In theory, cycling is cycling. The pulse of the music, the exhortations of your instructor and the presence of a group of like-minded exercisers do nothing to spin your pedals. In practice, though, the ingredients of a typical indoor cycling class somehow combine to lift workouts to heights that most participants wouldn't achieve on their own. The alchemy of group exercise is well known to runners and aerobics classes, but spinning has found a recipe so powerful that researchers studying it have been forced to re-evaluate their definition of “maximal” exercise – and sound a warning for beginners who may wander into a class unprepared.

The current incarnation of group indoor cycling dates back to 1987, when South African-born cyclist Jonathan Goldberg first organized training sessions in the style he later trademarked as ‘Spinning.’ These days, more than half of Canadian sports clubs offer group cycling classes, according to figures from the International Health, Racquet and Sportsclub Association. In the United States, 1.6 million people took classes in 2007, while a further 10.6 million pedaled solo. In a typical class, the instructor leads the class through a ride that varies dramatically in intensity, increasing and decreasing resistance to simulate hills and headwinds. Crucially, each person controls the resistance on her own bike – only the broad contours of the workout are synchronized.

‘Successful instructors turn out to be really good at motivating people to push harder,’ says Carl Foster, an exercise scientist at the University of Wisconsin, La Crosse, and past president of the American College of Sports Medicine.