How hard should I push when I do High-Intensity Interval Training (HIIT)?
Everyone knows about High-Intensity Interval Training (HIIT) these days, but it can still be a little confusing or intimidating when you’re just starting out. You know you're supposed to go hard, but how hard is too hard, how long is too long, and how often is too often? The industry doesn't do a great job of explaining all this, so let's give ourselves a framework for thinking about these questions.
Let's say you’ve got a hectic full-time job, haven’t done much exercise in the last couple of years, and now you're looking to do some HIIT. Surely there’s a difference between what you should do and what an experienced athlete should do, right? Surely there's more to it than that aspirational quote you just saw on Instagram, saying something like, “Train like an athlete to look like an athlete,” or, “Unleash the athlete in you”?
This isn’t a knock on those types of posts—on days when I’m struggling, they might get me going, too, and they’re probably not actually harmful if the goal is to just get you off the couch. But let’s dive a little deeper.
If you look up any definition of ‘athlete’, you’re not going to come up with ‘everyone’. Athletes dedicate their lives to pursuing perfection and optimizing performance, and a pole vaulter trains very differently to the way a tennis player does. With years of conditioning under their belt, athletes can withstand enormous amounts of intensity compared to the rest of us.
So no, most of us shouldn’t train like athletes, because we have neither the level of specific development nor the time to dedicate to recovery, nutrition, sleep, etc. So let’s move away from the mindset of wanting to smash ourselves all the time (the best athletes don’t actually do this, by the way), and look for a more rational, sustainable approach.
Okay, so how hard should you push? And how often?
As we've established, there's no definitive answer to this, as everyone is different. So to help you work out what's best for you, we're going to look at two important exercise principles which can function as guiding lights for all the exercise you choose to do from this point on. Bear with us.
Principle #1: There’s a sweet spot to the afterburn effect
Known in the literature as Excess Post-exercise Oxygen Consumption (don’t worry, you don’t need to remember that to get results), the afterburn effect is one of the key reasons why HIIT is so effective. By pushing yourself just a little bit harder than your body is comfortable handling, you create a disruption in your physiology. This disruption creates a little bit of oxygen debt, and sometimes some mechanical stress.
This may sound on the surface like a bad thing, but don't worry. This oxygen debt isn’t at all enough to be dangerous, but it is just offensive enough for your body to have to take notice. So as you go on with your day after a session, in the background, on a cellular level, your body needs to put some effort into recovering from this stress.
This means that over the 12 to 48 hours following a hard training session, your body is busy expending calories trying to return to homeostasis, which is fancy-talk for 'neutral'. Your metabolic rate basically has to stay elevated (i.e. higher than pre-exercise levels) throughout this recovery period—which is a significant reason why HIIT is so effective for fat loss, and also why you’ll often feel warmer than usual when you’re lying in bed if you did an HIIT session earlier in the day.
This sounds spectacular—and it is—but there’s a sweet spot that you need to know about. Across the board, researchers generally recommend keeping your HIIT workouts to about 15-25 minutes, and you should probably start out with about three sessions per week if you're new to this. That’s a good place to start looking for your afterburn effect sweet spot.
You see, HIIT workouts are supposed to be over quickly. The trouble is, the fitness industry took the idea that ‘high intensity is good for you’ and tainted it by making intense workouts that are longer, heavier and harder. Then in order to get people through it, they perpetuate an undertone of exercise as punishment, that ‘sweat is your fat crying’.
Interestingly, leading exercise scientists are actually researching just how small a dose of HIIT can be whilst still being effective. Some high-quality but small-scale studies (like this one) have found positive effects from as little as one minute of total high intensity ‘sprint’ time, in a 10-minute workout—which even included a couple of minutes each for warmup and cooldown!
HIIT is a heavily studied field in exercise science, and the key takeaway is this: you don't need to be constantly smashing hour-long sessions to enjoy the magical benefits.
Principle #2: Intensity is relative
There’s more to the picture than the ‘all or nothing’ approach that a lot of people take when it comes to exercise. In the literature, the recommendations are that during the 'high intensity' portion of your workout you should be pushing at around 85%-95% of your maximal output.
This number is best viewed as an estimate, and you don't need any fancy watches and devices to tell you you're in this zone. You should be out of breath, it should feel like a struggle, and at the end of your third set it should feel like you're pushing at an intensity of approximately 17 to 19 on a scale of 1 to 20, where 20 is your maximum possible intensity. There's no benefit to pushing to the point where you feel you might pass out or that you want to throw up. Nothing magical happens at 96%, and on a practical level, remember that there's never a need to chase that negative physical feedback from your body - associating 'good exercise' with those feelings is probably not a good thing for you in the long term.
We’d all be better off by remembering that intensity is relative to where you’re at right now, so your 90% is going to look quite different from the next person's. It’s personal—it’s even unique to the day (which is why when you train at Ritual, or with the Ritual FIT app, we always ask how you're feeling on a scale of one to 10 and adjust the coaching you receive accordingly).
Take Jill, who’s been doing HIIT consistently for the last three years. She might need to sprint up five flights of stairs to get her heart rate up, while Bob, who’s just started exercising again after three years of being sedentary, might only need to walk up those same five flights of stairs to get the same effect.
In the same vein, Jill might need to smash a session with jump squats and clap push-ups to hit the intensity she needs today, while Bob should work on stability, breathing, and a consistent pace with his bodyweight squats and straight-arm planks, and that would also bring him to the right intensity for him. Bob shouldn’t be trying to compete with Jill—her journey is hers, and his is his. This is why good coaching, whether in a gym or through an app, is important.
Keeping these two principles in mind will help you build a healthy relationship with exercise that will benefit you in the long run. With consistent practice, you’ll get much more than reduced body fat and increased lean muscle; you’ll enjoy better lung capacity, improved heart health, accelerated fat oxidation and increased insulin sensitivity (i.e. you’ll deal with sugar better), amongst other benefits. Your body will become more efficient overall.
By the way, if you’re interested in getting leaner, feeling stronger and looking fit, there’s absolutely nothing wrong with the more ‘traditional’ approach of lifting weights for an hour on some days, and doing ‘cardio’ for an hour on other days. It’s worked for many people because, well, it works.
What HIIT can offer, if done right, is similar results in a fraction of the weekly training time. There’s a lot of research (like this study) that shows similar performance improvements with HIIT versus traditional endurance style workouts, even where the time commitment in the HIIT group was literally less than a quarter of that of the traditional endurance training group.
That's nuts, and that's also why HIIT has blown up the way it has. It's simply the most efficient way to train. And now that you have the inside scoop, you'll be able to do your HIIT workouts the way they should be done, and reap the long-term rewards that this type of training brings.
So to get the improvement you want, push hard and challenge yourself relative to your own fitness level, but don’t feel like you need to push so hard and for so long that you feel like throwing up or passing out when it’s over. Do this consistently three times per week for a few months, and amazing things will start to happen.
Ian Tan is the Co-Founder of Ritual. He’s got an MSc in Strength and Conditioning, a background in psychology, and is a Certified Strength and Conditioning Specialist (CSCS).