Swiss Ball Russian Twist
This exercise works your whole core area, particularly your obliques.
Lie with your shoulders on a Swiss Ball and hold a medicine ball vertically above your head. Try and make sure you have a 90 degree angle at your knee and a straight line running through your knee, hip and shoulder.
Rotate slowly 90 degrees to one side. Your shoulder should roll on the ball and your arms should stay straight. Now explode back to the start point as fast as you can. Rotate slowly 90 degrees to the other side and repeat.
