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The multifidus is a long, thin muscle situated deep within the torso. Running parallel to the spine, it stretches from the axis, the second cervical vertebra from the top, to the sacrum, found at the base of the spine above the tailbone. This muscle is involved in vertebral stabilization during minor spinal movements and helps to reduce degeneration between individual vertebrae.
Made up of narrow bundles of muscle fibers known as fascicles that are staggered along its length, the multifidus is technically not one continuous muscle. As such, it has numerous points of origin and insertion as each fascicle arises from one vertebra and attaches at another that is one, two, or even three above it. Positioned just adjacent to the spinous processes of the vertebrae, the pair of bony protrusions jutting, prong-like, backward and downward from each vertebra, the multifidus fills the narrow vertical space to either side of the spine.
In the four separate spinal regions that this muscle spans — the cervical, thoracic, lumbar, and sacral — it arises from a different structure on the vertebrae contained in that segment. Those fascicles originating in the cervical spine, for instance, begin on the articular processes of C1-C5, rounded bony prominences on either side of each vertebra. The fascicles originating in the thoracic spine begin on the transverse processes, bony prongs found on either side of the thoracic vertebrae just anterior to, or in front of, the articular processes. In the lumbar region, fascicles arise from the mammillary processes, prongs located toward the posterior or back side of the lumbar vertebrae to the inside of the transverse processes.
Multifidus fascicles of the sacral spine have more complex origins. They arise from the back of the fused sacral bone on the sacral foramen, from the aponeurosis of the sacrospinalis, a sheet of connective tissue from which the erector spinae muscles arise, from the posterior superior iliac spine, a bony ridge on the back of the ilium bone in the hip, and from the posterior sacroiliac ligaments, which bind the ilium to the sacrum. As with the fascicles of the other three regions, the sacral fascicles each insert on the spinous process of a higher vertebra. Together, they function as one muscle binding each segment of the spine as well as connecting individual vertebrae.
It is this physiology that allows the function of the multifidus. This muscle stabilizes between segments, as it links the different regions of the spine and helps hold it together. Its tight, stiff structure also stabilizes and protects the joints separating individual vertebrae, so that they are less likely to become worn down via compression forces.