Here is Part II of my latest blog for Yoga Tune Up®. It includes a video demonstration of an exercise to strengthen the lower back and surrounding musculature.
http://www.yogatuneup.com/blog/2012/06/06/objection-your-plank-may-not-pass-the-lumbar-exam-part-2/
http://www.yogatuneup.com/blog/2012/06/06/objection-your-plank-may-not-pass-the-lumbar-exam-part-2/