Sit on exercise ball. Walk forward on ball and lie back on ball with shoulders and head hanging off. Switch position of feet by positioning bent leg under extended leg so waist is twisted with hip positioned sideways. Gently hyperextend back to contour of ball. Place hands behind neck or cross arms across chest.
Flex waist to raise upper torso. Return to original position and repeat. Position legs in opposite position and continue.
Certain individuals may need to keep their neck in neutral position with space between their chin and sternum. Some individuals may experience low back discomfort if hips are not bent, so they must use smaller ball size or lower their hip position on ball. Also see Spot Reduction Myth and Arm Position During Waist Exercises.
Movement can be made easier by positioning hips low on ball or placing arms further down away from head (eg: arms crossed on chest). See Arm Position During Waist Exercises.
Exercise can be made more challenging by using positioning ball lower on back and hips higher or adding additional weight. In which case, feet may be placed against base of wall for stability.