By 
LaShawn Merritt walks off the track Saturday after injuring his hamstring (AP)Like Floyd Mayweather in the ring, Rafael Nadal on clay or Joey Chestnut in a hotdog eating contest, USA Track and Field has long been unbeatable in the men's 400 meters.
The 12 fastest men in the history of the event were all Americans prior to this week. The U.S. swept the medals in both Athens and Beijing, captured gold at the last seven Olympics and produced at least one medal winner at every Olympics since 1920.
[ Related: Full video coverage of the 2012 Olympic Games ]
Each of those streaks ended with little drama Monday in London because no Americans were in the 400 meters final to challenge eventual champion Kirani James of Grenada. Defending Olympic gold medalist LaShawn Merritt pulled up in a preliminary heat Saturday after he aggravated a preexisting hamstring injury, and fellow Americans Bryshon Nellum and Tony McQuay finished third and fourth in their respective semifinal heats Sunday and didn't advance.
"I would never have imagined we'd never have anyone representing the U.S.," said former U.S. sprinter Quincy Watts, the 1992 gold medalist in the 400 meters. "It was a huge blow when LaShawn got hurt. LaShawn Merritt was going to be almost unbeatable this year. He's the guy with the No. 1 time. He's the guy who was running well all year. He's the guy who was going to be my pick."
The lingering question in the wake of this disappointment is this: Is the U.S.'s long run of 400-meter dominance now over? Or was 2012 merely a temporary blip that history will view as an aberration?
It's too soon to answer that question with any certainty, but the early signs suggest the U.S. is slipping a bit in its signature event and the rest of the world is catching up.
Although Merritt would have been the favorite to repeat in London had he been healthy, there are no guarantees he will still be a contender in Rio in four years. He'll be 30 by then, two years older than former Olympic champion Jeremy Wariner was in June when he finished sixth at the U.S. Olympic trials and failed to qualify individually for London.
The two most accomplished young 400-meter runners in the world are James, 19, and Luguelín Santos, 18, of the Dominican Republic. James, the reigning world champion in the 400 meters, won gold in a blazing 43.94 seconds Monday in London and Santos, the world junior champion, took silver.
[ Video: Carl Lewis' predictions for U.S. men's track and field team ]
It's possible the next great U.S. 400 runner capable of cracking 44 seconds is still in high school today. If not, the U.S. will have to hope guys like Nellum and McQuay continue to develop to the point where they can challenge the best in the world.
Nellum, the former sprinting phenom who has battled back after being shot in both legs four years ago, certainly has a chance if he can get fully healthy. The USC senior finished third at the U.S. trials in 44.80 seconds six weeks ago despite still not running full speed around the curves due to lingering complications from the three surgeries he has undergone.
McQuay, the NCAA champion from the University of Florida, also has shown promise. He ran 44.49 at the U.S. trials to finish second behind Merritt but went out too fast and faded late in his semifinal Sunday, perhaps a sign he was feeling the effects of a lengthy college season.
"These guys are huge young talents, but it's tough for guys who are still in college," Watts said. "You have to be at your best for Regionals, you have to be at your best for NCAAs, you have to be at your best for Trials, and then you have to come back and be at your best to try to get the gold. I experienced that and I know how rough and grueling this process is from a young athlete's perspective. It's disappointing for them, but this will help them in the future."
[ Video: Fast start can fuel track success ]
The good news for Nellum and McQuay is they will have a chance later this week to atone for not making the 400 meters finals. Both will be part of the U.S. 4x400-meter relay team that will be hoping to continue American domination of that event.
The U.S. would have been a heavy favorite to win gold if Merritt were healthy. Now the relay looks more wide-open than ever before.
More Olympics coverage on Yahoo! Sports:
Related Olympics coverage on Yahoo! Sports:
• Video: Who will step in for injured 400 star LaShawn Merritt?
• South African archer comes out after competition
• London catch-up: Day 9