The “action” of a ligament is similar to that of an antagonist muscle. If either is tight/taut, it restricts motion to the opposite side.
In the lumbar spine, three joints are located between each two adjacent vertebrae: one disc joint and two paired (left and right) facet joints.
The low back is defined by the lumbar spine, composed of 5 vertebrae, named L1-L5. The pelvis is defined by the bones of the pelvic girdle.
Table 1 shows average healthy ranges of axial motion of the cervical spine (head and neck), from the atlanto-occipital joint through the C7-T1 joint.
The disc joint is a cartilaginous joint that is composed of outer fibers called the annulus fibrosus that encircle the inner nucleus pulposus.
There is an old saying that no posture is bad unless you get stuck in it. The problem is that the thoracic spine often does get stuck in bad posture.