Lie on your back with your knees bent and one leg resting on a stability ball.
Tighten your abdominal muscles.
Slowly roll the ball away from you, straightening this leg and tightening the thigh muscle.
Keep your core muscles activated whilst you roll the ball back in towards you, bending this knee in to your chest.
Roll the ball in and out at a slow pace, ensuring your knee points up towards the ceiling throughout the exercise.
Keep your trunk and pelvis still and do not allow your lower back to arch as you repeat this movement.