There's probably no body part women obsess over more than their midsections. Because many women naturally hold onto body fat -- and bloat -- in this area, it can be one of the toughest to whip into shape.
"The muscles of men and women don't differ significantly though women tend to be longer waisted and wider through the pelvis," said Michele Olson, Ph.D., professor of exercise science at Auburn University in Alabama. "It's a challenge for us to develop nice flat, firm abs, especially after having a baby or as we get older."
A challenge, but not impossible. Here Olson describes the five best ab exercises for women based on her experience and how the "core" muscles of the abs, waist and back have responded to special testing by exercise scientists. "Doing five to 10 of each of these moves, two to three a week, is so much more effective than doing hundreds of crunches every day," she said.
Why it works: Tests show that even the deepest core muscles fire up during this exercise. At the same time, the hip muscles show a relatively low involvement so there's little risk of back injury. Olson said this move is especially good for new moms because it helps realign and strengthen the spine after giving birth.
How you do it: Kneel directly behind a large exercise ball with your palms on top of the ball, arms straight. Slowly roll the ball forward and away from you, stretching your body out as you go. Roll out as far out in front of you as you can while still maintaining balance and without losing control of the ball.
Hold at the end point for a moment then roll back to the start. Note: If you prefer, you can use an "ab wheel" instead of a ball.
How you do it: Lie on your back with your knees bent, feet together. Lift your bent legs off the floor so that your knees are aligned over your hips. Stretch your arms out at your sides a few inches above the floor, palms facing down. Pull your bellybutton in towards your spine.
Inhale, then exhale deeply as you lift your chin to your chest.
Keeping your core tight and arms rigid, pump your arms vigorously up and down a few inches; as you do this inhale deeply through the nose for five pumps and exhale through the mouth for five pumps until you have completed 100 pumps. As you get stronger, you can straighten your legs out.
How to do it: Lie on your left side with your left forearm on the floor perpendicular to your body and your right hand resting on your side. Stack your legs and hips and pull your bellybutton in towards your spine. Raise your body up by straightening and lengthening the waist so that you are balanced on your left forearm and the bottom of your left foot.
Hold that position for a slow count of 10. Repeat to the other side. As you get stronger, you can hold the position longer.
How to do it: Sit up tall on the floor with your legs out straight and palms down, fingertips pointing towards your feet. Maintain your tall and lifted spine as you lean back, balance on your sits-bones, and lift your legs up off the floor as high as you feel comfortable; use your fingertips to help balance and bend your knees as much as you need to accommodate for your level of strength and flexibility. Lift your arms up so your fingertips are alongside your knees.
Hold this pose for 10 slow counts as you focus on staying tall and keeping your chest lifted. As you get stronger, try to straighten your legs out.
How to do it: Lie on your back with your belly button pulled in towards your spine. Bring your knees in towards your chest and grasp your shins in your hands as you round your back to lift your head, neck and shoulders off the floor. Inhale deeply as you simultaneously extend your legs out straight about a foot off the floor and stretch your arms behind you so that your elbows are alongside your ears.
Exhale strongly as you sweep your arms around in a circle and pull your knees back into your chest.