This shouldn't have been troublesome, not at all. It was round two of the 110-meter hurdles. An easy round. U.S. hurdler Allen Johnson, the winner of four straight world titles, has been one of the most consistent track champions of the last eight years. He didn't have to win this race. He merely had to get over all the hurdles.

So it was a considerable problem and a thoroughly unusual circumstance when Johnson went under, rather than over, hurdle 10.

Johnson's quest for a second Olympic gold medal ended with a spectacular headfirst dive that would have been perfect at the baseball venue. After running square into the ninth hurdle with his lead foot, Johnson, a District native, was propelled forward. The impact knocked off his sunglasses and sent him skidding down the track, arms extended.

On a night American Allyson Felix, 18, continued the rampage of young U.S. stars by winning a silver medal in the women's 200 meters and Marion Jones easily qualified for the long jump final, Johnson saw the finish line only when he lifted his head off the track. After several seconds, the Lake Braddock High graduate buried his head in his hands.

"I fell down," Johnson said. "I don't know why."

Felix's performance was as notable, but far less perplexing. Wearing a silver necklace, silver earrings and shiny silver track shoes, Felix kept within striking distance of Jamaica's Veronica Campbell, 22, but couldn't quite run her down. Campbell earned the gold medal in 22.05 seconds. Felix finished in 22.18, setting a world junior record in the process.

Bahamian Debbie Ferguson, 28, won the bronze in 22.30. American Muna Lee finished tied for seventh in 22.87.

After the race, Felix fielded questions with the poise of someone 10 years her senior. She said she had matured, adapting to life as a track professional a year after tearing up high school tracks in Southern California. Despite having the slight build of a middle-distance runner, Felix, a minister's daughter, tore confidently through four grueling rounds.

By the time Wednesday's race rolled around, Felix believed she could win the gold.

"Definitely part of me is disappointed," she said. "That's what I came here for . . . [but] I have a lot of confidence. I'm very excited about the future."

She and young U.S. stars Justin Gatlin, Lauryn Williams and Jeremy Wariner have stolen the headlines this week. Johnson, meantime, joined a list of disappointed veterans wondering what happened. Stacy Dragila didn't qualify in the pole vault. Gail Devers got knocked out of the 100 hurdles. Angelo Taylor didn't advance in the 400 hurdles. John Godina failed to medal in the shot put.

"Allen is my idol," China's Liu Xiang said. "I do regret that he's not in the semifinal. Without Allen Johnson, the race is still fully open."

Johnson shed no tears Wednesday, though he wasn't exactly in the mood for talking. He unzipped the top of his U.S. team uniform and sauntered through the media zone, waving away most of the cameras and questions.

"I'm fairly disappointed," he said during a brief interview. "But, hey, it happens. I'll be watching the final -- unfortunately."

What seemed so uncomplicated quickly got complicated as Johnson, 33, rumbled down the course. He struck the third, fourth and fifth hurdles hard, nearly tripping at one point. But he righted himself, somehow, smoothly sailing over hurdles six through eight.

"I thought I was in control," he said. "I thought I had pretty much gotten things back together."

Johnson had not. He barreled into the ninth hurdle as Latvia's Stanislavs Olijars won the heat in 13.26 seconds. Johnson said it was only the second time in his career he had ever fallen in a race. The other time came two years ago in South Africa.

Johnson won the Olympic gold at the 1996 Summer Games in Atlanta in a time that remains the Olympic record: 12.95 seconds. He went to the 2000 Summer Games in Sydney favored to repeat, but a hamstring injury that flared up two weeks before the Olympics slowed him. He finished fourth. Afterward, he was disconsolate.

"It's just hard dealing with it -- going from the gold to nothing, it hurts," Johnson said then. "I feel like the rug was pulled out from under me. From this day forward in my career, I'm starting over. I have nothing."

Johnson proceeded to salve his disappointment with a pair of world titles, but he kept his focus on these Games. He wanted to become the third high hurdler in Olympic history, along with Roger Kingdom and Lee Calhoun, to win two golds.

Johnson came here the favorite. He not only had the fastest time of the year, but he had put together one of his most dominant seasons. He won the U.S. indoor championships, the world indoor championships and 10 other significant races.

There were hints, however, that even Johnson was susceptible to the occasional collapse.

Prior to Wednesday, he had just one other mystifying race: the final at the July U.S. Olympic trials. He finished third, barely qualifying for these Games, after having dominated the early rounds of the event. Afterward, he was angry and confused. He said he had no idea what had happened.

Wednesday, he insisted there was no connection between his baffling final at the trials and his meltdown in the Olympic race.

"Those were two isolated incidents," he said. "It just so happens they were the two biggest meets of the year."

"Part of me is disappointed. . . . [But] I have a lot of confidence. I'm very excited about the future," said Allyson Felix, 18, after finishing 2nd to Jamaica's Veronica Campbell, left, in women's 200 final."I fell down. I don't know why," said Allen Johnson after tumbling headfirst from mishap in 110-meter hurdles.