The thoracolumbar fascia, also known as the lumbar muscle fascia of the original lumbar and dorsal muscles, is divided into thick and thin layers, arranged in a muscle tissue sheath around the sacrospinalis muscle and the superficial short muscles of the back. This muscle fascia is thin and translucent in the thoracic and lumbar back. The lumbar and dorsal region becomes thick and very developed, presenting as a tendinous membrane with a milky white luster. It is located on the deep side of the deltoid, latissimus dorsi, and lower posterior serratus muscles, covering the sacrospinalis muscle and the superficial short muscles of the back. It transitions upwards to the nuchal muscle fascia.

Adhered downward to the iliac crest and the lateral spines of the sacrum. Internally adhered to the transverse processes of the lumbar vertebrae, intervertebral discs, supraspinous ligaments, and median sacral crest. Externally, it adheres to the costal angle of the rib and the intercostal muscle fascia in the thoracic and lumbar back, continues in the lumbar region along the tendinous sheath of the transversus abdominis muscle, and deeply heals on both sides of the sacrospinalis muscle with the thoracolumbar fascia (lumbar muscle fascia).

The deep layer is located on the deep surface of the sacrospinalis muscle, sticking upwards to the inferior margin of the 12th rib, downwards to the iliac crest, and internally to the transverse processes of the lumbar vertebrae. Externally, it merges with the superficial layer of the thoracolumbar fascia on both sides to form the insertion and origin tendons of the abdominal muscles. This layer of muscle fascia becomes very thick between the 12th rib and the first lumbar transverse process, known as the lumbocostal ligament (lumbocostal lig.).

The thoracolumbar fascia can be divided into three layers:

Superficial layer: thicker, located on the deep surface of the latissimus dorsi and lower serratus muscles, and on the surface of the erector spinae muscle. There is a space between the superficial layer of the thoracolumbar fascia and the erector spinae muscle, known as the subthoracolumbar fascial space, which contains cutaneous nerves, fat, and loose connective tissue. Normally, the superficial layer of the thoracolumbar fascia has the function of limiting the erector spinae muscle and improving its functional power, while the loose connective tissue under the thoracolumbar fascia plays a lubricating function between the thoracolumbar fascia and the erector spinae muscle. This layer of muscle fascia is also a common site for obtaining muscle fascia slices. The middle and upper layers of the thoracolumbar fascia are located between the erector spinae muscle and the quadratus lumborum muscle, originating from the 12th rib upwards and ending at the iliac crest downwards, attaching to the transverse processes on the inside, merging with the superficial layer on both sides of the erector spinae muscle to form the insertion and origin tendons of the abdominal muscles.

The superficial and middle layers of the thoracolumbar fascia, along with the transverse processes and spines of the intervertebral discs, form the osseous-muscle fascial compartment of the lumbar sacral region. This compartment contains the erector spinae muscle, the muscle group of the transverse process spine, the posterior and lateral branches of the lumbar nerve, and the nutritional blood vessels. The existence of this osseous-muscle fascial compartment may be one of the anatomical bases for low back pain.

The deep layer of the thoracolumbar fascia is located in front of the quadratus lumborum muscle, also known as the quadratus lumborum muscle fascia. It continues with the fascia of the psoas major muscle in front, and is also part of the abdominal muscle fascia. The fascia of the psoas major muscle and the iliac muscle fascia form the ilio-lumbar muscle fascia, enveloping the psoas major muscle and the iliac muscle, and continuing downwards to the lesser trochanter of the femur. Therefore, a cyst of the psoas major muscle can extend downwards along this muscle fascia to the inside of the femoral head. Due to inflammation and irritation, this muscle fascia thickens, and the cyst is confined in this space, sometimes reaching more than a thousand ml. The ilio-lumbar muscle fascia is also a key anatomical basis for the ilio-lumbar muscle fascial compartment syndrome, which plays a critical role in subacute damage to the psoas major muscle.

The thoracolumbar fascia thickens significantly in the lumbar region, which can be divided into superficial, middle, and deep layers. The superficial layer is thicker, located on the surface of the erector spinae muscle, combining with the insertion tendons of the latissimus dorsi and lower serratus muscles, and sticking downwards to the iliac crest and both sides of the sacral crest. Internally, it sticks to the transverse processes of the vertebral discs and supraspinous ligaments, and on both sides, it merges with the middle and upper layers to form the sheath of the erector spinae muscle; the middle and upper layers are located between the erector spinae muscle and the quadratus lumborum muscle, attaching to the tips of the transverse processes of the lumbar vertebrae and the tendons between the transverse processes on the inside, and merging with the deep layers on both sides of the quadratus lumborum muscle to form the sheath of the quadratus lumborum muscle; the deep layer is thinner, located in front of the quadratus lumborum muscle, also known as the quadratus lumborum muscle fascia. The three layers of muscle fascia converge at the inner margin of the quadratus lumborum muscle to become the origin of the internal oblique and transverse abdominal muscles. Because of the great mobility of the lumbar region, during excessive exercise, the thoracolumbar fascia is more prone to twists, which is one of the causes of lumbar and leg pain.