Not to derail the discussion (Well, okay, yeah. This is a derailment.) Here are the list of head injuries by sport in the US. Cycling was far and away number one. But other articles say "of course it is number one" since more people engage in it than many other sports AND people tend to me more cavalier about safety measures. Football mandates helmets, pads, etc. and is not played by nearly as many people who bike. Further, many people simply get on a bike and go for a ride, helmet be damned.
But among team sports, football is clearly the source of most head injuries. And the best numbers I could find were 2009.
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The following 20 sports/recreational activities represent the categories contributing to the highest number of estimated head injuries treated in U.S. hospital emergency rooms in 2009.
Cycling: 85,389
Football: 46,948
Baseball and Softball: 38,394
Basketball: 34,692
Water Sports (Diving, Scuba Diving, Surfing, Swimming, Water Polo, Water Skiing, Water Tubing): 28,716
Powered Recreational Vehicles (ATVs, Dune Buggies, Go-Carts, Mini bikes, Off-road): 26,606
Soccer: 24,184
Skateboards/Scooters: 23,114
Fitness/Exercise/Health Club: 18,012
Winter Sports (Skiing, Sledding, Snowboarding, Snowmobiling): 16,948
Horseback Riding: 14,466
Gymnastics/Dance/Cheerleading: 10,223
Golf: 10,035
Hockey: 8,145
Other Ball Sports and Balls, Unspecified: 6,883
Trampolines: 5,919
Rugby/Lacrosse: 5,794
Roller and Inline Skating: 3,320
Ice Skating: 4,608
http://www.aans.org/patient%20information/conditions%20and%20treatments/sports-related%20head%20injury.aspx