Sit on a chair with a ball under your affected foot.
Round your back and lower your head so that your weight goes through your tail bone.
Slowly begin to roll the ball away from you, straightening your affected knee and pulling your ankle up towards you.
Simultaneously lift your head up so that you are looking straight ahead.
Do not push into pain or tension.
Roll the ball back in, bending your knee, while lowering you head back down.
Perform the movement fluidly.