One of the secrets of the endurance of fitness trends, as opposed to fads, is creativity. Re-invention is key. Pilates, Zumba and indoor cycling don't refresh their class styles very often, and as consumers get accustomed to the class they also tend to get bored. Yoga, on the other hand, still attracts larger groups, but yoga folks reinvent themselves. They are always developing new forms of yoga, new presentations of yoga.