Using a ball for back extensions will give you more range of motion than you get on the floor and you'll also have a balance challenge since the ball is unstable. You may want to prop your feet against the wall to get more leverage.
- Lie down with the ball under the belly and hips, legs straight out behind you (or knees bent for a modification).
- Place the hands behind the head or under the chin - you can also keep the hands resting on the ball if you need a modification.
- Round down over the ball and then squeeze the lower back to lift the chest off the ball.
- Raise up until the body is straight (don't hyperextend), lower down and repeat for 1-3 sets of 10-16 reps.