WHAT HAPPENS WHEN...

The exercise theory behind the acronym promises major results from mini time slots – but what’s actually going on beneath the Lycra when you HIIT it hard?

...you do HIIT (besides DOMS)

GO HARD OR GO HOME

The clue is in the name. Moving at max intensity is the only way this will work. ‘You need to be at 90-100% effort for 15-30 seconds,’ says sports therapist Barry Sigrist.

‘By working anaerobically (sans oxygen for fuel), you’ll produce lactic acid, which releases adrenaline, helping to move fat around the body.’

REST ASSURED

A rest is as good as a rep. Yes, really. ‘The ideal ratio for a 20-minute session is 30 seconds on, 60 seconds off. Resting for so long might feel like cheating, but your muscles need time to renew their oxygen and glycogen levels,’ says Sigrist. ‘Too little rest and your body becomes catabolic, breaking down muscle tissue.’ Don’t need to tell us twice.

SWEAT LIFE

Sweating so much your moisturewicking leggings can’t cope? It’s all part of the HIIT experience. But don’t reach for a towel between sets. ‘For the body to lose heat through sweat, it must evaporate from the skin, so wiping it off will likely just increase overall sweat production.’ explains Dr John Dickinson, lecturer in sports and exercise science at the University of Kent. Wear that glow with pride.

GET LIT

To power a HIIT session, your body taps into muscle glycogen, not fat stores – calm down, if your goals include weight loss, this is a good thing. ‘HIIT triggers a release of human growth hormone and testosterone – both play a key role in metabolising fat,’ Sigrist explains. For best results, elicit a peak hormone response by incorporating compound movements into your sesh – think squats, lunges and (sorry) burpees.

SLOW BURN

Boom – your body is still burning calories while you’re busy shovelling in post-workout scrambled eggs. ‘The magic of HIIT takes place after your session,’ says Sigrist. ‘Your body has an “oxygen debt” to repay, and this creates something called excess post-exercise oxygen consumption.’ Simply put, you’re still burning calories for hours after you stop. You can amplify this effect by staying active, so consider HIITING it in the morning before work.