Soldiers participate in physical therapy while using a prosthetic brace called the Intrepid Dynamic Exoskeletal Orthosis (IDEO), which allows them to use and strengthen severely injured legs. John Moore/Getty Images hide caption

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Sean Morey, then with the Arizona Cardinals, celebrates after blocking a punt against the Seattle Seahawks in 2007. Morey, who suffers from post-concussion syndrome, retired from the NFL in 2010 on the advice of doctors. Stephen Dunn/Getty Images hide caption

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