If you’ve read just about any fitness blog, magazine or website in recent years, then you’ll likely have come across HIIT.
HIIT: high intensity interval training. Sprinting at maximum heartrate for a short period of time and then switching to a slower form of exercise for a couple of minutes to recover before starting the whole cycle again.
This type of training is all the rage because it is known to burn more calories in less time when compared with steady state cardio. And it’s great for our VO2 max, mitochondrial function and more.
But why? How does it work? What makes it so special?
What Happens When You Push it Hard
When you engage in HIIT, you start out by pushing hard and going at or near to your maximum heart rate. This is what makes all the difference, as now you are depleting your body of all of its readily available energy in order to drive those fast twitch muscle fibers. This is anaerobic training and it relies on ATP stored in the muscles, as well as glycogen.
After this, you then switch to your regular exercise at around 70% of your maximum heart rate. This is a steady pace that you can maintain, that burns fat using the aerobic system and that allows you to recover and reduce the lactate and other metabolites that build up in your blood during intensive exercise.
Welcome to After Burn
Steady state cardio is normally something you can maintain for a long time before you start to tire out and this is why a lot of people will exercise by running at a steady pace for 40-60 minutes.
If you do this after having done high intensity training however, you will be running at a point when you have very little available energy in your muscles and in your blood. All the glycogen has been used up and thus you have to rely even more on fat in order to keep going. Your body becomes more efficient at burning fat and you see greater benefits from the short amount of training that comes after.
But this isn’t even the best bit. What’s so good about HIIT is that this after burn effect continues for hours after you finish training. You’re now going about your usual activities with less glycogen, which means you’ll burn more fat even to do regular things like picking up a fork, or walking across the room!
