Athing Mu got tangled up in the middle of the track and started falling. One hand hit the ground, then the other. As she rolled onto her back, her bright pink shoes started pointing toward the sky.

With that, one of America's most promising runners saw her hopes of back-to-back Olympic titles in the 800m go down the drain, while sports fans got a refresher on just how unforgiving these US track trials really are.

The 22-year-old from New Jersey became the first big-name casualty of the trials Monday, victimised by a bunched-up pack in the backstretch of the first lap, to say nothing of the long-standing rule in the US that only the top three finishers at trials make the Olympics, regardless of their resume.

"I've coached it, I've preached it, I've watched it," Mu's coach, Bobby Kersee, told The Associated Press. 

"And here's another indication that regardless of how good we are, we can leave some better athletes home than other countries have. It's part of our American way."

Mu filed an appeal and USA Track and Field officials sorted through the replays deep into the night, but eventually denied the protest. Kersee said Mu got spiked, had track burns and hurt her ankle.

"She's going to be licking her wounds for a couple of days," Kersee said.

Mu got back to her feet and finished, but was more than 22 seconds behind the winner, Nia Akins, who ran 1 minute, 57.36 seconds. Mu was choking back tears as she headed quickly off the track and through the tunnel after the race. She did not do interviews.

She was racing on the outside in a tightly bunched pack and looked to be veering to her left toward the eventual third-place finisher, Juliette Whitaker, when she tripped and went tumbling, leaving three runners behind her flailing as they jumped over and around her.

Mu is hardly the first athlete to have this happen. One of the more memorable and heartbreaking moments on this track came eight years ago in the same event, when Alysia Montano, looking to return to the Olympics, got tripped up in the home stretch and stayed down on the track crying.

"I have a little mama bear feeling," said Montano, who is at the track this week doing in-house interviews over the PA system. "But the race is brutal sometimes. It's two laps, a tight race and everyone's feeling scrappy to try to figure out what position they want to get into."

The Olympic trials marked Mu's first meet of the year after dealing with injuries all season. She looked to be in good form in her first two rounds, and Kersee said her season was coming together.

But in the 800 final, she barely made it half a lap.

Despite the fall, Mu could still go to Paris as part of the US relay pool; she was a key part of the team's gold-medal win in the 4x400 three years ago in Tokyo.

After winning NCAA, national, world and Olympic championships all before turning 21, Mu won a bronze medal at the worlds last year and, afterward, conceded she needed a break from all the pressure, social media and other demands that came along with being tagged as one of track's great new stars.

In interviews leading into this week's meet in Eugene, she said she had rediscovered her love for the sport and was looking forward to the quest to become a back-to-back champion.

She has dominated this distance thanks, in part, to a long, loping stride, and that might be what cost her in a race where she came in as the favourite.

"I heard it and I was just like 'OK, keep running, it wasn't you,'" second-place finisher Allie Wilson said of the commotion that resulted in Mu's tumble. 

"That, unfortunately, is part of racing. Things like that can happen."

AP