To get Rihanna's abs, try -
The Plank: Lie face down on a mat resting on the forearms, palms flat on the floor. Push off the floor, raising up onto toes and resting on the elbows. Keep your back flat, in a straight line from head to heels.
Tilt your pelvis and contract your abdominals to prevent your rear end from sticking up in the air or sagging in the middle. Hold for 20 to 60 seconds or however long you can.
