What a shame, but such a graceful exit from the sport. Here's part of the statement/article from USA Gymnastics, you can read the remainder, including an overview of her competitive career and links to various videos at the following link: http://usagym.org/pages/features/120603_johnson/index.html:

Shawn Johnson of West Des Moines, Iowa, who has four Olympic and three World career medals, today announced her retirement from competitive gymnastics. Her stellar career includes three World gold medals and one gold and three silver Olympic medals, and her gymnastics prowess and outgoing personality made her a fan favorite. Johnson, who has been on the comeback trail since 2011, was forced to cut her efforts to make her second Olympic team short due to recurring knee problems.

"I still have the heart, drive, and desire to compete and help the USA at the London Olympics," said Johnson, who also won two senior U.S. all-around titles. "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.

"Being given the opportunity again by Martha Karolyi and USA Gymnastics to represent the USA has been the greatest honor of this comeback," said Johnson. "Thanks to all of my sponsors for allowing me to represent them, and their support has been a dream come true. Also, my agent, Sheryl Shade, has truly given me the chance to pursue my biggest dream while sharing my story with the world. And to my fans and friends, I will be forever grateful. To my parents, Teri and Doug, I thank you for the love you have given me. To my coaches, Liang and Li Qiao (Chow), thank you for nurturing me throughout my life in the sport of gymnastics and the lessons of sportsmanship.

"I am announcing this 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," said Johnson. "I have had the honor to train and compete with them, and I am excited to join the gymnastics family and fans in cheering them on to victory. I will be there with every ounce of pride and energy to cheer on my girls and Team USA."

"Shawn is one of our country's most popular gymnasts, and she has been an integral part of the USA's success," said Steve Penny, president of USA Gymnastics. "Her personality, smile and powerful gymnastics won both medals and fans. Shawn contributed to two of our country's most successful teams in recent history – the 2007 World Championships Team, which won the USA's second World team title, and the 2008 Olympic silver-medal team. As an individual, she won two World gold medals, including the coveted all-around title, and three Olympic medals. We wish Shawn the best of success as she begins her next adventure and future involvement in the sport."

"Shawn has been an outstanding representative of USA Gymnastics throughout her career," said Peter Vidmar, chairman of the Board for USA Gymnastics and 1984 Olympic gold medalist. "I have tremendous respect for Shawn's dedication and hard work during her return to gymnastics for a run at her second Olympics. And while we celebrate her competitive achievements, we also appreciate everything she has done as a role model and ambassador for our sport. We look forward to having her continue to be a part of the gymnastics movement while she pursues the next chapter of her remarkable career."

"Shawn has brought much joy, grace, energy and great success to the gymnastics world," said Liang Chow, who has coached Johnson since she began in gymnastics. "My wife, Li, and I have enjoyed the many years we have had working with her. Shawn's dedication to gymnastics and her work ethic make her a great example for future generations to follow. Although her gymnastics career is over and we will miss her, we believe an even brighter future is waiting for her."

"Shawn will always hold a special place in gymnastics and my heart," said Martha Karolyi, the women's national team coordinator. "She always showed the joy of doing gymnastics. Shawn was an excellent student in training and a fierce competitor in the arena. I will miss her, but I am confident that she will be very successful in any endeavor that she decides to pursue."

Johnson is considering her plans for the future that include college, volunteer work and continuing to be involved in gymnastics. Johnson will make the trip to London this summer on behalf of some sponsors and have the opportunity to cheer for the USA.

"Looking to the future, I have many goals and plans I want to pursue," said Johnson. "I am going to college, want to work with philanthropic foundations, and hope to inspire new generations to follow their own dreams and pursue their passions. I also will continue to be involved in the growth of gymnastics because it has been an important part of my life for 15 years. I am very excited about starting the next part of my journey."