Considerable research has shown that high-intensity interval training (HIIT) increases fat oxidisation, strengthens your heart, and drastically improves your anaerobic capacity. HIIT is any form of short-term exercise that uses short bouts of high-intensity effort followed by short bouts of rest. Sessions last between four and 60 minutes and can increase your metabolism for up to 48 hours.
Sounds like a magic pill for exercise – but do you know what’s really happening to your body when you do HIIT?
Nigel Stepto, Associate Professor in Exercise Physiology at Victoria University, writes:
Initial stages of HIIT
During a strenuous bout of HIIT your body is working overtime, borrowing from a whole host of energy systems to keep you from collapsing. It’s this overload of training stress that forces your body to become almost superhuman – even if it’s only for a few seconds.
A key component of this, says Stepto, is your body utilising your ATP-PC energy system. “Because all-out training uses the anaerobic system, in the first five to 10 seconds your body starts to pull fuel from your glycogen stores, then it switches over to your aerobic system,” says Stepto. “Because you’re giving your body a very limited time to rest it’s forced to pull fuel from a whole range of energy systems.”
Five minutes after HIIT
Okay, so you’ve just completed one of the most intense workouts of your life – five minutes afterwards, you’re likely still to be sucking wind, have an elevated heart rate and the adrenaline is still pumping. This is your body attempting to return a sense of balance, or homeostasis, says Stepto. “Your body is trying to normalise lactate levels almost immediately, and you’re likely to be breathing pretty hard with a high body temperature. This is your body attempting to return to a state of homeostasis.”
30 minutes after HIIT
Your workout is all done for the day, but even half an hour after HIIT, your body is still reeling from the stress you placed on it. After 30 minutes your heart rate and body temperature should’ve returned to normal, but you’ve suddenly developed a raging appetite. This is your body attempting to replace the glycogen fuel stores you used during training. Stepto recommends for those who train regularly, this is an ideal time to feed your body carbohydrates for recovery.
Six hours after HIIT
Still feeling that workout? While you probably aren’t, your body definitely is. Even six hours after your HIIT session, your metabolism is still firing away. This is all to do with your body’s sensitivity to insulin, says Stepto. Insulin sensitivity controls what nutrients your body takes from food and moves them into muscle or fat cells. The less insulin your body requires, the more likely you are to store excess glycogen as fat.
“High-intensity training has been shown to improve your insulin sensitivity, which controls the amount of glucose your body’s cells can take,” says Stepto.
“Even six hours after a strenuous session your metabolism is still primed to process fuel you put into your body.”
48 hours after HIIT
If you train regularly, the short nature of HIIT means it’s probably unlikely that you’ll still be sore 48 hours after your workout – but that doesn’t mean it’s stopped positively impacting your body, says Stepto. “Studies have shown that up to 48 hours after HIIT a person is still more insulin sensitive,” says Stepto. “That’s why for people looking to lose weight we recommend training no less than every second day, so people can take advantage of that insulin resistance overlap.”