The holidays are in full swing, meaning healthy eating and steady exercise fall off of your list of priorities. Instead, all we can think about are festive treats and comfort food, which is completely normal, but we don't want to give up healthy habits entirely. To stay on track, feel energized, and strong, we asked our fitness guru, Berto Calkins, to create a 5-minute core workout, exclusively for you. Berto takes you step-by-step through a series of ab movements that target different core muscles. After you complete this quick yet effective exercise, you will feel accomplished, motivate...