Ball Side Flexion with Medicine Ball Show Images Show All Exercises     Primary Muscle Group: Abdominal - Internal and External Obliques Muscle Groups Worked in This Exercise: Side Abdominal - Internal and External Obliques, Deep Abdominal Muscles (transverse abdominis), and lower back (erector spinae) Preparation: Lie on your side over the stability ball so your left hip is resting on the ball. Spread your legs for good solid balance. Execution: Hold a medicine ball over your head and curl up towards the ceiling. Lay back down across the ball and repeat the movement. Repeat with the other side. Here are a few suggested alternative exercises: Ab Crunch Ab Crunch - Machine Ab Crunch Obliques Ball Crunch Traditional Double Crunch Dumbbell Side Bends Kneeling Cable Crunch Kneeling Oblique Twist Lateral Bend with Barbell