Lie over a stability ball with your hands on the floor and the ball under your thighs.
You may feel more stable with the ball closer to your hips.
Keeping your back straight tighten your abdominal muscles.
Alternating legs, lift one leg up behind you, keeping your knee at 90 degrees.
Think about pushing the sole of your elevated foot up towards the ceiling.
Control the movement as you bring your knee back down towards the floor, then repeat with your other leg.
Ensure you do not arch your back or twist your hips as you do this.
