Online gyms were introduced in the US in 2007. For the past four years, they’ve also been catching on in Europe. Fitness enthusiasts can choose among many different kinds of courses, including step classes, Pilates and weight training. Major providers also offer personal trainers to help maintain customer loyalty. It’s a real boon for people who don’t have time to go to an actual gym. But experts suggest that absolute newcomers should first do a few classes with a real-life trainer.