Getting a flat stomach means more than just working the six-pack ab muscles. To really slim down your waistline, you'll need to work the muscles at the insides of the midsection. The internal core of the abdominals includes the internal obliques, a corset-shaped set of muscles at the sides and back, located underneath the external obliques. Under those is the transverse abdominus, the innermost set of abdominal muscles responsible mostly for breathing. Ideally, you should do exercises that work all parts of the abdominals to prevent injury to your lower back, but you can work on isolating the obliques using a number of different exercises.
Among common abdominal exercises, the Captain's Chair is the most effective for working the obliques, according to the American Council on Exercise. To do the exercise, place your forearms on the vertical bars of a captain's chair at the gym and grip the hand grips. Then brace your core as you lift your legs upward, either bending your knees or keeping them straight. If you don't have the proper equipment to do the Captain's Chair, you could also hang from a horizontal door frame or bar and do a similar exercise that will work the obliques.
Dumbbell Fly on Ball
Stability ball exercises can be great for the core -- and a few will work the obliques and transverse abdominus, as well as muscles in the back, chest and arms. One such exercise is the dumbbell fly on an exercise ball. Grasp a set of dumbbells and position your body on top of the ball, with your feet flat on the floor and your shoulders and upper back resting on top of the ball. Squeeze your buttocks and abdominals to hold your torso parallel to the floor. Start with the dumbbells straight above your chest and the knuckles of your hands facing one another. Then "fly" your arms out to the sides. Move your arms back to the starting position, all the while keeping your torso parallel to the floor. By the time you get to 12 repetitions, your arms and abs will probably be burning.
Any exercise that forces your body into the plank position -- which may look like the up position of a push-up -- will work the core muscles. As such, simply doing planks will help your cause. To ramp it up a bit, try it on a stability ball. Start by resting your midsection over the top of the ball, with your feet resting on the floor for stability. Place your hands on the floor and then walk your hands forward, causing the ball to roll underneath you as you move forward. Stop when your feet rest on top of the ball, holding your body in a flat plank position. Then walk your hands backward to start it all over again.
Exercises with a twisting motion may also help engage the internal oblique muscles. The bicycle crunch is another of the most effective exercises among the most common ab workouts, according to the American Council on Exercise. Lie on your back, bend your knees and rest your feet on the floor. Then place your hand behind your ears and raise one knee to meet the opposite elbow. Quickly move back to starting and touch the other knee to its opposite elbow.
Nicole Vulcan has been a journalist since 1997, covering parenting and fitness for The Oregonian, careers for CareerAddict, and travel, gardening and fitness for Black Hills Woman and other publications. Vulcan holds a Bachelor of Arts in English and journalism from the University of Minnesota. She's also a lifelong athlete and is pursuing certification as a personal trainer.