Target Muscles: internal and external obliques
Action: Contract your abdominals and alternate dropping your knees to each side of your body, leaving a few inches of clearance between your calves and the floor. One knee drop on each side equals a single repetition. Do three sets of 12 to 15 reps.
Stability Ball Roll-Up
Target Muscles: rectus abdominis (lower segment)
Set Up: Lie face down on a stability ball with your hands flat on the floor and your wrists directly under your shoulders. Walk your hands forward until your shins are resting on the surface of the ball.
Action: Contract your abdominals as you pull your knees toward your chest to move the ball closer to your body, pointing your tailbone toward the ceiling. Pause before extending your legs back to the starting position. Do three sets of 10 reps.
Target Muscles: transverse abdominis
Set Up: Lie facing downward on a mat. Press up onto your forearms, ensuring that your elbows are directly under your shoulders and your body forms a straight line from the top of your head to your feet.
Action:Contact your abdominals and hold the position for 60 seconds. Keep your eyes on the floor below your head and don’t let your hips drop toward the ground. Rest for 30 seconds before performing the move two more times for a total of three reps.
(via Oxygen Women’s Fitness | Training | Beach Body Abs Bonus)