Sitting trunk flexion and rotation with resistance band (Trunk rotation/extension strengthening, with band (low), in front, sitting on stability ball; 01)

Sit on the ball with feet hip width apart.
With both hands, hold onto a piece of resistance band that is tied to a point at approximately ground level.
Activate your core stability muscles and bend forwards and to one side so your hands are near the outside of your foot.
As you straighten up, rotate back and away in the other direction, pulling against the resistance of the band.
Repeat the movement both ways.