The basic anatomy of the vertebrae and disc is presented. Instrinsic spinal stability is provided by the intervertebral discs and ligaments, extrinsic stability by the muscles. In the sagittal plane the axis of motion is related to the disc; in the horizontal plane it is determined by facet orientation.
The vertebral end-plates are most susceptible to injury, followed by the vertebral body and the disc. The end-plates tolerate greater dynamic than static forces. Muscle action during lifting decreases the load on the intervertebral discs by transforming the thorax and abdomen into semirigid-walled cylinders. Interdiscal pressure is greatest while sitting, less in standing, and least reclining.
Bracing provides incomplete immobilization, but substitutes for abdominal muscle action. The inflated corset decreases disc pressure by increasing intra-abdominal and intrathoracic pressures. Abdominal strengthening exercises can be of benefit in low back disorders by the same mechanism.