There’s a reason group cycling classes draw in such a crowd: There’s nothing quite like the endorphin high you get as you sweat through hill climbs, race through intervals, then catch that last gust of tailwind as you zoom toward the finish — all on a specially designed indoor bike that mimics the feel of a true road bike. Group cycling burns an incredible number of calories and builds cardiovascular fitness like nobody’s business, but it’s also low-impact enough for anyone to try.