By Elizabeth A. Murray, Mount St. Joseph University
The true back muscles are divided into four major subdivisions. The first set are the splenius capitis and splenius cervicis muscles. The next group or layer of true back muscles are collectively called the erector spinae muscles. The third major group are the transversospinalis, and the fourth set of muscles along the spine are generally called the segmental complex.
Splenius Capitis and Splenius Cervicis Muscles
The first set of the true back muscles are the splenius capitis and splenius cervicis muscles—and as the names imply, the splenius capitis attaches to the skull, while the splenius cervicis attaches to the transverse processes of cervical vertebrae. The splenius complex is found only in the neck—and reflect a clear example of naming as a human construct, because this pair of muscles really has a continuous origin along the spinous processes of upper thoracic and cervical vertebrae.
Early anatomists decided to differentiate them due to their dissimilar insertions. When one looks at the splenius muscles together, they can really see the continuous origin. Splenius means ‘plaster’, like an old term for ‘bandage’, and early anatomists must have thought the pair looked like a bandage on the neck.
The small void or window superior to the splenius capitis isn’t a cutaway—deeper muscles can actually be seen through that void. When acting together, the splenius capitis and cervicis extend the head and neck; when acting unilaterally, they aid lateral flexion and head rotation to the same side, which is ipsilateral action.
This article comes directly from content in the video series How We Move: The Gross Anatomy of Motion. Watch it now, on Wondrium.
Erector Spinae Muscles
The next group or layer of true back muscles are collectively called the erector spinae muscles, since they help keep the spine straight. These constitute the bulk of the large, longitudinal blocks of muscle that can be easily felt on either side of the spine. Because of their differential development in various areas of the back, as well as for their different insertions, anatomists subdivided them longitudinally into three subunits.
First, the iliocostalis group, which are the most laterally located; then the longissimus group, which are intermediately located; and finally, the spinalis, which are most medially located, adjacent to the spinous processes of the vertebrae.
Iliocostalis, Longissimus and Spinalis
Iliocostalis are the part of the erector spinae that go from the ilium of the pelvis up to ribs—costal means ‘pertaining to the rib’. Longissimus goes the long way—all the way from the low back up to the skull. Spinalis is the part of the erector spinae group that is most well developed in the thoracic region, sitting closest to the spine.
To achieve the precise naming of locations that we really need in anatomy and medicine, anatomists also wanted a way to convey not just the lateral to medial subdivision of the erector spinae, but also the superior to inferior breakdown.
So, for example, some of the muscles have a capitis region—like the longissimus capitis; some muscles have cervicis regions—like the spinalis cervicis; and still others have thoracis and/or lumborum regions.
These blend very naturally in the body, and there are no dotted lines or landmarks indicating where iliocostalis lumborum ends and iliocostalis thoracis begins.
The Transversospinalis
The transversospinalis are so named because they connect transverse and spinous processes of vertebrae. Of course, not those on the same vertebra, since if a muscle originated on a transverse process and inserted on the spinous process of the same vertebra, it wouldn’t cross a joint.
Each transversospinalis muscle originates on the transverse process of one vertebra and inserts on the spinous process of another. As a result, they have an oblique orientation, as they cross one or more intervertebral joints. Since they are posterior to the vertebrae, they extend, support, and stabilize the spine, but their oblique orientation also allows them to have a role in rotation of the spine.
There’s not a whole lot of movement between any two adjacent vertebrae, but stacking 24 of them allows the accumulation of slight rotations between adjacent pairs, adding up to a much greater range of motion within the spine, as a whole. However, it is important to remember that the major rotators of the spine are abdominal wall muscles.
Subunits of Transversospinalis
Transversospinalis is a group name, and like the erector spinae, these are subdivided into three subunits: the semispinalis, the multifidus, and the rotatores.
These are tucked in the relatively narrow trough between spinous processes and transverse processes, so rather than being considered from lateral to medial, these subunits are discussed in layers from superficial to deep. In general, the semispinalis muscles of the transversospinalis group span five or more vertebral segments, while the multifidus are deep to them and span three to five vertebral levels. The deepest members of the transversospinalis group are the rotatores that span just one or two vertebral levels.
The Segmental Complex
In addition to the splenius muscles in the neck, the erector spinae that run the length of the spine, and the transversospinalis group, there is a fourth set of muscles along the spine—generally called the segmental complex, as they span just one vertebral segment.
These are subdivided into the interspinales muscles, a tiny pair of muscles that run between adjacent spinous processes, and the intertransversarii muscles that connect adjacent transverse processes.
Up to this point, all of the true back muscles are supplied by the dorsal rami of spinal nerves—however, relatively recent studies have demonstrated that the intertransversarii are supplied by ventral rami.
Common Questions about Major Subdivisions of True Back Muscles
The four major subdivisions of the true back muscles are: the splenius capitis and splenius cervicis muscles, the erector spinae muscles, the transversospinalis, and the segmental complex.
The erector spinae muscles help keep the spine straight. These constitute the bulk of the large, longitudinal blocks of muscle that can be easily felt on either side of the spine. Because of their differential development in various areas of the back, as well as for their different insertions, anatomists subdivided them longitudinally into three subunits: the iliocostalis group, the longissimus group, and the spinalis.
Iliocostalis are the part of the erector spinae that go from the ilium of the pelvis up to ribs—costal means ‘pertaining to the rib’. Longissimus goes the long way—all the way from the low back up to the skull. Spinalis is the part of the erector spinae group that is most well developed in the thoracic region, sitting closest to the spine.
