sfrider 

This is interesting, but I wonder about even lower MHR percentages, like 55-65%. I did a 20mi hike a few days ago up in Point Reyes, and carried the Edge 305 with HR strap just to get the data in with the rest in TC. Just about the entire time (7h) was spent at 55-65%, only occasionally getting up to 70 and slightly above on the steeper grades (like going up over Mt Wittenberg). The amazing thing is how closely grade and HR% follow, they're virtually identical, until the last two miles or so (when I was starting to feel severe muscle fatigue) where they diverged a bit (HR up). Energy consumption was probably in the 5000 kcal range, but since HR was so low I went out on a limb and guessed I was going mostly on subcutaneous fat. I ate one clif bar all day on the trail and was just fine. A little diluted cytomax to keep salts in check and blood sugar up just in case it turned out to be a hot day (it was nice and cool). So the fat % must have been really big, at least 70% or I would have bonked or started feeling symptoms. I did carry enough food that I could simply take a 3-hour food break and siesta before taking the shortest route back, so it wasn't as reckless at it may seem.

It's a different activity for sure, but I'm still really curious what low-HR exercise is fueled by. I did have the worst case of DOMS I've had in many years BTW, man I could barely walk yesterday. The Edge estimated 6900 kcal; yeah right, even though hiking is more demanding than bicycling at the same speed, and more demanding downhill than on flat ground, that's still way the heck off!

I can't seem to find anything on this anywhere, most of the hiking resource still live in a world where you need to replete every calorie (instead of viewing subcutaneous fat as a fuel tank that can be repleted much later), that lactic acid buildup causes DOMS, etc.