Posterior tilt and mini squats in two point (with stability ball) ("Squat" Core/hip extension strengthening, kneeling behind stability ball)

Kneel with your hands on a stability ball in front of you.
Drop your hips back and bend your knees slightly, as if you are about to sit down.
Half way down, return to the starting position by tightening your buttocks and pushing your hips forwards.
Once upright, flatten your lower back and recruit your abdominal muscles.
Hold this position before squatting back down again.