Tests can be administered. Requirements can be imposed. But when it comes to the NFL's ability to protect players from the dangers of concussions, the league faces a formidable obstacle: a locker-room culture that values strength above all.
14hKevin Van Valkenburg
25mAdam Schefter and Tania Ganguli
1hBonnie D. Ford
18hDan Arritt, Special to ESPN.com
5hSam Khan Jr.