Could injury risk mean the end of high intensity exercise?

Go hard or go home, right? But go too hard and you could be going to the hospital instead.

HIIT (high-intensity interval training) boasts more than 37 million Google search results and almost six million hashtags on Instagram, so it’s safe to say that it’s kind of a big deal.

With its ability to boost your fitness in half the time as traditional workouts — thanks to a mix of all-out physical effort and slower-paced recovery periods — HIIT has become the go-to method for anyone looking to sculpt a six-pack or train for a major exercise feat.

But underneath all the hype, a rebellion is brewing.

Earlier this year, researchers from Rutgers University in the US found that people who engaged in HIIT were far more likely to sustain a knee or shoulder injury, and in 2018, global fitness chain Les Mills called for stricter guidelines, with head of research Bryce Hastings declaring: “There is only so much HIIT a regular exerciser can do in one week before the effects are compromised.”

It’s this potential for harm that has spawned the rise of anti-HIIT acronyms like LISS (low-intensity steady state cardio), LIIT (low-intensity interval training) and MISS (moderate-intensity steady state cardio).

It’s also birthed a subset of fusion classes like HIIT yoga and HIIT Pilates. But with so many purported benefits, one has to ask: do HIIT’s downsides really outweigh the positives?

Fact or fad?

“HIIT has been an integral part of training athletes for close to a century,” explains associate professor Timothy Fairchild from Murdoch University.

“The format involves repeating high levels of physical exertion followed by lower levels of physical exertion multiple times, and it can be used in everything from running and biking, to swimming and gym work.”

During the high-intensity intervals (think burpees, sprints or all-out pedalling), your heart rate is pushed to up to 90 per cent of its maximum capacity, and it’s these intense bursts of physical activity that encourage your body to blast fat and increase your endurance.

“To get the best results out of your training in the shortest amount of time, you need to put your body under a certain amount of strain,” notes PT and YBell inventor Aaron Laurence.

“The greatest benefits of training come from higher levels of intensity, so if you hit that zone as often as possible in a session, weight loss, improved cardio, increased muscle endurance and better overall movement and agility will follow.”

As well as giving your fitness a serious leg-up, HIIT also improves your heart health and, according to recent studies, it could also stave off bowel cancer, improve your memory and help manage chronic diseases like type 2 diabetes.

The catch? It’s not always done right.

Too fast,too furious

Although there’s truth to the age-old adage, ‘no pain, no gain,’ in HIIT this can be taken too far. In a recent study published in the American Journal of Medicine, scientists reported that high-intensity spin classes put beginner exercisers at risk of developing rhabdomyolysis, a condition where muscle breaks down and enters the bloodstream, causing an extreme amount of pain.

A separate Les Mills-backed study revealed spending more than 40 minutes a week at your maximum heart rate could reduce your performance and increase your risk of injury. And then there’s the risk of overtraining.

“Overtraining can happen with all exercise formats, but HIIT makes people particularly susceptible to this,” notes Prof Fairchild.

Laurence agrees. “A lot of people think if they do more sessions, they’ll see more results, but this is where you get into trouble,” he warns.

“You can’t do HIIT every day — you need to eat and sleep accordingly, and have a proper recovery program otherwise you’ll hit a wall.”

As well as causing you to feel mentally and physically fatigued, too much HIIT can also elevate your levels of the stress hormone cortisol by activating your fight-or-flight response. While cortisol isn’t-inherently bad, elevated levels, caused by overdoing it with HIIT or simply not finding ways to relax, can slow your metabolism, compromise your immune system and bring your exercise gains to a standstill — valid concerns that have led to the rise of cortisol-conscious workouts.

The balancing act based on the idea that less isn’t always more, cortisol-conscious workouts like yoga or long, heart-pumping walks are designed to give your body a chance to recover while also increasing your fitness. The experts say this is the balance you should be aiming for.

“HIIT is too popular and effective to be ruled out completely, but more and more people are realising they can’t sustain that level of effort every session,” Laurence says.

According to Prof Fairchild, the key to a balanced training program is variability, so look to complement your HIIT sessions with low- or mid-intensity workouts and always take time to rest.

“Your body needs to recover after every training session,” he tips.

“If you’re training a particular muscle and it requires two days to repair but you train every day, the muscle won’t adapt and will stay in a chronic state of disrepair. So if you’re determined to have your HIIT cake and eat it, too, just remember to slow down.”


Brush up on your fitness lingo with this workout glossary


What it means: High-intensity interval training.

What it does: Using timed bursts of intense physical activity and short recovery periods, HIIT is designed to spike your heart rate, which triggers a fat-burning response.


What it means: Low-intensity steady-state cardio.

What it does: Rather than spiking your heart rate, the purpose of LISS is to maintain a low-intensity pace during cardio exercises like hiking, walking or steady cycling. This gives your body time to recover and also increases your cardio fitness.


What it means: Low-intensity interval training.

What it does: If a HIIT interval goes from jogging to sprinting, LIIT goes from walking to jogging. Because the exercises are less intense, you’re able to do them for longer.


What it means: Moderate-intensity steady-state cardio.

What it does: MISS can be anything from a long Sunday run to a heart-pumping steady swim. These workouts focus on building endurance and burning fat and are less demanding than HIIT but slightly more demanding than LISS.

3 low-intensity workouts


During your one-on-one assisted-stretching time with a flexologist, you’ll work on increasing your range of motion, easing aches and improving circulation. There are studios in NSW and


Online memberships give you access to training videos and workouts that are all easy on your joints and beginner-friendly,


Classes use an interval format, but they’re low-impact and always include a juicy stretch session at the end. Find a location near you in QLD, NSW or VIC, or sign up for online workouts.

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