For men who’ve been immersed in football to the exclusion of all else since high school, leaving the league can be crushing, as they have no idea how other people live, much less find work. And the league’s younger alumni are “five times as likely” as their non-NFL age counterparts to have problems with mobility and strength, from walking stairs to lifting objects. But despite the risks, which the players know well, the NFL’s tough-guy mentality resists change, and players have bristled against new rules designed to lighten the impact.