The thing with hiking is that while average total calorie-output may be low, muscle-exertion (intensity) is high. So HR is only a partially accurate indicator. Rather than pushing smoothly at 25-40% of maximum muscle-force continuously like on a bike, you're pushing your muscles at 50-80% max with each step on a climb. The higher peak forces is why you're more sore for the same total calories burned as a bike ride.

I doubt you even build up lactic-acid on a hike. DOMS is primary microtears in the muscle and bound-up Z-bands. Additionally leaking cytoplasm resulting in elevated levels of CPK signaling repair efforts.

http://www.drmirkin.com/fitness/1346.html " but now we know that lactic acid has nothing to do it"
http://pubmedcentral.org/articlerend...?artid=1474216