Deadbug

Lie on your back, with your hips and knees bent to 90°. Raise both arms toward the ceiling. Pull your lower back to the floor to eliminate the gap. Start by pressing one leg out, and tapping the heel to the floor. "As you extend one leg, exhale as much as you can, keeping your lower back glued to the floor," Dunham says. When you can’t exhale any more, pull your knee back to the starting position. Make this more difficult by holding weight in your hands, or by lowering opposite arm and leg.