Traditional gyms don’t do much in the way of building community or routine. “You pay a fee and the gym is kind of hoping that you don’t show up,” says Jason Kelly, the New York bureau chief of Bloomberg News, and author of the book 2016 book Sweat Equity: Inside the New Economy of Mind and Body. “They already have your money.” Over the past decade, that solo-gym experience has been supplanted by pay-by-class group fitness. “A lot of people do better if they feel like someone’s telling them what to do,” says Kelly. These boutique fitness companies have grown like ivy, snaking their way through strip malls and blossoming in cities. They offer highly-specialized group workouts, like spinning in a dark studio with blaring music, or pilates in a dark studio with blaring music, or yoga in a dark studio with blaring music—you get the idea. Between 2013 and 2017, boutique memberships in the U.S. grew by 121%, boosting the fitness industry’s worth to $25.8 billion. Traditional gym memberships only increased by 15%. Read more at Quartz.