When I was a kid we used to make up all sorts of games. In particular, we would stand on the monkey bars at recess and jump and see how far we could land. Children are fearless when it comes to their physical limitations. When they see a challenge instead of saying, "Oh, I can't jump that far", they say "Lets see how far I can jump."  We have to learn to think like a child, we have to learn how to play again. So the 2nd part of this answer, which I have already touched on it a bit, is FEAR. Most of the time we let fear get in the way of us overcoming a physical challenge.  As we grow older, and we haven't done a particular task in a long time, we become scared.  We have to learn to over come our fears of physical activity.