Alternate leg extension prone over a gym ball (Core stabilization, lifting leg (alternate), prone on stability ball)

Kneel on the floor with a stability ball in front of you.
Roll forwards over the ball until you are lying on it with the ball under your hips.
Make sure your hands are on the floor under your shoulders.
Keep your feet on the ground.
Slowly lift one leg out straight behind you so that your leg is in a straight line with your back.
Do not arch your back.
Lower your leg back down and repeat with the other leg.