The muscles of the erector spinae group are covered in the lumbar region by the thoracolumbar fascia, in the lumbar and thoracic regions by the serratus posterior muscles and in the neck by the splenius muscles.
In the lumbar region, the thoracolumbar fascia is trilaminar and encloses erector spinae posteriorly and quadratus lumborum anteriorly (Fig. 8.28). At the lateral margins of both muscles, the laminae of the fascia fuse and give attachment to the transversus abdominis (Fig. 8.28).
Serratus posterior superior and inferior (Figs 8.29 & 8.30) are weak respiratory muscles, which connect lower cervical and upper thoracic spines with upper ribs, and lower thoracic and upper lumbar spines with lower ribs, respectively. They are supplied by branches of anterior rami of thoracic spinal nerves (intercostal nerves).
Splenius (Fig. 8.31) attaches medially to the lower part of the ligamentum nuchae and to the upper thoracic spinous processes. The superior part, splenius capitis, attaches laterally to the mastoid process and superior nuchal line. The inferior part, splenius cervicis, attaches laterally to the transverse processes of the upper cervical vertebrae.
On each side, erector spinae (Fig. 8.33) occupy the groove between the spinous and transverse processes of vertebrae. The muscle attaches inferiorly to the posterior surface of the sacrum and the posterior part of the iliac crest. In the lumbar region it forms a prominent vertical muscle mass (Fig. 8.37) and attaches to the spinous and transverse processes of the lumbar vertebrae. In the upper lumbar region the muscle divides into three columns. The lateral column forms iliocostalis, which attaches to the angles of the ribs and transverse processes of the lower cervical vertebrae. The intermediate column forms the longissimus, which attaches to the ribs and the transverse processes of the thoracic and cervical vertebrae. The medial part of the erector spinae forms the spinalis, which attaches to the spinous processes of the upper lumbar, thoracic and lower cervical vertebrae.
The muscles of this group lie obliquely (Fig. 8.32), covered by erector spinae (Fig. 8.7). They interconnect transverse processes and spinous processes of vertebrae at higher levels. Semispinalis forms the longest and most superficial member of this group and interconnects lower thoracic transverse processes and upper thoracic and cervical spinous processes. Multifidus lies deep to semispinalis. Its fibres attach the transverse process of each vertebra to the spinous processes of one to three vertebrae above. The deepest fibres of transversospinalis form the rotatores, which connect the lamina of one vertebra to the transverse process of the vertebra below. The rotatores are best developed in the thoracic region.
The transversospinalis muscles are supplied by the posterior rami of the spinal nerves.
Simultaneous contraction of the deep muscles of both sides extends the vertebral column and regulates or prevents flexion. Unilateral contraction produces lateral flexion and rotation, assisted by the abdominal oblique muscles. The deep muscles have a very important role in achieving and maintaining the fully upright posture.