Plyometric exercises are high-intensity, forceful moves designed to increase muscle speed and power. Originally used in training drills for athletes, plyometrics are finding their way into gym workouts and group exercise classes.
Plyometrics often involve jumping. In a step class, for instance, a plyometric approach would be to jump off and onto a platform rather than stepping on and off. Some standard resistance exercises, such as push-ups, can also be "plyometric-ized."
On the down side, plyometrics can be hard on the joints, and should be avoided if you've had injury to the knees, ankles or other joints.
Fortunately, some lower-risk plyometric programs do exist, such as the Urban Rebounding classes offered at many Bally Total Fitness clubs. (A rebounder is a trampoline-like apparatus that absorbs some of the impact.)