Being a cyclist I've learned some of the rudimentary ideas about general fitness. One of the things I've learned about is the idea of heart rate zones. Below a certain heart rate most of the energy you get comes from burning fat, above that is the so-called aerobic zone. This implies in this range that most of the energy comes from your lungs taking in oxygen and processing that with your food to generate energy (I think, roughly). Above that is the anaerobic zone, where you can't take in enough oxygen to meet your energy requirements and you get lactic acid build-up. There's one thing that doesn't make sense to me though, aren't there localized effects when you're working certain muscles only?
Anyway, it may be because it's the beginning of the season, but on my ride the other day I was below my aerobic zone but my legs were burning and I just didn't have energy. I know the body is complex, but I'm thinking that because I'm working primarily my thighs I was anaerobic in my legs, even though my heart rate didn't get into the aerobic zone. Is this possible? I read a book once that talked about this but I don't have it anymore. I suppose the answer is to strengthen my legs, maybe do intervals or hills and just get out cycling more. That also happened when I first started cycling, any other ideas?