The knee injury that sparked Shawn Johnson's comeback put an end to it, too.

The Olympic gold medalist announced her retirement from competitive gymnastics Sunday, four days before the start of the U.S. championships. She has had repeated setbacks with her left knee, seriously damaged in a January 2010 ski accident, and she feared she was putting her long-term health at risk by continuing with her attempts to make a second Olympic team.

"It just little by little gets worse and worse," Johnson told The Associated Press. "My body is to the point where I need time to rest and retire so I can be healthy for the rest of my life. It's hard to wrap my mind around. Gymnastics has been my entire life."

The decision did not come easily. After winning four medals at the 2008 Games - only Michael Phelps, Natalie Coughlin and Nastia Liukin left Beijing with more - Johnson took the next two years off. She won "Dancing with the Stars" and, with her bubbly personality and girl-next-door looks, became a bona fide celebrity.