http://well.blogs.nytimes.com/2014/03/31/with-the-thrills-come-extreme-risks/?_php=true&_type=blogs&_r=0

At the recent annual meeting of the American Association of
Orthopedic Surgeons, Dr. Sabesan reported that more than 40,000 such injuries
occur annually among participants in seven extreme sports, including
skateboarding, snowboarding, mountain biking and motocross. Her analysis showed
that 83 percent were head injuries and 17 percent neck injuries, with 2.5
percent described as severe, resulting in potential lifelong disability or
death.

“The level of competition and injuries we’re seeing keep rising,” Dr. Sabesan
said in an interview. “Many do recover, but not necessarily without long-term
consequences.”