Lie prone on ball with feet against base of wall. Place arms to sides or clasp hands behind hips or low back.
Raise torso off of ball by hyperextending spine. Lower torso onto ball allowing spine to flex. Repeat.
If no wall is available, hips can be positioned lower on ball and toes can be positioned out wide on floor for balance. Synergists only work partial range of motion through terminal extension.
Movement can be made easier by positioning hips low on ball.
Exercise can be made more challenging by positioning ball lower toward hips or positioning hands behind head or arms to sides of head. See Arm Position During Waist Exercises.
Synergists (see comments)