The human body is supported on FACET JOINTS / ZYGOAPOPHYSIAL JOINTS: the BACK RACK system works on FACET JOINTS.
Articular surfaces are small just 1.6 cm, they stabilise the lumbar spine in compression to DE-COMPRESS and prevent excessive bending and gliding movements between adjacent vertebrae. Joint surfaces are permanently pre-stressed in the body / ligamentum flavum/,thus the protection of intervertebral discs. In the upper lumbar spine they are vertical,but more oblique at L4-5 and L5-S1.Thats why lower lumbar z..joints resist about 20% of the compressive force acting PERPENDICULAR to the mid – plane of the discs,while the upper levels resists only half as much.Lower margins of the articular surfaces suffice to lower compression.Pathological disc narrowing can lead to small hollows eburnated bone in the laminae and osteopathic spurs around the margins of the inferior process. In exceptional cases the FACET JOINTS can transmit up to 70% of the spine compression force from one vertebra to another .Therefore even in HEALTHY spines extremely lordotic postures and backwards bending motions can cause substantial compressive forces through zygopophysil joint.
Flexion movements occur in the superior antero-medial regions of joint surfaces. Lumbar facet joints are best at resisting forces acting perpendicular in plane of the disc,thus severely limiting the range of axial rotation in the lumbar spine.Greatest contact stress occurs in the superior-posterior margins of the joint surfaces,they are capable of resisting shering forces of 1 kN each.Small axial rotation happens when the facet joints are asymmetrical in the horizontal plane.Muscle forces acting as pulling down spinuos process cause to increase the intervertebral shear force resisted by facet joints/door knocker mechanism/. That helps to LOCK the facet joints and increase stabilization in flexed postures.
Human body weight causes MOTION along the spine thus loosening it and all relevant structures mechanisms ; eg PAIN,STIFFNESS etc..