Bob Karch is the chairman and a professor in the Department of Health and Fitness at American University. Here, he helps cross-country champion Steve Hallinan check his heart rate while cross-training on the bike. Allison Aubrey/NPR hide caption

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University of Florida quarterback Tim Tebow embraces his mother, Pam, during a November 2009 pre-game ceremony before an NCAA college football game against Florida State. The two are set to appear in a controversial anti-abortion ad that will air during the Super Bowl. Phil Sandlin/AP hide caption

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Lowell Bailey (in purple), Bill Demong (bib 61) and Tim Burke (bib 37) competed against each other when they were 9 and 10 years old — and now they're headed to the Winter Olympics in Vancouver. Courtesy of the Demong family hide caption

itoggle caption Courtesy of the Demong family

Ski cross skiers at the Nature Valley World Cup ski cross final in Lake Placid, N.Y. Graham Smith/NPR hide caption

itoggle caption Graham Smith/NPR