Sit sideways on a bench and grasp the edges of the bench for support. With bent knees, raise your legs slightly and then straighten them. Lean backward as balance requires. Next, lift your knees up towards your chest while keeping your lower legs pointed downward and flexing (tightening) your abs as hard as you can. Keep continuous tension on the abs throughout this movement. This works both upper and lower abdominal muscles.
Here are a few suggested alternative exercises:Ab CrunchAb Crunch ObliquesBall Crunch TraditionalDouble CrunchDumbbell Side BendsLateral Bend with Barbell