Actually, i read a paper somewhere that states this to be the case. The mind is wired to get you to cease activity before reaching its actual limit - IIRC, something along the lines of evolutationary coding to save something in case a saber tooth came looking for you. Peak activity involves training the body to actually overcome the brain's "safety shutdown" and actually go to the limit. Apparently, once you do this a few times, it becomes easier to actually repeat this in the future.

It is amazing the sorts of pain you can teach your body to ignore. I've been kicked full-on in the nuts in a sparring match and actually finished 3 more minutes till the round ended before collapsing in pain. Now I need to learn to do the same with the lactic burn in the legs for cycling.