The "core" of your body is where your center of gravity is located — around your trunk. Strong core muscles provide support to your spine for everything you do — whether you're walking, lifting, standing or sitting.

Over time, a weak core can leave you susceptible to poor posture and injury. For instance, if you have weak core muscles, you may be more likely to experience nagging lower back pain.

But you can work to strengthen your core muscles, just as you would your biceps or triceps to strengthen your arms. In the following slides, Charles Davis, a physical therapist at Mayo Clinic, Rochester, Minn., shows you how.

To work your body's core, engage your deepest abdominal muscle — your transversus abdominis — by coughing once. The muscle you feel contracting deep in your abdomen is your transversus abdominis.

The transversus abdominis isn't the only muscle that makes up your body's core. But by concentrating on keeping this muscle contracted throughout these exercises, the rest of your core muscles get a workout, too.

Once you've learned to contract your abdominal muscles, you can begin the following exercises. As you start out, do each exercise five times. As you get stronger, gradually increase to 10 to 15 repetitions.

This exercise works many of your core muscles in combination. Lie on your back with your knees bent (A). Keep your back in a neutral position — not overly arched and not pressed into the floor. Avoid tilting your hips up. Cough to activate your transversus abdominis.

Holding the contraction in your abdominal muscles, raise your hips off the floor (B). Align your hips with your knees and shoulders. Hold this position for as long as it takes to complete three deep breaths — about five to eight seconds. Return to your starting position and repeat. For a challenge, try alternately extending one knee while maintaining the bridge position.

With each of these exercises, coordinate your breathing with the activation of your transversus abdominis. If you don't, you won't benefit from these exercises.

To work your muscles more completely, vary your routine:

Opposite hand on opposite knee. Push your right hand against your left knee while pulling your knee toward your hand (A). You'll be pushing and pulling across the center of your body. Repeat this exercise using your other hand and leg.

Hand on outside of knee. Place your left hand along the side of your left knee (B). Use your hand to push your leg in toward the center of your body. At the same time, create resistance by pushing your knee away from the center. Repeat using your other hand and leg

To work your muscles more completely, use these variations:

Opposite hands on opposite knees. Place each hand on the opposite knee, toward the inside of your knee (A). Your arms will cross over each other. Push your hands against your knees and create resistance by pulling your knees in toward your hands.

Hands on outside of knees. Place your hands along the sides of your knees (B). Use your hands to push your legs in toward the center of your body. At the same time, create resistance by pushing your knees away from the center.