Looking for a challenging abs exercise? Check it out: The Prone Oblique Roll. Get in plank position with your shins about hip-width apart on a stability ball and your hands shoulder-width apart on the floor (A). Keeping your feet on the ball, draw your right knee toward your right shoulder (the left just comes along for the ride) (B). Return to center. GET MORE MOVES: http://ow.ly/8AxAe read more..