Why Gymnast Shawn Johnson Is Dropping Out of the 2012 Olympics

After leading the U.S. gymnastics team to the silver medal in 2008, and winning a gold one herself, Shawn Johnson is bowing out of this year's summer games in London -- and all the Olympics, for the foreseeable future.

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"I still have the heart, drive and desire to compete and help the USA at the London Olympics," said Johnson, 20, announcing her retirement in a statement on Sunday. "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."

Johnson, a fan favorite in Beijing, won the gold for the balance beam and the silver for the floor exercise and the all-around; capitalizing on her Olympic victory, a la Apollo Ohno, she later ventured into reality TV territority to win the mirror-ball trophy on Dancing With the Stars in 2009.

In January 2010, she injured her left knee in a ski accident and things did not improve in time for London. In recent weeks, she sat out from training because of her swollen knee.

"It's been a really hard decision. How can you tell yourself, 'No, I think it's time to say it's finally done,'" she told The Associated Press. "I'd like to be 30 and have kids and run around with them. It became more about my future life than this future one moment. I'm looking at the bigger picture of things."