The harmful side effects of too much HIIT It's one of lockdown's most popular workouts, but a new study says too much high intensity interval training could have harmful side effects 30 April 2021 • 11:00am HIIT is often seen as the ideal way to get fit in as little time as possible Credit: Thomas_EyeDesign/Getty Images 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.