Many of their acolytes are busy women who not only find it more efficient to exercise at home, but more comfortable. Lorrayne Schaeffer, 41, a computer engineer in San Diego, exercises only at home, and only with Ms. Friedrich. "I feel intimidated at a gym," she said. "At home I can do it on my own time not have to compete for the equipment -- just me and Cathe and the crew."
According to Jill Ross, director of product acquisition at Collage Video, 8.5 million fitness DVDs were sold in the United States from July 2006 through June 2007; the average home video user is about 45, and most instructors don't hit their stride with viewers until around age 37. "You need a certain commanding presence," she said. "If Oprah was 22, would she be fun to watch?" (Incidentally, Ms. Ross said, brunet video stars generally are more popular than blond ones, because they are perceived as more serious.)