Weather conditions also have to be just right. A windless, cloudy, 50-degree day is ideal, Joyner said. The course, which has to be a loop to meet record regulations, also needs to be flat. That probably explains why the last few records have fallen at London and Berlin, which are devoid of hills.
The New York City marathon, which is a point-to-point course, is far too hilly and its weather is too inconsistent to be conducive to a sub-two hour marathon. So far, no one has run faster than 2:07:43 in the five-borough race.
Still, the fact that a growing number of people continue to sign up to run New York and other marathons suggests that a two-hour marathon is becoming within reach. Instead of playing soccer or basketball, athletes with the most running potential are now more likely to choose running as their sport.
And even though two hours is an arbitrary barrier to strive for, the push to break it helps highlight the amazing potential of the human body.
"Whenever people have attempted to place limits on what humans can do, humans always do a little better," Joyner said. "If you make barriers, people are going to try to break them, whether it's going to the moon, flying across the Atlantic, trying to set an age-group record or becoming the oldest person to do something. It's who we are."