Andrea Metcalf demonstrates a bottom push-up.
Bottom Push-Ups
Starting on your elbows and knees, press one foot toward the ceiling. As you begin, your knee should be parallel to the floor; extend it 3 to 6 inches higher. Keep your abs tight to prevent arching of the lower back. Perform 30 reps on each leg.

Looking for a greater challenge? Move one arm behind your back and work that waistline at the same time.