Pilates is popular with stars and athletes. It concentrates on the body's "core" and is a great way to tone, stretch, and strengthen your muscles without straining the joints. It is sometimes used in physical therapy and rehab. Your local gym may have pilates mat classes, but make sure the instructors have experience and more training than a weekend workshop. If you're really interested, and can spend more money, look for a studio or facility with highly trained instructors and special pilates equipment.