Exercise ball crunch
Lie on your back on an exercise ball, with your hips toward the front of the ball. Place your hands behind your head, with your elbows pointing out the sides. Tighten your abs, and use them to pull your head, shoulders and upper-back up and forward. Your lower back should stay pressed to the ball. Your hands should support your head, without pulling it forward (keeping your elbows straight out to the sides will help you avoid pulling on your neck). Pause, then slowly lower yourself to the starting position.
Lie on your back and lift your legs straight up in the air with a slight bend in the knees. Place your arms flat on the floor straight out from your sides. Keeping your abs tight, ease your legs down to the right as far as you can while keeping your lower back flat on the floor. Use your abs to pull your legs back to the starting position. Then lower your legs to the left.
Lie on your back with your hands behind your head. Raise your legs directly above your hips, maintaining a slight bend in your knees throughout the exercise. Tighten your abs, and use them to raise your torso upward (your lower back should remain flat on the ground). As you lift, move your left leg toward your head and move your right leg away from your head almost to the ground. Simultaneously twist your torso slightly, pulling your right shoulder toward your left leg. Then switch legs and arms, pulling your right leg toward your head and your left shoulder toward your right leg, and moving your left leg toward the ground. Keep your legs and shoulders off the ground throughout the exercise.
Plank (Double- and single-leg)
Begin on your knees in a modified push-up position, resting on your forearms instead of your hands. Angle your forearms so that your hands are clasped lightly in front of you, and align your elbows under your shoulders. Tighten your abs and lift your knees off the floor until your body is in a straight line from the top of your head to your heels. For a single-leg plank, lift one leg a few inches off the floor so that you're supported by the opposite leg. Hold for the recommended length of time.
Medicine ball front raise and twist
Stand with your feet shoulder-width apart and a slight bend in your knees. Hold a light medicine ball in front of you with both hands. Keeping your abs tight, your eyes straight ahead and your shoulders down, lift your arms straight up and forward as if reaching for the wall in front of you. Stop when your arms are parallel to the floor. Then, keeping your lower body and arms still, twist your torso to the left as far as you can. Return to the center, and smoothly lower your arms to the starting position. Repeat, twisting your torso to the right.
Medicine ball crunch
Lie on your back the floor or an exercise ball. If you use a ball, your hips should be toward the front of the ball. Your lower back should stay pressed to the ground or ball. Hold a medicine ball on your chest, just below your collarbone. Tighten your abs, and use them to pull your head, shoulders and upper-back up and forward. Keeping your abs contracted, lower yourself to the starting position.
Lie on your back with your hands behind your head. Lift both legs straight up in the air, maintaining a slight bend at your knees. Tighten your abs, and use them to raise your hips straight up off the ground. Hold for one second, then slowly lower to the starting position.
Sit on the ground with your upper body upright and your knees bent with your feet flat on the floor. Tighten your abs and slowly lean back, while lifting your left leg in the air and straightening it. Stop when both your torso and your leg are at a 45-degree angle to the ground. Make sure you have your balance, then lift and straighten your right leg until itt's parallel to your left. You can use your arms to stabilize yourself, but once you have your balance, lift them up until they are parallel to your legs. Hold for the suggested amount of time.