"Push-ups fire into all four abdominal muscle groups to flatten your abs, strengthen your back, and work your chest, shoulders, and even the front of your legs," Hilliard says.
To advance the movement, she recommends a cobra push-up. Start off in a high plank, either on your toes or knees, with your feet 6in apart. Walk your hands about one foot ahead of you and bend both elbows simultaneously, lowering your body as low as you can with the goal of tapping your elbows to the floor. Make sure you feel this in your triceps, lats, and obliques -- not your shoulders. Return to a straight-arm plank by pressing your palms into the mat and engaging through the backs of your arms.
Repeat eight to 10 repetitions at a time, moving slowly to reap the benefits of this killer exercise.