Spinalis Muscle: Location, Function, and Back Pain

The spinalis is the smallest and most medial column of the erector spinae, the powerful group of back muscles that runs along each side of your spine from the pelvis to the base of the skull. Sandwiched between the spinous processes and its much larger neighbors, the longissimus and iliocostalis, the spinalis is easy to overlook in anatomy diagrams and even easier to ignore in the gym. Yet this slender muscle strip plays a genuine role in extending and stabilizing the vertebral column, and its condition shows up in imaging studies of scoliosis, chronic back pain, and age-related spinal degeneration.

Where the Spinalis Sits in the Erector Spinae

The erector spinae is not a single muscle but a group of three parallel columns that lie in the groove on either side of the vertebral spinous processes. From lateral to medial, the three columns are the iliocostalis (closest to the ribs), the longissimus (the largest and most central), and the spinalis (right next to the spinous processes themselves). All three columns share a common tendinous origin from the sacrum, iliac crest, and lower lumbar spinous processes before fanning upward and splitting into their distinct bundles.

The spinalis attaches primarily to spinous processes at both ends, unlike the longissimus and iliocostalis, which have more lateral attachments to transverse processes and ribs. This close-to-midline positioning means the spinalis has a short lever arm. It is essentially pulling the spine backward right at the center of rotation rather than from a wider angle. That geometry gives it less mechanical advantage for generating large extension forces but a more direct line of action for fine segmental control.

Three Subdivisions

The spinalis is typically described as having three parts, each named for the region of the spine where it is most prominent:

Spinalis thoracis: The most clearly defined portion. It originates from the spinous processes of the upper lumbar and lower thoracic vertebrae and inserts into the spinous processes of the mid-to-upper thoracic vertebrae. When people refer to “the spinalis” without qualification, they usually mean this part.
Spinalis cervicis: Often poorly developed or even absent. When present, it connects spinous processes in the lower cervical and upper thoracic region. Many anatomy texts note that it blends into surrounding muscles so thoroughly that it is hard to isolate during dissection.
Spinalis capitis: This portion is so intermingled with the semispinalis capitis that most anatomists consider it inseparable. In practice, it is not usually treated as a distinct functional unit.

The thoracic subdivision is the one that matters most in clinical and rehabilitation contexts, because it has the most consistent architecture and is the segment visible on cross-sectional imaging of the mid-back.

How Small It Really Is

One of the most striking things about the spinalis is its size relative to the other erector spinae columns. Detailed cadaver work measuring the paraspinal muscles found that the longissimus had the greatest average cross-sectional area at about 10.4 cm², the iliocostalis came in around 9.2 cm², and even the small multifidus registered measurable area, while the erector spinae as a whole showed very small force-generating cross-sections relative to what you might expect for such a long muscle group.¹ The spinalis is the slenderest column of the three. Its small cross-sectional area means it contributes relatively little raw force to spinal extension compared with the longissimus or iliocostalis. Some anatomists have described it as almost vestigial in the lumbar region, where the other erector spinae columns dominate.

This size difference has practical consequences. Strengthening exercises aimed at the erector spinae, like back extensions or deadlifts, load the longissimus and iliocostalis far more than the spinalis simply because those muscles have more tissue to recruit. The spinalis responds to training like any other skeletal muscle, but its contribution to total back-extension torque remains proportionally modest.

What the Spinalis Does

The primary action of the spinalis is spinal extension, meaning it helps you arch your back or stand upright from a bent-forward position. Because it attaches to spinous processes on both ends, its line of pull is almost perfectly posterior, making it effective at resisting forward flexion. When you are simply standing still, the spinalis contributes to the tonic postural activity that keeps your thoracic spine from rounding forward under gravity.

Beyond extension, the spinalis plays a small role in lateral flexion. Contracting the spinalis on one side helps tilt the spine toward that side, though the longissimus and iliocostalis are much stronger lateral flexors due to their more lateral positions. The spinalis has essentially no rotational leverage because its attachments run parallel to the spine’s axis of rotation. Rotation is handled by deeper muscles like the rotatores and by the obliques in the abdominal wall.

Research comparing how the erector spinae muscles activate in upright versus quadrupedal postures shows that the overall erector spinae group works harder to maintain extension in quadrupeds, where the spine is held horizontally against gravity, than in upright humans.² For humans standing upright, the erector spinae columns, including the spinalis, are active at relatively low levels during quiet standing and ramp up during forward bending, lifting, or explosive extension movements.

Fiber Types and Fatigue

The paraspinal muscles, including all three erector spinae columns, are postural muscles. They have to stay active for hours while you sit, stand, and walk, so you would expect them to contain a high proportion of slow-twitch fibers that resist fatigue. In healthy people, that is exactly what researchers find: the lumbar paraspinal muscles are dominated by slow-twitch, oxidative fibers designed for endurance.

In people with chronic low back pain, though, the picture shifts. Biopsies of lumbar paraspinal muscles show a significantly higher proportion of fast-twitch glycolytic fibers and a lower proportion of slow-twitch fibers compared with pain-free controls, with no change in the size of individual fibers.³ A more glycolytic fiber profile means the muscles fatigue faster, which could feed a vicious cycle: the muscles tire quickly, provide less support, and the spine is left more vulnerable to strain. Whether this fiber-type shift is a cause of back pain or a consequence of it (through disuse or guarding behavior) remains debated.

Because the spinalis sits so close to the spine and has a relatively small cross-section, even modest changes in its endurance capacity could matter for local segmental stability. It is difficult to biopsy the spinalis in isolation, so most fiber-type data comes from the broader paraspinal group, but the physiological principle applies across the erector spinae columns.

The Spinalis in Scoliosis

One area where the spinalis receives specific attention is scoliosis research. In adolescent idiopathic scoliosis, the spine curves laterally, and the muscles on each side of the curve become asymmetric. Imaging studies have found that intramuscular fat infiltration in the spinalis, longissimus, and multifidus is consistently greater on the concave side of the curve in scoliosis patients. In one study, fat asymmetries in these muscles were 13% to 57% larger on the convex side in adolescents with scoliosis, compared with just 1% to 20% asymmetry in controls.⁴

This fat infiltration is not just cosmetic on an MRI scan. Fat replacing muscle tissue means there is less functional contractile tissue available. When the spinalis on one side of the spine is infiltrated with fat while the other side remains relatively healthy, the asymmetry in pull can reinforce the curvature. Whether the muscle changes drive the curve progression or result from it is a chicken-and-egg question that researchers are still working through, but the association is consistent across multiple imaging studies.

Therapeutic approaches targeting these muscle imbalances have shown some promise. A case study of a child with neuromuscular scoliosis found that robotic horseback-riding therapy improved spinal alignment and increased paraspinal muscle cross-sectional area and symmetry.⁵ That is a single case, not strong evidence by itself, but it illustrates the broader idea that rebalancing muscle volume and activation on either side of the spine is a rehabilitative goal in scoliosis management.

Back Pain, Trigger Points, and Fatty Degeneration

Chronic back pain is the context where the spinalis and its erector spinae neighbors get the most clinical attention. The paraspinal muscles as a group are frequent sites of myofascial trigger points, those tender, taut knots in muscle that refer pain when pressed. A systematic review and meta-analysis found that active trigger points were present in all assessed spinal muscles in people diagnosed with various spinal pain disorders.⁶ Latent trigger points, the kind you might have without knowing it, were not consistently more common in pain patients than in healthy people. The clinical takeaway is that active trigger points in the paraspinal muscles are a genuine feature of spinal pain, not just a normal finding that everyone has.

Fatty infiltration is another marker of trouble. As paraspinal muscles degenerate, whether from chronic pain, inactivity, aging, or nerve damage, fat gradually replaces muscle tissue. MRI-based studies of patients with lumbar spinal stenosis have developed indices to quantify this fatty replacement and have found that it correlates with pain and disability levels.⁷ The spinalis, being the smallest column, may lose a larger fraction of its functional tissue before anyone notices on imaging, simply because there was less tissue to begin with.

How Spinal Surgery Affects the Paraspinal Muscles

Posterior lumbar surgery, the kind where the surgeon approaches the spine from the back, requires retracting the paraspinal muscles to reach the vertebrae. This retraction is not gentle. The muscles are pulled aside, sometimes for hours, with metal retractors that compress the tissue and restrict blood flow. Experimental work in animal models has shown that this retraction causes significant muscle degeneration, elevated markers of muscle damage, and neurogenic injury to the muscle fibers.⁸ The severity of damage was worst when retraction was prolonged without breaks, and intermittent release of the retractors reduced the injury.

For the spinalis specifically, this is relevant because its medial position means it is directly in the path of surgical access to the spinous processes and laminae. Any midline posterior approach will involve displacing the spinalis. Surgeons have become increasingly aware that postoperative back muscle dysfunction, sometimes called “failed back surgery syndrome” when it persists, can stem partly from this muscle trauma rather than from the bony or disc problem that prompted the surgery. Minimally invasive techniques that use smaller incisions and tubular retractors aim to reduce the collateral damage to the paraspinal muscles, including the spinalis.

Posture, Sitting, and Everyday Load

You do not need a dramatic injury or a surgical procedure for the paraspinal muscles to come under strain. Prolonged sitting, especially in a slumped posture, places a sustained low-level demand on the erector spinae that the muscles are not well designed to handle in a shortened, rounded position. Research on adolescents found that body discomfort increased significantly after just 30 minutes of slumped sitting, and this increase was particularly pronounced in those who already had chronic low back pain.⁹

When you slump, the thoracic spine rounds forward, and the erector spinae muscles, including the spinalis thoracis, are stretched into a lengthened position where they have to work eccentrically to prevent further rounding. Over hours, this sustained eccentric load can fatigue the muscles and contribute to discomfort. The spinalis, positioned right along the midline, is directly loaded by this rounding because its job is to resist exactly that motion. Standing desks, regular movement breaks, and exercises that strengthen thoracic extension are all practical ways to reduce the cumulative strain on these muscles.

Training the Spinalis

No exercise isolates the spinalis. Because it works alongside the longissimus and iliocostalis, and because all three columns share a common tendon of origin, any movement that targets the erector spinae will recruit the spinalis to some degree. Back extensions on a Roman chair, deadlifts, good mornings, and prone trunk lifts all load the spinalis along with the rest of the erector spinae group.

If your goal is to bias the spinalis as much as possible, the relevant variable is keeping the movement close to pure sagittal-plane extension with minimal lateral or rotational components. The spinalis has almost no rotational leverage, so exercises that add a twist, like rotational cable pulls, will shift the demand toward the obliques and deeper spinal rotators without meaningfully increasing spinalis activation. Slow, controlled back extensions through a full range of thoracic flexion and extension, with the load centered rather than offset, come closest to maximizing the spinalis contribution relative to the other erector spinae columns.

For people recovering from back pain or surgery, rehabilitating the erector spinae often starts with low-load isometric holds in a neutral spine position and gradually progresses to dynamic extension exercises. The fiber-type research mentioned earlier suggests that endurance-oriented training, using lighter loads and longer time under tension, may help restore the slow-twitch fiber profile that healthy paraspinal muscles display. High-rep back extensions, sustained plank variations that emphasize a neutral thoracic position, and swimming are all commonly recommended by physical therapists working on paraspinal reconditioning.

Why the Spinalis Gets Overlooked

Anatomy textbooks dedicate pages to the erector spinae, but the spinalis often gets a brief mention as the smallest column before the discussion moves on to the longissimus and iliocostalis, which generate most of the group’s force. Clinical imaging studies frequently measure paraspinal muscle cross-sectional area as a single composite or break it into the multifidus and the erector spinae as a whole, rather than separating the spinalis from its neighbors. Even the detailed cadaver study that meticulously measured individual paraspinal compartments did not report the spinalis as a separate entity in its cross-sectional data, instead folding it into the erector spinae measurement or noting it as too small to meaningfully separate.¹⁰

This neglect does not mean the spinalis is unimportant. It means the muscle’s role is more about contributing to a team than acting as a solo player. The erector spinae function as a coordinated unit, and the spinalis adds its small but consistent pull right at the midline, helping to distribute extension forces evenly across the spinous processes. When the spinalis degenerates, infiltrates with fat, or atrophies asymmetrically, the effects show up not as isolated spinalis weakness but as part of a broader pattern of paraspinal dysfunction that can manifest as pain, postural imbalance, or reduced spinal stiffness. Understanding the spinalis as one thread in a tightly woven group, rather than as an independent mover, gives a more accurate picture of how your back actually works.