Planks work almost every muscle in your body. Some consider it the ultimate exercise. It will definitely work your entire core, including all of the big three that we discussed earlier.
Begin in a push up position, balanced on your hands and toes. Your hands will be directly under your shoulders. Your feet will be hip-width apart.
Push all the way up until your arms are fully extended and lock your elbows.
Keep your body in a straight line and brace your core (draw your bellybutton in toward your spine).