When I was on my mission, my mission president told us about training fleas. When you want to train fleas, you put them in a jar. The fleas will try to jump out of the jar, so you put a lid on it. The fleas will bump up against the lid for a while and after a while they will stop hitting the top the lid. You can unscrew the lid and the fleas won't jump out anymore. Their muscles haven't changed--they have just set a mental block on what they think they can do.
(Just ignore the fact that this is a PlayStation commercial)
I think we are capable of amazing things and that we just limit ourselves. We just need the proper perspective to see that we have such an enormous potential.