This is an excerpt from Developing the Core by NSCA -National Strength & Conditioning Association & Jeffrey M. Willardson.
Lie faceup on the floor with your legs bent. Place the arms and hands across the chest. Your upper back should be slightly off the ground to maintain constant tension on the target muscles. Bring your knees up toward your chest, bending them at a 90-degree angle. Contract your abs to raise your hips up off the floor slightly, raising your legs in the process. Return to the start position, and continue for the desired number of repetitions.
To increase intensity, place hands behind the head or overhead.
Assume a quadruped (all-fours) position, chin up, spine in a neutral position. Simultaneously extend your right leg and left arm so they are parallel to the floor. Do not allow the hips to rotate outward. Hold this position for the desired amount of time, and then repeat with the opposite arm and leg. Continue for the desired number of repetitions, alternating sides with each repetition.
Lie on your back with your arms out to the sides and palms flat on the floor. Keeping your legs straight and feet together, raise your thighs so that they are perpendicular with the ground. Keeping your upper back pressed to the floor, slowly lower your legs directly to the right. Raise your legs back to the start position, and repeat the process on your left. Alternate from side to side for the desired number of repetitions.
Reverse Pendulum Medicine Ball Twister: Bend your knees and perform the reverse pendulum movement as described. If the movement becomes easy, place a medicine ball between your knees or thighs.
Read more from Developing the Core by the National Strength and Conditioning Association (NSCA) and edited by Jeffrey Willardson.