The P90X workout program is a form of cross training, mixing everything from weight lifting, to yoga to cardio. It's just one of dozens of workout DVDs sold by Beachbody, a company that makes $400 million a year. CEO Carl Daikeler told "Nightline" P90X was originally designed for people already in shape: "P90X started as 'we're gonna sell fitness to the fit, basically,' he said.

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