MotoGP rider. Aside from the obvious difficulty attaining the necessary skill to do what they do, if you look at how they interact with the bike and realize that they never really sit on it, nor can they hang on with their hands as much as you'd think, the fact that they have the strength to ride one for 45 minutes under the loads they generate is somewhat hard to understand. Then, consider that the helmet buffeting for most of each lap is bad enough that they really have a hard time seeing their markers... then it rains and they can't see much of anything...
JR
http://www.youtube.com/watch?v=IqrGbiGYs6A&feature=related