Build a bulletproof lower back without a single plate.
Stop at a straight line. Don't crank into a big arch at the top.
Draping your hips over the ball lets your spine round fully at the bottom, so the lower back extends through a longer arc than most benches allow, working the erectors from a true stretch to a straight line. The glutes fire alongside to drive hip extension, especially if you squeeze them before you lift. It earns its place as spinal-erector work you can do anywhere with a ball and a wall, no rack of plates required.
The ball gives a longer range at the bottom and a comfier pivot on the hips, while the bench holds you more rigidly and is easier to load. Start on the ball to groove the movement, use the bench when you want heavier progression.
Anchor your feet firmly against a wall or heavy object and set your hips on the crown of the ball before you start. Most instability comes from a lazy foot position, not the ball itself.
No, stop when your body forms a straight line. Cranking into a big arch shifts stress into the spine rather than strengthening it, the money is in the controlled lift from the rounded bottom position.
