BOSU Ball Kneeling Ab CrunchCheck out the video
Place a Bosu Ball on the floor in front of you, ball-side up. Position both your knees on the ball with your hands on the floor beneath you for support.
Lift and extend your right leg behind you, reaching your heel back and up toward the ceiling.
At the same time, raise your left arm off the floor and extend it out in front of you. Pull your left elbow and right knee together and repeat.