General Topography and Classification
The deep back muscles, also referred to as the intrinsic back muscles or spinal musculature, constitute a complex anatomical structure. This entire muscle group is securely covered by a dense connective tissue layer—the thoracolumbar fascia (fascia thoracolumbalis).
For ease of study and understanding of biomechanics, the deep muscles are topographically divided into three layers:
- First layer (superficial deep layer);
- Second layer (middle deep layer);
- Third layer (deepest layer).
First Layer: Erector Spinae Muscle
The primary and largest element of the first (superficial deep) layer is the erector spinae muscle (musculus erector spinae). It is the longest and functionally most powerful muscle along the entire length of the back.
It has a single massive origin in the lower part of the trunk. Its attachment points for the origin are:
- posterior part of the iliac crest;
- sacrum;
- spinous processes of the lumbar vertebrae.
Topographic Parts of m. erector spinae
Ascending from its sites of origin, the erector spinae muscle divides into three distinct parts (tracts).
- Iliocostalis muscle (m. iliocostalis)
Divided into lumbar (lumborum), thoracic (thoracis), and cervical (cervicis) parts. Its main function is extension of the vertebral column and lateral flexion. Innervation is provided by the dorsal rami of spinal nerves—rr. dorsales nn. spinalium (segments C8–L1).
- Longissimus muscle (m. longissimus)
It also has three sections: thoracic (thoracis), cervical (cervicis), and cranial (capitis). Muscle fascicles attach to the transverse processes of the vertebrae, and in the head region, to the mastoid process. This part extends the vertebral column and head, and rotates them to its side.
- Spinalis muscle (m. spinalis)
Like the previous part, it includes thoracic (thoracis), cervical (cervicis), and cranial (capitis) divisions. The primary function of this muscle tract is extension of the vertebral column.
Briefly: answers to topical questions
Which muscles comprise the first layer of the deep back muscles?
The first layer of the deep back muscles includes the erector spinae muscle (m. erector spinae). It is the longest and most powerful back muscle and is topographically divided into three parts:
- Iliocostalis muscle (m. iliocostalis) — includes the lumbar, thoracic, and cervical parts.
- Longissimus muscle (m. longissimus) — consists of the thoracic, cervical, and cranial parts.
- Spinalis muscle (m. spinalis) — also includes the thoracic, cervical, and cranial parts.
What is the innervation of the spinalis muscle (m. spinalis)?
The spinalis muscle (m. spinalis) is part of the erector spinae muscle (m. erector spinae), which is innervated by the dorsal rami of spinal nerves (rr. dorsales nn. spinalium).
To remember the three parts of the erector spinae muscle (Iliocostalis, Longissimus, Spinalis), use the mnemonic.
Class and exam questions
What structure covers the intrinsic muscles of the back?
Deep muscles of the back lie beneath a dense aponeurotic covering—the thoracolumbar fascia (fascia thoracolumbalis).
Name the most powerful and longest muscle of the back.
This is the erector spinae muscle (m. erector spinae), which belongs to the first superficial layer of the deep muscles.
Where does m. erector spinae originate?
It originates from the posterior part of the iliac crest, the sacrum, and the spinous processes of the lumbar vertebrae.
To which structures does the longissimus muscle (m. longissimus) attach?
Its fascicles attach to the transverse processes of the vertebrae and to the mastoid process of the skull.
What else to study on this topic
- Anatomy and Functions of the Second Layer of Deep Back Muscles
- Structure of the Third Layer of Deep Spinal Muscles
- Thoracolumbar Fascia (fascia thoracolumbalis): Layers and Biomechanics
- Innervation Features of the Back Musculature by Dorsal Rami of Spinal Nerves
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