The plank is good for your core, but adding a jumping jack motion with your feet pushes this move to a whole new level. The plank jack is a great cardio move; it raises your heart rate while working your lower and upper body.
Start in a standard plank position, with your shoulders over your wrists, your body in one straight line, and your feet together.
Like the motion of a jumping jack, jump your legs wide and then back together. Keep your pelvis steady and don’t let your butt rise toward the ceiling.