The exercise theory behind the acronym promises major results from mini time slots – but what’s actually going on beneath the Lycra when you hit a HIIT class hard?
1. Your lungs
The clue is in the name. Moving at max intensity is the only way this will work – which is why you’re out of breath. “You need to be at 90 to 100 percent effort for 15 to 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.”
2. Your muscles
You need the power, but mini-breaks are just as important. 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.” You don’t need to tell us twice.
3. Your skin
Sweating so much your moisture-wicking leggings can’t cope? [Ed’s Note: Check out the Dry Motion tech in these Asics tights.] 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, a lecturer in sports and exercise science. Wear that glow with pride.
4. Your hormones
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.
5. Your pudge
Boom – your body is still burning kilojoules while you’re busy shovelling in post-workout chocolate Superfoods Super Oats. “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 ’joules for hours after you stop. You can amplify this effect by staying active, so consider HIITing it in the morning before work.