Lie on ball on side of torso, waist, and hip. Position legs outward with feet on floor or against floor board.
Raise side of torso up by lateral flexing waist. Lower torso back on ball and repeat. Position other side on ball and repeat with opposite side.
Movement can be made easier by positioning hip low on ball. For less resistance, place upper arm over hip.
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. Movement can also be performed with added weight.