Crain clipped her rear wheel on a ramp and lost control over the front of her handle bars. She slammed her chest into the next ramp and remained on the ground for about a minute.
Crain managed to stand up and finish the ride, but was listed as DNF on the result sheet.
There was no immediate word on how badly Crain was hurt. If she's able to continue, she would be given the 16th and final seed in the women's competition for races on Thursday.
Crain was a late replacement on the U.S. team for Arielle Martin, who was hospitalized after a crash during a final training run on July 30 in California.