[...] what's driving sales of these garments? "It's like this competitive thing we have with other women," says Mary Pantier, a 40-year-old yoga instructor in Erie, Colo., who accidentally flashed her Spanx, worn under her workout ensemble, while in a downward-dog pose in class. The great-grandmother of shapewear, the corset, was "the most controversial garment in the history of fashion," says Valerie Steele, director and chief curator at the Museum at the Fashion Institute of Technology, who wrote a book about the rigid, uncomfortable garment.