Turn a wobbly ball into a lower-back builder.
Extend to neutral, not beyond. Hyper-arching at the top is where backs get cranky.
Rounding forward over the curve of the ball takes the lower back through a range no flat bench can match, so the erectors are trained from a fully lengthened position back to neutral. The glutes assist the lift, driving hip extension so the spine is not doing the job alone. It is one of the most accessible ways to build a resilient lower back, needing nothing but a ball and something to brace your feet against.
They are near-identical movements, both extending the spine over the ball. The key with either is the finish, come up to a straight line and stop rather than cranking into an arch.
Under your hips and lower stomach, so your torso can round down over the front of the ball. If the ball sits under your chest you lose the range that makes the exercise worthwhile.
Once bodyweight gets easy, hold a small plate against your chest. Progress slowly though, the stretched bottom position is demanding and quality reps beat loaded ugly ones.
