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The thoracolumbar fascia is a layer of soft tissue in the middle and lower back to which many of the back muscles attach. Alternately known as the lumbodorsal fascia, it is found to the outside of and in between the deep muscles of the lower back, both surrounding them and separating them from one another. It forms three layers: an anterior or deep layer, a middle layer, and a posterior or superficial layer. Made up mostly of collagen and other elastic fibers, it not only encapsulates and partitions the muscle but acts of connective tissue joining the muscle to the bones of the pelvis, spine, and ribcage.
Visible in anatomy drawings as silvery-white tissue alongside the red fibers of the back muscles, the thoracolumbar fascia has a large surface area, extending from the bottom of the neck to the ischium bone in the lower posterior pelvis. If the skin and fat covering the back were removed to reveal the outermost layer of muscles, a diamond-shaped patch of the thoracolumbar fascia would be visible in the center of the lower back. The two sides of the large latissimus dorsi muscle above and the two sides of the gluteus maximus muscle below appear to form a large X, with the thoracolumbar fascia as the intersection of these muscles in the center. This patch of fascia covers the erector spinae muscles in the lumbar region and attaches to the edges of the ilium and ischium bones in the pelvis below.
Removing the superficial back muscles — the latissimus dorsi on either side of the mid-back and the trapezius muscle of the upper back — reveals that the thoracolumbar fascia like the erector spinae group extends upward past the thoracic region of the spine, or that within the ribcage. Crossing underneath the serratus posterior muscle, whose fibers angle inward off the ninth through twelfth ribs, it continues its ascent to either side of the vertebral column. The posterior layer of fascia then passes beneath the rhomboid major and minor muscles, found between the shoulder blades, before blending with the fibers of the nuchal fascia of the neck.
Beneath the erector spinae muscles, which extend the spine backward, is the middle layer of the thoracolumbar fascia. This layer penetrates the back muscles from either side and separates the erector spinae group — the iliocostalis, longissimus, and spinalis muscles — from the quadratus lumborum muscle. The quadratus lumborum runs between the hipbones and the sides of the lumbar spine, and it flexes the spine laterally, or sideways, and depresses or pulls downward on the rib cage. Deep to the quadratus lumborum is the anterior layer of the thoracolumbar fascia, which runs deep to or in front of that muscle and approaches the spine from either side. It separates the quadratus lumborum from the psoas major muscle of the anterior torso.
The thoracolumbar fascia, while not muscle, is considered significant to movements of the torso and hips. As it links the muscles of the hips and legs to those of the back and shoulder girdle, for instance by joining the gluteus maximus, a powerful extensor of the hip and leg, to the latissimus dorsi, which draws the arms and shoulder blades down and back, the fascia transfers movement forces between those muscle groups. In other words, it contributes to the integration of movements in the upper and lower body, such as helping the glutes and lats contract in combination during spinal rotation.