Primary Muscle Group: Low back
Muscle groups worked in this exercise: Gluteus Maximus, Semitendinosus, Semimembranosus, Quadratus Lumborum, Iliocostalis Lumborum.
Preparation: Kneel on the floor and bring the ball up close to you. Lie forward on the ball with your abdomen right on top of the ball and place both hands on the floor in front of you. Extend your legs behind you and balance upon the balls of your feet.
Execution: Lift your left leg as high as you can while exhaling. As you lower your leg, inhale. Then repeat with the other leg. It's important to keep your torso stable and prevent wobbling. Alternate legs after completing all repetitions.
Comments: You'll notice that the ball will try to shift as you lift and lower your legs - this forces your abdominal muscles to stabilize your torso. Remember to pull your navel in and up towards your spine.