Slide show: Core exercises with a fitness ball

One way to strengthen your core muscles is to do core exercises with a fitness ball. You can begin a core exercise routine with a fitness ball, or use one to liven up your current exercise routine.

It's best to use a fitness ball that's the right size for you. Balls are sized according to their diameter, with the correct size based on your height. The ball fits properly if your hip, knee and ankle line up at a 90-degree angle or slightly greater when you're sitting on the ball. Most people do best with a slightly deflated 55-centimeter (cm) or 65-cm ball.

The softer the ball, the less difficult the exercise will be. If you're just starting out, consider using a larger, softer ball.

Perform each of the following core exercises five times. As you get stronger, gradually increase to 10 to 15 repetitions.

Before you start strengthening your core muscles with a fitness ball, locate your deepest abdominal muscle — the transversus abdominis — by coughing once. The muscle you feel contracting deep in your abdomen when you cough is the transversus abdominis.

This muscle isn't the only muscle that makes up your body's core. But keeping it contracted when you do these exercises helps strengthen all of your core muscles.

• Hold the fitness ball in front of you so that your arms are parallel to the floor. Bend your knees and reach out (A). Keep your back straight, and don't let your knees extend beyond your feet. You should see your shoelaces if you look down.
• From the starting position (A), cough and hold to activate your transversus abdominis. Rotate your trunk and reach with the ball toward your left (B). Hold for three deep breaths — about five to eight seconds. Use your core muscles to pull yourself back to the starting position. Rotate to your right.
• Vary this exercise by holding the ball in a downward position (C) or an upward position (D).

• Starting out behind the ball, position yourself with your stomach resting on the ball and your feet touching the floor just behind the ball. Lean forward until you touch the floor with your hands.
• Keep leaning forward while walking your hands farther away from the ball.
• Stop when you feel the ball reach your upper thighs. Your feet will be suspended above the floor, and you'll be balancing on your hands and the ball. Keep your shoulders directly above your hands. Contract your abdominal muscles and hold the position for as long as you can maintain your balance and form.
• Challenge yourself by working the ball farther down your legs, to your shins and then to your feet.

Attempt this exercise only after you've mastered the plank exercise.

• Start in the plank position (A). Cough and hold to activate your transversus abdominis. Make sure your hips are square and your shoulders are in alignment with your hands.
• Push your body weight forward until your shoulders are in front of your hands (B).
• Hold this position for three deep breaths before returning to the start position.

Attempt this exercise only after you've mastered the plank exercise.

• Start in the plank position (A). Cough and hold to activate your transversus abdominis.
• Press your knees into the ball (B).Then use your abdominal muscles to bring your knees into your chest.
• Hold this position for three deep breaths before returning to the start position.

• Sit up tall on the ball with your feet resting on the floor, about hip-width apart. Keep your back straight and your head in alignment. Fold your arms across your chest so that your hands rest on the front of your opposite shoulders.
• Cough and hold to activate your transversus abdominis, then slowly lean backward until you feel your abdominal muscles kick in. Remember to keep breathing through the exercise.
• Hold this position for three deep breaths before returning to the start position.

• Lie on your back on the floor with your legs resting on top of the ball. Cough and hold to activate your transversus abdominis. Raise your hips and buttocks off the floor into a bridge (A).
• Hold this position for three deep breaths before returning to the start position. Besides your core muscles, you'll feel the muscles along your backside — the gluteals and hamstrings — contract to keep you in place.
• After you've mastered the bridge, challenge yourself and test your balance by raising first one leg and then the other a few inches off the ball (B).

• Begin in the bridge position (A). Keep your core muscles contracted and breathe deeply throughout the exercise.
• Keep your hips off the floor while you pull the ball closer to your buttocks with your feet (B).
• Dig your heels into the ball for traction and to engage the muscles along the back of your thighs (hamstrings).
• Hold for three deep breaths before returning to the bridge position.

• Lie on your back on the floor and rest your legs on top of the ball. Your legs should be about hip-distance apart. Cough and hold to activate your transversus abdominis muscle.
• To help protect your lower back, focus on pulling your bellybutton in toward your spine and keeping your abdominal muscles contracted.
• Squeeze your legs together — the ball will create resistance for your inner thigh muscles (A).
• Next, use your core muscles to raise the ball off the floor (B). Hold for three deep breaths and return to the start position.

• Lie on your back on the floor, your legs draped about hip-distance apart over the ball. Activate your transversus abdominis and focus on pulling your bellybutton in toward your spine. Grasp the ball firmly with your legs and use your core muscles to lift the ball off the floor (A).
• Keep your core muscles engaged and continue to grasp the ball between your legs while slowly letting your legs fall to your right (B).
• Before you reach the floor, use your core muscles to pull your legs back to the start position. Keep your shoulders on the floor throughout this exercise. Repeat on your left side.

• Lie on your right side, with the ball between your legs (A).
• Tighten your core muscles and raise your legs, with the ball, a few inches off the floor (B). Hold for three deep breaths before returning to the start position.
• Protect your neck by resting your head on your hand. If this places too much strain on your neck, straighten your arm along the floor and rest your head directly on it. This removes any pressure on the side of your neck.

This is an advanced exercise, so try it only after you've gotten the feel of working with the fitness ball.

• To challenge your balance, kneel on top of the ball. Kneeling on the ball forces your body's stabilizing muscles to work extra hard to keep your balance in check.
• Focus on maintaining your balance, and remember to keep breathing throughout the exercise. Hold the position for as long as you can maintain your balance and form.