This exercise works the upper part of the rectus abdominis.
Lie down into supine position with your knees up, your feet on the ground (but you may also place your legs on a bench). Arms should be next to your body in a straight position, face forward. Slowly lift your shoulder in such way that your shoulder blades should elevate from the ground at least 3-5 cm. Hold this position for a second then go back to the starting position and repeat the exercise in a proper repetition number. Inhale when you go back to the ground, exhale when you elevate. During the crunches try to keep your waist on the ground.