Primitive man was extremely robust out of necessity, because running after food animals kept the leg and back muscles toned and balanced.  The hamstrings and the quads attaching to the bottom part of the pelvis have a profound effect on the structure and function of the lower back.  Modern man assumes a seated position for much longer periods of time, which has the effect of shortening the hamstrings on the back of the thighs.  The hamstrings, when tight, will pull the pelvis into a posterior or ‘flat back’ position, which causes the lumbar curve to flatten out and results in the reduction of facet joint articulation in the lower back.  In this circumstance, the soft tissues of the lower back strain strenuously to hold the lumbar spine together and to keep the pelvis from posteriorly rotating any further.  The most common form of a dysfunctional lower back will have a compromised articulation between L5 and the sacral base complicated by a flattened or posterior back.