To the happy, you-gotta-move-with-it music, Karen Lauer pedals fiercely on her stationary bike at the front of the room, urging participants in her spin class to do the same. As the song trails off, ...

Spinning might look about the same as outdoor cycling or riding a stationary bike, but in many ways, it’s a far more intense workout—and one of the easiest to overdo. First, there aren’t many (if any) ...