Usain Bolt is down to his last, blazing curtain call

Muhammad Ali stood alone on many fronts, but Joe Frazier, George Foreman and a few others still stood toe-to-toe with him in the ring. Jack Nicklaus contended with Arnold Palmer on the front end of his career and Tom Watson on the back end.Usain Bolt? Nobody has been a match for him, on or off the track.The man who reshaped the record book and saved his sport is saying goodbye. His sprints through the 100 meters and Jamaica's 4x100 relay at the world championships, which begin Friday, are expect