Gymnast Shawn Johnson, who won four Olympic medals, including a gold, and was the all-around world champion in 2007, announced on Sunday that she has retired, the Los Angeles Times reported.
The 20-year-old Johnson said an injured knee had prevented her from properly preparing for the London Olympics.
"Finally, time ran out," she told the Times.
Her announcement came four days before the start of the U.S. gymnastics championships.
After winning four medals at the 2008 Games, including a gold in the balance beam, Johnson took the next two years off. She won "Dancing with the Stars," and she considered competing in London until she injured her knee in January 2010 in a skiing accident.
The long layoff along with the torn ACL, MCL and meniscus never allowed her to return to full strength. She tried to get ready for the 2012 Olympics, but her knee would not let her train properly.