The thoracolumbar fascia is a large, roughly diamond-shaped area of connective tissue constituted by the thoracic and lumbar parts of the deep fascia enclosing the intrinsic back muscles. The deep fascia of the back attaches medially to the nuchal ligament, the tips of the spinous processes of the vertebrae, the supraspinous ligament, and changes its name at the thoracic and lumbar regions to the thoracolumbar fascia before inserting into the median crest of the sacrum as depicted in the figure below.
The thoracolumbar fascia, like most other deep fasciae surrounds the deep muscles of the back and lumbar regions, and divides the muscles into compartments. It extends laterally from the spinous processes and forms a thin covering for the deep muscles in the thoracic region and a strong thick covering for muscles in the lumbar region where it is widest. It acts as an attachment site for the trapezius muscle, latissimus dorsi, gluteus maximus and the hamstrings group of muscles thus playing an important role in body movements. Also, it houses the quadratus lumborum, transversospinalis, spinal erectors (Erector spinae) and multifidus muscles, and their tendons, among other intrinsic muscles of the posterior thoracic and lumbar regions.