in some way. If doing slightly lower-intensity cardio for longer is more enjoyable to a person, and hence they do more of it, they are more likely to a) sustain it and b) lose weight. I enjoy both, so I do both, and doing what you enjoy is the less stressful and more sustainable approach.
4) Why HIIT is good, and why that doesn’t mean it’s the solution for everyone
Now I have finished my ranting, I want to talk about why HIIT is great, but also where individual differences come in.
- HIIT can make a bodyweight workout very hard. Explosive bodyweight moves can build strength and stamina that transfer to most sporting activities (including endurance) and weight-lifting.
- HIIT can provide a very hard workout in a short space of time.
- HIIT is superior for building muscle: the more vigorously you move your limbs, the more muscle activation you achieve, and hence the more your body will react.
- HIIT can hold your attention very well – you can cram lots of different exciting exercises in, which keeps you on the ball.
- In endurance sports, it builds speed and lactate tolerance: for example, running intervals at short- and middle-distance paces will in turn improve your performance in long distances.
I really see the benefits of HIIT in circuit-training and running. The speed and speed-endurance sessions have boosted my long-distance times a lot, and the circuit-training has increased my strength in my weight-training activities. I also love jumping around and love trying new variations of exercises. Plus I love to sweat!
HOWEVER… just because something is the best way for you and you really enjoy it, doesn’t mean that other people should do the same. For example, keeping fit just to stay healthy does not require you to get up to such high intensities. There are various ways of doing things, and what you should do is up to YOU. Fit it around your lifestyle, what you enjoy, and what you are trying to achieve.
- 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, 661-666.
- Børsheim, E., & Bahr, R. (2003). Effect of exercise intensity, duration and mode on post-exercise oxygen consumption. Sport Medicine, 33, 1037-1060.