Place ball at low back area and place feet wide to stabilize the body. Cross arms over chest. In a controlled motion, keeping neck straight, curl upper body up while turning your torso to the left. Contract abdominals fully without compromising form. While maintaining the controlled motion, return to starting position. Do not allow muscle to relax before next repetition. After completing one set on the left, switch to your right side. Focus on really twisting your torso and feeling the contraction when you are in the up position.
Here are a few suggested alternative exercises:Ab Crunch - MachineBall Crunch TraditionalDouble CrunchDumbbell Side BendsKneeling Cable CrunchKneeling Oblique TwistLateral Bend with Barbell