The Plank - You are Doing it Wrong

I am in the position to see hundreds of people do planks on a regular basis.  More than half tend to do this simple exercise wrong, completely negating the positive benefit.  It is all about hip control and the ability to stabilize the pelvis in a neutral position.  Most people take a lazy approach due to core weakness or a poor core stabilization pattern.  As a result, the exercise becomes useless and only serves to reinforce poor movement and muscle recruitment patterns.  The image below is from an article by Laura Williams (http://www.sheknows.com/health-and-wellness/articles/991097/6-exercises-you-are-doing-wrong).  The two images at the bottom illustrate what I most commonly see.  I have added the yellow lines to illustrate what I am trying to explain.  In this case, the core muscles must engage to rotate the top of the pelvis back (posterior tilt) in order to achieve the proper alignment (top picture).  If the core is too weak to engage, or if the person has a faulty muscle recruitment pattern, the top of the pelvis will rotate forward.  In the bottom picture on the left, the pelvis has rotated forward (anterior pelvic tilt) and now all of the stabilization in the plank is being done by the passive structures of the spine (ouch!).  In the picture on the bottom right, the pelvis is rotated forward, but instead of using the spine to stabilize, the rear is lifted in the air in order to maintain the anterior tilt of the pelvis.  Both of these are common compensations in the plank position for someone with a weak core or poor core stabilization pattern.  In the properly performed plank, the key is to rotate the pelvis posteriorly by engaging the abs, keeping the back flat as in the top picture.  Incidentally, the two compensations you see in the bottom are often repeated in sit-ups, squatting, deadlifts, push-ups, walking running etc...