This exercise uses a ball to isolate and work the lower abdominal muscles.
Steps
Lay down as if you were performing a pushup. Place your feet and shins over an exercise or stability ball.
Keeping your back straight and supporting your weight on your hands, pull your knees towards your chest, so the ball rolls forward under your ankles.
Crunch your abs and then roll the ball back to starting position straighten your legs.
Tips
Keep your back straight as you perform this exercise.