About 135,000 or 65 percent of sports- and recreation-related traumatic brain injuries treated in U.S. emergency rooms each year occur in young people ages 5 to 18, according to a study published this week in the Centers for Disease Control and Prevention's Morbidity and Mortality Weekly Report (MMWR).

Approximately 8 percent or more than 10,000 of these youths were hospitalized, the study said. The study defines traumatic brain injuries, including concussions, as a blow or bump to the head that disrupts the way the brain normally works.

Among 5 to 18-year-olds, horseback riding, ice skating, riding all-terrain vehicles, hockey and tobogganing/sledding were the activities responsible for the highest percentage of visits to the emergency room for traumatic brain injuries.

CDC researchers also found that many young people sustain head injuries while bicycling, during playground activities, and while playing football, basketball and soccer.

Just how dangerous head injuries can be was evidenced this week, when a 12-year-old softball player in Michigan was knocked unconscious by a ball that hit her in the head during practice. She died a day later, police and family said.

Margaret Ruth "Maggie" Hilbrands was hit by a ground ball during a routine infield drill on Monday. The Grand Rapids girl died Tuesday at DeVos Children's Hospital. Click here to read that story.

For the CDC study, researchers examined data from the National Electronic Injury Surveillance System All Injury Program (NEISS-AIP) for the years 2001 to 2005 and looked at the overall number of traumatic brain injury-related emergency department visits and the activities associated with those visits.

The "Heads Up: Concussion in Youth Sports" tool kit can be ordered and downloaded free-of-charge online at: http://www.cdc.gov/ConcussionInYouthSports.