Sit straight on a stability ball holding a resistance band attached to one side.
Hold your arms our in front and lift one leg slightly off the floor.
Do not allow your body to slouch or lean.
Keeping your stability, rotate away from the band, pulling against the resistance with the strength in your stomach muscles.
Control the movement back to the start position and repeat.
To rotate the other way, you will have to have the band attached at the other side.
