by Patrick Carney

Strengthen your core and look slimmer with these six exercises.

Stability balls are large, puncture-resistant balls, and look similar to a big beach ball. Originally, they were designed for physical therapy, but the bounce to exercise equipment was obvious. Just ask Robin Berghman, an occupational therapist. She uses the ball at work, with her patients, and at home, when she exercises.

"People who have never used one before think that it's easy. But after a few minutes on a stability ball, they start feeling it. Stability balls, just by design, require you to use many muscles—not just for the exercise, but to maintain balance."

Stability balls are a great way to work the body's core, which includes the abdomen, the muscles along your spine, and your hips. In addition to strengthening your core, these stability ball exercises will help you prevent injury, improve your posture, and appear slimmer.

1. Hip Bridge
Muscles worked: Gluteus maximus (your butt), which will strengthen your hips

1. Sit down on the ball and walk your feet out until your upper back is leaning against the ball.
2. Spread your feet slightly wider than hip width, and cross your arms over your chest.
3. Squeeze your glutes together, so that your hips rise to knee height. Pause for one second and then slowly release, letting your hips fall 10-12 inches.

2. Crunch
Muscles worked: Abdominals

1. Sit on the ball and roll yourself forward, until the middle of your back is on the ball. Keep your feet hip width apart.
2. Cross your arms over your chest, and place your fists between your chin and your chest. This will keep your head and neck aligned.
3. Contract your stomach to lift your shoulders off the ball. Hold for a second, and then lower your shoulders back onto the ball.

3. Plank
Muscles worked: Center and sides (obliques) of abdominals

1. Lie facedown over the ball, with both your hands and feet on the floor.
2. Walk on your hands away from the ball, and let it roll under you.
3. Stop when your knees are on the stability ball. You'll now be in a push-up position.
4. Tighten your abdomen and hold for three deep breaths, or as long as you can keep your back straight.
5. Roll yourself back to the original position. When you're ready for it, try rolling out further, so just your ankles are on the ball.

4. Alternate Superman
Muscles worked: Almost all core muscles, including the small muscles that hold your spine together

1. Lie facedown over the ball, balancing on your toes and fingertips.
2. Raise your right leg and left arm, so that they are parallel to the ground. Hold your arm straight out from your body.
3. Don't look up or let your chin drop to the ball. This will keep your neck in alignment with your spine.
4. Hold this position for 3 to 10 seconds.
5. Lower your arm and leg, and repeat with your left leg and right arm.

5. Back Extension
Muscles worked: Lower back

1. Lie facedown on the ball, with your hips centered on the ball. Extend your legs and plant your feet about shoulder-width apart. You can also put your feet against a wall for more stability.
2. Put your hands behind your head.
3. Bring your chest and head up from the ball. Bring your chest up as high as you feel comfortable and hold for a few seconds.
4. Slowly lower your chest and head back to the ball.

6. Ball Twists
Muscles worked: Obliques and chest

1. Standing straight, hold the ball by your palms, at arm's length from your body. Keep your knees slightly bent.
2. Squeeze the ball and keep squeezing throughout exercise.
3. Slowly twist your shoulders to the left, while keeping your hips straight.
4. Return to center, and then twist your shoulders right.