Take my instructor for example. Great guy, incredibly friendly, and with his beard has a slightly santa'ish look. He's not in great shape, due to a series of injuries throughout his life. Still very strong, but lacking in cardio. He moves at the speed you'd expect of somebody his age. Not slow, but certainly not with the spring of youth.
But then he does a technique.
Ever see Clone wars? Remember when yoda suddenly starts kicking ass? Its basicaly like that. This man who shouldnt be able to move in those ways, suddenly is faster, stronger, and more precise than anybody i've seen. The burst only lasts for a few moments, then he's back to himself again.
The effects of years and years. hundred of hours performing similar techniques. They add up. The body learns to do things in a certain way, and continues to do them that way even after logic says you shouldnt be able to.
Old Karate Guys scare me.