Trunk rotation in sitting (stability ball)

Sit upright on a firm surface with your feet hip width apart.
Sit far enough back so that your thighs are supported on the surface.
Hold a stability ball in front of you so that it is not resting on your knees.
Rotate your body round with the stability ball to place it as far behind you as you can.
Bring the stability ball back to your starting position before rotating to the opposite side.