“Lord, whom shall we go away to? You have sayings of everlasting life"- John 6:68
WHAT has science offered?
Gotta say I love HIIT cardio. I love how it spares my muslce but after my weight training low intensity is the way to go.
Why are you doing HIIT right after lifting? By that point in time you should have low enough glycogen that you could get away with doing a light cardio for 20-30mins.
I mean, it's just my opinion, but I would recommend doing HIIT on off-days.
Speed * Time = Distance
So in the end, W = Force * Distance (W=Fd)
So, if you really want to burn the most calories, just go the most distance that you can running, walking, or sprinting. I think the purpose of HIIT is to burn up only glycogen, hence the 18 minute suggestion. I'm sure there's more involved in mechanical and metabolic efficiency which is probably why HIIT is a good idea. It may not burn as much calories, but the fat burning potential is higher.
When you are working near your maximum heartrate, a lot of your calories are coming from lactic fermenation which is very inefficient, but effective way to use glycogen. I think it yields like 4 ATP per glucose molecule. When you're just jogging along and have enough oxygen (aerobic) your body can do the oxidative phosphorylation and get like 34 or something ATP per glucose molecule.
Just a little more pseudo-science for you guys (I don't claim to know what I'm talking about). When you deplete glycogen stores, your body is going to be throwing everything in the furnace so to speak. This is when it gets catabolic and you start using protein for energy. If I were to recommend HIIT I would say do it in the afternoon on non-workout days. That way it will burn up all of your free floating glycogen and then when you're done and go home, your body will have the time to prepare fat for use in oxidative phosphorylation. If I understand this correctly, its kind of similar to ketogenesis. I would use it to quickly "burn" up glycogen and let the ketogenesis occur for the next couple of hours while I sleep. Also, I wouldn't want to eat any carbs afterwards to enjoy the full benefits. The same idea, just different is to not eat carbs before dinner. That way, when you sleep, you don't have free circulating glycogen, and your body must use fat for energy.
This is just how I understand it with my limited basic biology and physics knowledge.
I've hacked that last thread to pieces. What it comes down to is that the HIIT burns more calories because it uses glycogen less effectively, so therefore, you have to use more to do the same work. Also, HIIT sets your body up for ketogenesis by depleting glycogen stores.