There was a great article recently in The Economist about group exercise, and how it is easier with friends. Basically the idea is that people can push themselves harder in a group than when they are by themselves....mostly because of hormonal responses. They go on to explain a possible evolutionary benefits, such as hunting in groups. The researchers in the study used rowers, and if you are into that sort of thing, be sure to check out the Indo Row program with Engine Room Fitness. But the results would likely be just as applicable to spinning, indoor cycling, old fashioned step aerobics, or just about any synchronized group workout. Good news for group workouts huh?