Lie on ball on side of torso, waist, and hip. Position legs outward with feet on floor or against floor board. Hold weight behind or above head.
Raise side of torso up by laterally flexing waist. Lower torso back on ball and repeat. Position other side on ball and repeat with opposite side.
Exercise can be performed without added weight until more resistance is needed. Movement can also be made easier by positioning hip low on ball. In contrast, exercise can be made more challenging by positioning hip higher on ball. In which case, feet may be placed against base of wall for stability. See alternative form. Also see Spot Reduction Myth and Arm Position During Waist Exercises.