well, i refer to the 'burning' as stimulation, most likely wrong, lol. i think its latic acids.
i was just thinking about this the other day. when i squat for 5 reps, i feel no burning, but get bigger and stronger. if u just did 3x15 of leg extentions, i dont think youd get as much growth but get a hell of alot of burning.
in the end, i feel like the burning is a good thing, even if it means nothing, i kind of like it. like when i drink pop really fast it hurts. i like that, i think im twisted