Sit on exercise ball with medicine ball in hands. Walk forward on ball and lie back on ball with shoulders and head hanging off and knees and hips bent. Gently hyperextend back to contour of ball. Position medicine ball over upper chest with arms extended.
Keeping arms extended and medicine ball over upper chest, flex waist to raise upper torso. Return to original position. Repeat.
If ball is positioned lower than upper chest (eg: over waist), effective resistance dramatically diminishes. See how length of lever arm affects resistance. Movement can be made easier by positioning hips low on ball. In contrast, exercise can be made more challenging by using positioning ball lower on back and hips higher. 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. See Arm Position During Waist Exercises and Spot Reduction Myth.