Planks are a great core exercise for a reason – they’re HARD.
Your hands should be under your shoulders, with your toes planted in the ground. Don’t lock your knees, or hyperextend them. Gaze at a spot on the floor about a foot beyond your hands, and keep your body in a straight line, activating your core and using your glutes to stabilize you.
Hold the position for 20 seconds. As you get stronger, you can increase your time – go as long as you can without compromising your form, or breathing.