A Big Bounce

The Chicago Bulls and the Olympic Training Center in Colorado Springs use them. The key to the balls' success: no back support. Either you sit up straight, or you fall off. When you do it right, it improves you posture and strengthens your back muscles.

The ball is useful for blowing off steam; you bounce to release anger, or stretch and roll when you're tense. "It's like sitting on a big Prozac," says an anonymous ball-riding veteran.

If you do try the ball, go slow; even trained athletes are sore after the first day on it. Warm up by rotating your upper body and rolling your hips. Then try these muscle-loosening exercises, recommended by Deby Harper, a consultant for the Fitness Co. of Scottsdale, Ariz., which distributes the balls for office use (480-443-9611, $15 to $45):

Upper Back and Neck

Place both hands palms down on the ball behind you. Slowly bring your head toward your chest, and gently tuck your chin into your chest. Hold your 10 to 15 seconds, breathing deeply. Relax as you gently lift through your upper torso, and slowly return to the starting position.

Shoulder and Upper Back

Face your desk with feet flat on the floor. Slowly lower your upper body toward the edge of the desk, gently placing your forehead on the edge. Reach your opposite end of the desk by walking your fingers outward. Hold the stretch as you breathe deeply. Relax and return to a seated position.

Hamstring Stretch

Sit with feet flat and knees bent. Keeping feet on the floor, slowly straighten your knees until you feel a comfortable pull behind your thigh (not on your back). Hold 60 seconds. Repeat.