Arlington, Va. — I started playing football at eight years old. Back then, if you fell and hit your head, the coach would ask if you were all right and to shake it off, and then you went back out on the field. There wasn’t a protocol to check if you had a concussion or possible brain injury. I went on to play college football, and after graduation, I played in my first NFL season in 2005. In 2010, I attended a brain-trauma workshop at the NFL Players Association’s annual conference in Florida. A doctor compared images of a healthy…