General Topography and Classification
The deep back muscles, also referred to as the intrinsic back muscles or the musculature of the spine, form a complex anatomical functional unit. This entire muscle group is securely covered by a dense connective tissue layer—the thoracolumbar fascia (fascia thoracolumbalis).
For clarity of study and biomechanical understanding, the deep muscles are topographically divided into three layers:
- First layer (superficial deep layer);
- Second layer (intermediate 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). This is the longest and functionally most powerful muscle along the entire length of the back.
It has a single, massive origin in the lower trunk. Its points of fixation for its origin include:
- posterior part of the iliac crest;
- sacrum;
- spinous processes of the lumbar vertebrae.
Topographical Parts of m. erector spinae
Ascending superiorly from its origins, the erector spinae divides into three separate columns (tracts).
- Iliocostalis muscle (m. iliocostalis)
Divided into lumbar (lumborum), thoracic (thoracis), and cervical (cervicis) parts. Its main function is extension of the spine and lateral flexion to the same side. Innervation is provided by the dorsal rami of spinal nerves—rr. dorsales nn. spinalium (segments C8–L1).
- Longissimus muscle (m. longissimus)
Also features three subdivisions: thoracic (thoracis), cervical (cervicis), and capitis (capitis). The 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 toward the same side.
- Spinalis muscle (m. spinalis)
Like the previous part, it includes thoracic (thoracis), cervical (cervicis), and head (capitis) regions. The main function of this muscular tract is extension of the vertebral column.