Forget the Gym: This Service Charges Less the More You Workout

The fitness chain business model, as is widely known, relies on you not working out. A gym sells more memberships than it can comfortably handle, based on the assumption that most of its members won't show up. But what if there were a gym where you paid less per month the more you worked out?

That's kind of the concept behind Fitmob, a website and iOS app that launched Thursday, except that Fitmob shuns gym locations in favor of outdoor classes and indoor spaces rented by the hour. Anyone can sign up to run a class, which gets a star rating and reviews from participants