And Tough Mudder! And Bootcamp! Every day, it seems, a new must-try program pops up at your gym or on TV or in your cousin’s garage. Many are absurd (we’re looking at you, Shake Weight). But some are well worth your time and sweat. The best of them keep your motivation high, challenge muscles you didn’t know existed, and help shed that layer of skinny-fat once and for all. Here, our seven favorite (and least crazy) exercise crazes currently making America’s heart race.