"It's all downhill from here" is a phrase that is supposed to be comforting, but according to this article, that wasn't an aphorism any runner came up with. This brief essay on what makes downhill running so hard started out with the story of Grete Waitz, one of the most famous female runners in the world. She was running the Boston Marathon in 1982 and was a good three minutes ahead of the nearest competitor. All of a sudden, with only 5 miles left to go of a 26 mile race, she hit a downhill stretch and her legs completely gave out.
Apparently, if you haven't done too much downhill running, your legs will not be used to "eccentric contractions" in your quadriceps. Since running downhill is basically like a continual falling onto your own feet (a considerable amount farther than what normal running is) your quads are being subjected to much more extreme stress.
This causes them to be significantly weakened; sometimes up to 25% weaker after just 45 minutes of downhill running! Researchers have calculated that this amount of strain can take a good two weeks to fully recover.
All is not lost, though. Like most things, practice makes perfect. Er . . . well, at least better. If you are training for a race like the Boston Marathon where there are extensive downhill stretches, you can build up your quads in preparation. By responsibly training those muscles for the added stress of downhill running, you can be ready to take the hills on and avoid muscle failure like Waitz experienced on her trip downtown.