Martin's family said in a statement late Sunday that the medal contender "had a few setbacks" after she was upgraded from intensive care to a trauma unit. Martin had a severe laceration to her liver and a collapsed right lung in the crash during a final practice July 30.
USA Cycling did not announce the injury until Wednesday, when it said that 19-year-old Brooke Crain would replace Martin on the Olympic roster. The team left for London the same day.
The BMX competition begins Wednesday with seeding runs at the Olympic Park.