Lie down on back, crossing one leg over the other while positioning legs up approximately 4-8 inches off floor keeping legs straight. Place arms along side of body with hands under glutes. In a controlled motion, keeping legs straight, split legs apart as far as possible. While maintaining the controlled motion, draw legs together switching legs alternating one on top of other. Repeat to desired number of repetitions.
Here are a few suggested alternative exercises:Ab CrunchAb Crunch ObliquesBall Crunch TraditionalDouble CrunchDumbbell Side BendsLateral Bend with Barbell