Fig. 1.

The spinal ligamentous structures considered by Panjabi, complemented by inclusion of the thoracolumbar fascia (TLF). Black and dark gray: the spinal ligamentous structures already included in the hypothesis; i.e. disc annulus (DA), facet capsules (FC), and the spinal ligaments (SL) which are connecting the vertebral bodies, arcs and their processes of adjacent vertebrae. White: the TLF, proposed in this letter for inclusion into the hypothesis and consisting of a posterior layer (P-TLF) covering the erector spinae (ES), middle layer (M-TLF) on the posterior side of quadratus lumborum (QL), and anterior layer (A-TLF) on the anterior side of QL and posterior to psoas (PS). The supraspinous ligament (SSL) is formed by the TLF as well. Note: axes of spinal movements usually traverse the central area between DA and both FCs, including their respective outlines. The TLF structures therefore tend to have a larger distance from the axes of spinal movements than the spinal ligamentous structures