Sit back and low in the saddle. Before you pedal another mile (or even start out) in this high-energy stationary-bike class, ask the instructor to help you adjust your seat and handlebars. Sitting too high and hunched close to the handlebars puts pressure on your knees. "Plus, you'll be doing the majority of your work with your quadriceps," says Johnny G, creator of the Spinning program. Sit a little lower and farther back and your glutes, calves, and hamstrings will do the work―especially if you keep your heels down (no toe-pointing).
Also remember to...
Stay in control of your jumps. When doing jumps―quick intervals of standing and sitting―many people never sit or stand fully; they move forward and backward in a push-up-like motion, which is hard on the back and the elbows. Doing this means you are "out of control," says Johnny G, and you should either slow your jumps to half-time (do one for every two the instructor does) or put more resistance on your flywheel, which will force you to pedal more slowly and have more control over each revolution.