The multifidus muscle is a series of small, triangular muscle bundles located on either side of the spinal column that make up the 2nd layer of the deep back muscles. Composed of 3 layers total, the deep back muscles are collectively called the intrinsic layer. Yet another name for this group is the transversospinal layer.
Above the transversospinal (deep) layer of back muscles is the superficial layer. The superficial layer is also known as the extrinsic muscles or the "erector spinae," or simply the paraspinal muscles.
Back to the multifidus. This muscle extends lengthwise down much of the spine and has 4 parts: Cervical (neck), thoracic (corresponding approximately with the upper and mid back area), lumbar (your low back) and sacral (corresponding to the sacrum, which is below your low back.)
At the cervical, thoracic and lumbar regions, one end of the multifidus attaches onto the transverse processes of a spinal vertebra, and the other to the spinous process on the vertebra 2-4 levels higher.
Transverse processes are extensions of bone that emanate from the body of the vertebra on either side. Similar to transverse processes, spinous processes are projections of bone that arise from the back of the vertebral body. Processes provide attachment sites for muscles like the multifidus and others.
In these combined regions of cervical, thoracic and lumbar, the muscle spans between C-4 to L-5. It is thickest at the lumbar spine.
At the sacrum, the multifidus originates at the back of this triangular-shaped bone. It also originates on the medial (facing the center of your back rather than toward the outside) surface of your posterior superior iliac spine. Similar to (but not the same as) the processes discussed above, the posterior superior iliac spine is a protrusion of bone that emanates from the top of the back part of your hip bone. It faces inward toward your sacrum bone. The multifidus also originates from your sacroiliac ligaments. The sacral part of the mutifidus muscle attaches onto the spinous processes of vertebrae above it.
Multifidus Muscle Roles
As the name implies, the general role of the back extensor muscles — of which the multifidus is one — is to extend the back, which is analogous to making an arching movement. These muscles also contribute to side bending (tilting) and rotation (aka twisting.) When both sides of the extensors (and the multifidus) contract, the result is back extension. When only one side contracts side bending and rotation motions are produced.
But the multifidus also stabilizes the vertebrae as the spine moves. It is thought that the unique design of the multifidus endows it with extra strength.
Multifidus strength has been the subject of some interesting research in recent years. Investigators have looked at the types of fibers that comprise this little muscle — and the way in which these fiber types contribute to spinal stability. McDonald, et. al report on several such studies in their review: "The lumbar multifidus: Does the evidence support clinical beliefs?" They say one researcher found that the multifidus provides about 2/3 of the stiffness at the L4/L5 intervertebral joint. McDonald and his team mention other studies demonstrating that a multifidus contraction controls the motion of uninjured low back joints, and increases the stiffness (and therefore the stability at) injured low back joints.
Further, experts categorize multifidus fiber types by layers. The deepest layer, they suggest, contributes more strength and stability to the spine than do superficial layers. One reason cited for this includes the fact that the deep layer only spans 2 vertebral segments (as opposed to up to 4 with the other layers). The resulting shorter "excursion" of the deep layer of the multifidus means that when the muscle contracts, it contributes to more compression type motion at the spinal joint it affects — but less rotation — compared to other back extensor muscles (including the more superficial layers of the multifidus itself.)
Another reason the deepest layer of the multifidus muscle provides more strength is that it tends to "co-contract" with the deepest abdominal muscle in front — the transverse abdominal. Core muscles are noted for their contribution to spinal stability and often play an important role in home exercise programs given to spine patients in physical therapy.