06/03/2012 at 05:00 PM EDT
, one of the most popular gymnasts in recent memory, has announced her retirement from the sport. She was hoping to make the 2012 Olympic team.
"I still have the heart, drive, and desire to compete and help the USA at the London Olympics," Johnson – who had been making a comeback from recurring knee problems – announced in a statement
"Unfortunately, it has become obvious that my left knee is not able to sustain the demands of gymnastics any longer," said the gymnast, 20, who won four Olympic and three World career medals. "All I can do now is gracefully retire and thank everyone who has believed in me and my journey."
In addition to her gymnastics accolades, she also won her season
of Dancing with the Stars
Johnson added that she made the announcement now "so the focus for the next three weeks will be on the incredibly talented gymnasts who are trying to make our 2012 Olympic Team." She will still attend the Games to cheer on her former teammates.
The gymnast initially injured her knee in a January 2010 ski accident and feared her long-term health would be at risk if she continued competing, the Associated Press reports
. When she tried to increase her training in preparation for London, her knee did not hold up, and continued training for the Games could mean a total knee reconstruction in the future.
"It's been a really hard decision. How can you tell yourself, 'No, I think it's time to say it's finally done,' " Johnson told the AP. "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."
"It's weird, for the first time in my career I came up short. But I feel like I succeeded as well," added Johnson. "It almost came too easy the first time. It was a humbling experience this time around."
The Olympian, who helped the American team earn a silver medal in Beijing in 2008, won silver in the all-around, silver on the floor exercise, and gold on the balance beam – the only U.S. woman to do so apart from Shannon Miller in 1996.