Timing of concussion diagnosis is related to head impact exposure prior to injury - Beckwith, J G; Greenwald, R M; Chu, J J; Crisco, J J; Rowson, S; Duma, S M; Broglio, S P; McAllister, T W; Guskiewicz, K M; Mihalik, J P; Anderson, S; Schnebel, B; Brolinson, P G; Collins, M W (Med Sci Sports Exerc, 2013)