Hip thrust on a stability ball - Version 2 ("Bridge" Core/gluteals strengthening isometric, supine on stability ball)

Lie with your middle back resting on the ball and your feet flat on the floor hips width apart.
Cross your arms over your chest.

Engage your core muscles and tighten your buttock muscles to lift your hips up into the bridge position.
To make the exercise harder, walk your feet further away from the ball but only do this if you can maintain good control.