Some seasons it’s cardio, other moments glorify the upper body, a few years ago it was all about the yoga butt. But now, like trans fats or Marc Jacobs handbags, core strength is taking up space in the zeitgeist. I’ve been inundated with the same message in yoga studios, at my chiropractor’s office, and even at my local knitting café: pull in the belly, line up the spine. They say it improves posture, eliminates pain, and, of course, makes you––whatever your gender––look smokingly hot in a sweater.