The world-record pace for the marathon continues to improve for both men and women. For men, the record pace for the marathon is now about as fast as the record pace for the 10,000-meter run just after World War II. Today, champion athletes are running more than four times farther at speeds of well under five minutes per mile. How can this be? Are humans simply built better or is there something else behind the mind-blowing speeds on the racetrack? Michael Joyner, M.D.