Shawn Johnson

Four-time Olympic medal winner Shawn Johnson announced her retirement from gymnastics Sunday.

Johnson, 20, failed to make the 2012 Olympic team after suffering from recurring knee problems.

"I still have the heart, drive, and desire to compete and help the USA at the London Olympics," Johnson said in a statement posted on her Facebook page.  "Unfortunately, it has become obvious that my left knee is not able to sustain the demands of gymnastics any longer. All I can do now is gracefully retire and thank everyone who has believed in me and my journey."

Check out photos of Shawn Johnson

Johnson, who also won three World career medals, still plans to travel to London to cheer on her former teammates. She first injured her knee in a January 2010 ski accident and feared her long-term health would be at risk if she continued to compete.

In addition to winning a gold medal on the balance beam, silver on the floor exercise, silver all-around and helping the American team win silver, Johnson also won her season of Dancing with the Stars back in 2009.