One of my students broke both wrists in university mountain biking a few year's back... that was a pretty good excuse not to get his essays in. He seemed remarkably pleased about it!
Mind you he was a half decent rider - I have just watched the XC video, and there is some really poor riding there - no line selection, completely stupid use of the breaks, we're not even getting anywhere near 'flow' - just people with no idea how to ride an mountain bike. Very entertaining though...