[citation needed] Williams suggested that humans, in evolving to stand erect, severely deformed the vertebral column, redistributing body weight to the posterior aspect of the intervertebral discs in the lumbar spine.
Williams emphasized the universality of this problem: "The fifth lumbar disc has ruptured in the majority of all persons by the age of twenty...." He suggested that although most people at this age have not experienced severe low back pain, they will, in all likelihood be subject to mild bouts of low back pain which can be attributed to the ruptured disc.
[citation needed] The primary benefit is supposed to be the opening of the intervertebral foramen, the stretching of ligamentous structures, and the distraction of the apophyseal joints.
Williams said: "The exercises outlined will accomplish a proper balance between the flexor and the extensor groups of postural muscles...".
One review stated that "Perhaps no two methods of physical therapy treatment for low back pain are so contradictory in both theory and practice.
"[10] In contrast to Williams, McKenzie suggested that all spinal pain can be attributed to alteration of the position of the disc's nucleus pulposus, in relationship to the surrounding annulus; mechanical deformation of the soft tissue about the spine which has undergone adaptive shortening; or mechanical deformation of soft tissue caused by postural stress.
McKenzie concluded that a continually flexed lifestyle may cause the nucleus to migrate more posteriorly, resulting in low back pain.