Here are some plank variations.  Those of you who know me well know I’m a perfectionist and so I am going to correct my own form.  During the point and flex variation, when both feet are moving, my free leg shouldn’t be lifting and lowering.  This is an indication that my glutes were not firing the whole time.  My free leg needs to be active throughout the whole exercise to help lift the leg up.  The only part that should be moving is the ankles and the shoulders are a reaction to the movement.   But after these exercises your abs will love you!