Answer from David Arrand: The only similarity between the two is that they are both performed on a bike and involve mostly lower body exercise,and even then,different machines. I cycle outdoors and I teach indoor cycling. Although both spinning and RPM indoor styles adopt methods to try to simulate outdoor cycling as much as possible, it just isn't possible.However because they do basically work the same body parts, and are great cardio exercises,each compliments the other greatly and worked together they will help you achieve perfection in both.

Answer from Ken Moody: Celia, consider that when you're on a bike outside you must: stabilize the bike; steer; brake; and cover actual ground. (tires against a surface) Spinning involves none of this. The bike is stationary, so no stabilization is necessary. And because the bike is going nowhere, no steering is necessary, and no braking is necessary. But the most important difference between Spinning/Indoor cycling and outdoor cycling is that in Spinning there is an almost 40 pound silver wheel that creates that smooth ride we all experience. It also creates momentum in a way a bike on a road does/can not. And I agree that because both activities utilize mostly the same muscles, although not in exactly the same way, both "feed" each other beautifully.