London Fletcher
Credit: Wesley Hitt / Getty Images

"You certainly wouldn't expect a linebacker to be in the league as long as he's been," Dr. Riley Williams says of the active NFL leader in consecutive games played (247), attributing London Fletcher's success at 38 in part to his training habits. But Williams also notes Fletcher's ability to overcome injuries. "He may be a guy where genetics play a little bit more of a role, because he seems to bounce back and heal very quickly."