The back muscles that lie deep into the thoracolumbar fascia are called intrinsic or true back muscles. These muscles are divided into four layers: superficial, intermediate, deep, and deepest layers.

Superficial Layer:

The superficial layer consists primarily of the splenius muscles, which include the splenius capitis and splenius cervicis. These muscles are mainly responsible for the head and cervical spine movements, including extension, rotation, and lateral bending. The splenius capitis extends from the upper thoracic and lower cervical vertebrae to the skull, while the splenius cervicis stretches between the thoracic and cervical vertebrae.

Intermediate Layer:

The intermediate layer includes the erector spinae muscles, a group of three long, column-like muscles: the iliocostalis, longissimus, and spinalis. These muscles run almost the entire length of the spine, from the sacrum and iliac crest to the ribs and vertebrae up to the base of the skull. They are crucial for maintaining posture and providing the strength to extend the vertebral column. The erector spinae muscles also assist in lateral flexion and, when acting unilaterally, aid in bending the spine to the side.

Deep and Deepest Layers:

The deep layer comprises the transversospinalis muscles, including the semispinalis, multifidus, and rotatores. These shorter muscles are primarily involved in stabilizing the spine, aiding in extension, and allowing for controlled rotation and bending of the vertebral column. The deepest layer consists of minor deep muscles like the interspinales and intertransversarii, which span between adjacent vertebrae and contribute to the stability and localized movements of the spine. Additionally, the levatores costarum, found in this layer, assist in elevating the ribs, which is particularly important during deep inhalation.