To do Plank Jacks, start at the top of a push up with your hands under your shoulders and your feet together. Your body should be in a nice straight line from your head to your heels.
Brace your abs then jump your feet out wide to each side as if doing a Jumping Jack. As you jump, do not let your butt go way up in the air or your hips sag way down toward the ground.
Quickly jump your feet back in together, holding the nice plank position. Do not shrug your shoulders or bend your arms as you go. Also, do not let your hands move forward in front of your shoulders.
Repeat, jumping them back out. Keep your core engaged as you jump your feet in and out. Do not let your butt go up in the air or your hips sag as you jump your feet out wide and then back together.
Beginners may need to start by stepping one foot at a time out wide and then back together. You can also add some variety to this move and make it more challenging for your inner thighs by doing it with sliders or towels under your feet.