40 min moderate jog, but would it be fair to say a 20 minute HIIT session at maximum effort would require at least twice the effort, therefore twice the energy expenditure with the added bonus of EPOC?
But effort doesn't directly equate to calories burnt. Imagine sprinting as fast as you can until you drop through exhaustion. You would be done in under a minute. Maximum effort but only 30-40 calories burned tops. Because you could only keep it going for 1 minute!
In a HIIT session, of that 20 mins most of it is spent at a moderate speed with only maybe 5 minutes total spent at the fast speed. A high rate of calorie burn during those 5 minutes, yes, but there's only so many calories you can burn in 5 minutes! Let's say you burn 10 cals per min in the easy intervals and 20 cals per min in the hard intervals. At the end you'll have burnt 250 cals, only 50 more than if you just stayed at the slow speed throughout. I'll give you 50 cals for EPOC and you're looking at 300 total cals. Now a 40 min jog burning, say, 12 cals per min you've burned 480 cals.
Intense, yes, because of the hard intervals taking you above your anaerobic threshold and pushing your muscles, heart and lungs hard and great for fitness. But the hard parts aren't sustained long enough to make a massive difference to calorie burn.
I should say that I think the ideal programme would include HIIT and longer slower stuff both for variety and to train different aspects of aerobic and anaerobic fitness. But I think people are misled into thinking they have to do HIIT all the time because it's "better for fat loss" when it isn't. And frankly it's bloody hard and most people can't sustain it on a regular basis.
<message edited by CheekyChappie on 30 January 2012 12:52>