This exercise activates all of your abdominal muscles.

1. Lie face up with your left knee bent, left foot flat on the floor, and right leg extended toward the ceiling.
2. Reach toward the ceiling with your the left arm and keep your right arm down by your side.
3. Without moving your hips or shoulders, open your raised leg to the right and raised an arm to the left.
4. Now, concentrating on your abs, return your raised leg and arm to the center. Do 2-5 reps, then switch sides and repeat.

Source: Health