Head Impact Exposure in Youth Football Part 2: Elementary School Ages 9 to 12 and the effect of Practice Structure by Cobb BR, Urban JE, Davenport EM, Rowson S, Duma S, Maldjian JA, Whitlow CT, Powers AK, Stitzel JD. in Annals of Biomed Engineering; July 2013 online.
Additional coverage through the New York Times.