But wait, there’s more: through these infomercials, P90X has grown into a major player in exercise DVDs, one of the few growth spots in an otherwise shrinking DVD market. Beachbody, the Santa Monica company behind P90X, has expanded into workout DVDs and infomercials tailored to particular audiences. Its Body Gospel, for instance, is aimed at Christians. There is also Tony & the Folks for seniors and TurboFire for women. On top of that is a range of supplements and fitness gear.

Mr. Horton may be the face (and biceps) of P90X, but the man behind the curtain is Carl Daikeler, who has been plying the infomercial trade since the 1980s. His first production was for an industry that isn’t exactly known for its quads: accounting. Later, he produced infomercials for all kinds of pitches, be they dating services or eight-minute abdominal workouts.

His breakout idea was to create a workout program that was so hard that he dared TV viewers to try it. In 2002, he and his business partner, Jon Congdon, took that pitch to Mr. Horton, who had starred in an exercise video called Power 90. The result, released in 2005, was P90X — X for “extreme.”

The early P90X infomercials bombed. But that changed when, at Mr. Daikeler’s urging, customers like “Dallas C.” and “Kristy M.” began sending in before-and-after pictures, now featured on the company’s infomercials and Web site. More than three million copies have been sold since then, with sales increasing every year through 2010 (they are currently running even with last year), company officials said.

Now Mr. Daikeler, 47, wants to more than double his annual sales to $1 billion. To do so, he will have to move beyond the buff clientele who have embraced P90X to an even bigger market: Americans who are overweight or nowhere near as fit as they need to be to keep up with P90X.