Lie on your back with your knees bent and your feet flat on the floor.
Tighten your abdominal muscles.
Slowly slide one leg down to lie straight on the floor.
Keep your core muscles activated whilst you slide the same leg back up to the starting position.
Do this slowly and make sure your knee continues to pointing straight up to the ceiling throughout the exercise.
Ensure you keep your trunk and pelvis still and do not allow your lower back to arch as you repeat this movement.