The intrinsic (deep) back muscles lie underneath the superficial and intermediate muscle layers. They work together to maintain posture, stabilize the vertebral column, and move the spine and head. 

Almost all are supplied by the posterior (dorsal) rami of the spinal nerves.

Functionally, they allow the body to extend, rotate, and laterally flex the back while maintaining  stability during movement.

Anatomically, they are divided into three main layers:

1. Superficial (outer) layer.

2. Intermediate (middle) layer.

3. Deep (inner) layer.

These muscles are covered by a layer called deep fascia, which helps organize and support them.

Splenius capitis

The splenius capitis rotates the head to the same side and helps extend the head and neck when both sides work together.

• Origin: Lower part of the ligamentum nuchae and spinous processes of C7–T3/4.

• Insertion: Mastoid process and occipital bone.

• Innervation: Posterior rami of C3–C4 spinal nerves.

Splenius cervicis

The splenius cervicis rotates the head and neck to the same side and assists in extension.

• Origin: Spinous processes of T3–T6.

• Insertion: Transverse processes of C1–C3/4.

• Innervation: Posterior rami of lower cervical spinal nerves.

Semispinalis

The semispinalis is the largest and most superficial muscle of the deep layer. It extends the head, cervical spine, and thoracic spine. Moreover, it laterally flexes the spine (to the same side) and rotates the head/neck/spine to the opposite side.

It is divided into three parts based on where it attaches:

• Semispinalis thoracis.

• Semispinalis cervicis.

• Semispinalis capitis.

Semispinalis fibers run upward and cross 4–6 vertebral levels, giving them a strong effect on the movement of the upper spine and head.

• Origin: Transverse processes of C4–T10.

• Insertion: Spinous processes of C2–T4 and the occipital bone.

• Innervation: Posterior rami of spinal nerves.

Multifidus

The multifidus lies deep to the semispinalis and is most highly developed in the lumbar region, where spinal stability is crucial.

This muscle spans the entire spine but is thickest in the lower back.

It not only stabilizes the vertebral column but also assists with extension of the spine, rotation to the opposite side, lateral flexion, and maintaining posture during movement.

• Origin: Sacrum, iliac spine, and vertebral processes.

• Insertion: Spinous processes 2–4 vertebrae above each origin point.

• Innervation: Posterior rami of spinal nerves.

Rotatores

The rotatores are the deepest and smallest muscles of the transversospinales group. They are found mainly in the thoracic spine, where vertebral rotation is most possible.

There are two types of rotatores:

• Rotatores breves – span one vertebral level

• Rotatores longi – span two vertebral levels

They assist in extension and rotation of the vertebral column, provide significant stability, and are important for proprioception (helping the brain know the spine’s position).

• Origin: Transverse processes of vertebrae.

• Insertion: Spinous processes 1–2 vertebrae above.

• Innervation: Posterior rami of spinal nerves.

Interspinales

The interspinales muscles lie between the spinous processes of adjacent vertebrae. They help extend the spine, especially in the neck and lower back and play a key role in stabilizing vertebral alignment during larger movements made by the bigger muscles.

In the thoracic region, they are often small or absent, since that part of the spine naturally has less movement.

• Origin: Spinous process of one vertebra.

• Insertion: Spinous process of the vertebra directly above.

• Innervation: Posterior (dorsal) rami of spinal nerves.

Levatores costarum

The levatores costarum are small, triangular muscles located in the thoracic region.
They extend from the transverse processes of C7–T11 to the rib directly below.

They elevate the ribs, assisting in inspiration (breathing in), slightly help with rotation and lateral bending of the thoracic spine, and contribute to stabilization of the thoracic vertebrae.

• Origin: Transverse processes of C7–T11.

• Insertion: The rib immediately below the origin point
  (occasionally they attach to the rib two levels below),

• Innervation: Posterior (dorsal) rami of spinal nerves.

Intertransversarii

The intertransversarii muscles are found between the transverse processes of vertebrae. 

They assist with lateral flexion of the spine (side bending), help stabilize the spine by preventing excessive lateral motion, and provide proprioceptive feedback, helping the body sense spinal position.

Like the interspinales, they are well developed in the cervical and lumbar regions and often absent or very small in the thoracic spine.

• Origin: Transverse process of one vertebra.

• Insertion: Transverse process of the vertebra above.

• Innervation: Posterior (dorsal) rami of spinal nerves.

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