Can you have too much of a good thing? When it comes to high intensity interval training (HIIT), the answer could be yes.
HIIT is characterised by short, intense bursts of exercise, working at around 80 per cent of your maximum heart rate with brief rest periods in between. It is proven to be one of the most effective ways to improve your aerobic fitness in a short period of time. Multiple studies show that it significantly increases maximum oxygen consumption (V̇O₂ max), which is a good predictor of overall health.
One piece of research, conducted in 2016, followed two groups of participants over 12 weeks; the first group did intense 10 minute interval training workouts and the second did 50 minutes of lower intensity continuous training, but both reaped the same benefits despite the drastically different time commitments. By this logic, HIIT is seen as the ideal way to get fit in as little time as possible; some HIIT workouts last just five minutes, meaning you get a lot of fitness bang for your buck.
Despite its proven benefits, however, a new study from the Swedish School of Sport and Health Sciences suggests that excessive HIIT could have a negative impact on your health and performance. When 11 healthy participants completed strenuous HIIT sessions on an exercise bike five times in a week, not only did their performance plateau but they experienced a marked decline in the function of their mitochondria (the energy generators inside cells) and a disturbance in their blood sugar control.
When I put this to Dr Jean-Philippe Walhin, a lecturer in exercise physiology at the University of Bath, he says that HIIT is a safe and effective method of training as long as you don’t overdo it and intersperse it with other forms of exercise.
“This study has its limitations – participants were working at 95 per cent of their maximum heart rate, with long intervals, and were overloaded with training sessions over a period of just three weeks,” he tells me. “In real life you can listen to your body. If you’ve done a strenuous session one day, take the next one off or go for a gentle walk or jog instead.”
The small study lasted four weeks and the intensity and frequency of participant’s training was increased over this period. In the first two weeks the participants – five male, six female – saw an increase in their fitness levels. However, by the third week they were training more intensively (for intervals of up to eight minutes at max effort) and more frequently (five times per week).
At this point, their physical performance stagnated and researchers recorded reduced “intrinsic mitochondrial respiration”, an indication of how efficiently their mitochondria were working.
For many home workout devotees, five HIIT sessions a week may not seem excessive at first. As HIIT is touted as the most effective exercise for fat loss there’s a tendency for people to overdo it, see it as a short term fix or to assume that, as HIIT sessions are short and often involve only exercises that use your body weight as resistance, they’re not putting much strain on your body.
These statements are untrue; the idea that HIIT is more effective than other exercise for fat loss is unproven and, according to personal trainer Alice Liveing, it can actually make you more injury prone.
“I’ve lost count of the number of times clients or friends have come to me and said they have niggling injuries from intense HIIT,” she says. “Because you work as hard as you can for a very short time in a HIIT class, it’s easy for form to suffer. The focus is on going all out, rather than the quality of the movements.”
While most studies regarding HIIT are conducted on treadmills or static bikes, it’s HIIT-style body weight interval training that has become especially popular over lockdown; this usually involves plyometric moves (like squat jumps) that can put additional strain on your joints, especially if you’re a beginner.
Despite this, as Walhin says, the majority of people who exercise recreationally are more likely to undertrain than to overtrain. A scenario like that in the above study, in which participants rapidly ramped up their training regime over a short period of time, is unlikely to apply to the average at-home HIIT-er.
To avoid potential issues, HIIT devotees just need to put quality over quantity, allow enough time for recovery and alternate high impact workouts with gentle exercise or mobility training.
“Variety and recovery are key and it’s important to increase your training in a sensible way and not overdo it,” Walhin says. “You wouldn’t train the same group of muscles every day or, if you’re a runner, do a half marathon five times a week.”
Liveing also emphasises the importance of recovery. “It takes around 24 to 48 hours to fully recover from a HIIT workout, so if you’re doing one each day you’re not giving your body a chance,” she says. “While it might help you reach short term goals, excessive HIIT won’t serve you long term.”
Alice Liveing’s fitness app Give Me Strength launches on 1 June