Here’s a killer ab exercise from Shape’s Best Tested Ab Moves Workout: The Bridge with Leg Extension.

Lie on your back with your knees bent and both feet on the floor. Anchor yourself with your hands at your side. Pull your navel toward your spine, contracting your abs. Lift your body up so only your shoulder blades are touching the floor, making sure your body forms a straight line. Hold this pose for 30 seconds.

Then, while you’re still in the lifted position, extend your left leg out at hip height. Hold, then switch to lift your right leg, keeping your hips elevated the whole time. Alternate back and forth 8 times. Rest, then do another set. You’ll feel the burn in your abs (including your obliques) and lower back.