Erector spinae is a large, complex, powerful mass of muscle consisting of several parts running the length of the vertebral column.
Figure. The erector spinae muscle mass with its constituent parts shown.
In the lumbar region, it has a broad belly with a well-defined lateral border, but as it extends upwards it divides into three parallel columns, each of which is divided into three parts according to their relative positions.
Erector spinae arises inferiorly from a strong, thick, flat tendon attached along a U-shaped line around the origin of multifidus. The medial limb arises from the spinous processes of Tl 1 to L5, spreading onto the supraspinous ligaments and associated median sacral crest. The lateral limb attaches to the lateral sacral crest, the sacrotuberous, sacrococcygeal and posterior sacroiliac ligaments, and the posterior pan of the iliac crest medial to internal oblique. Deep to the lateral limb erector spinae has a fleshy attachment to the iliac tuberosity and the inner lip of the iliac crest. From this extensive origin the muscle fibres pass upwards deep to latissimus dorsi splitting into three columns.
The most lateral of the three columns, iliocostalis can be divided into lumbar, thoracic and cervical parts. Iliocostalis lumborum inserts by six slips into the inferior borders of the lower six ribs near their angles. Medial to each slip arises iliocostalis thoracis which inserts near the angles of the upper six ribs and the transverse process of C7. Finally, iliocostalis cervicis arises medial to the slips of thoracis to insert into the posterior tubercles of the transverse processes of C4-C7.
The intermediate column of erector spinae and is the longest and thickest. It can be divided into thoracic, cervical and capitis parts. Longissimus thoracis runs from the transverse and accessory processes of the lumbar vertebrae and adjacent thoracolumbar fascia to insert by two sets of slips to the transverse processes of all 12 thoracic vertebrae and adjacent regions of the lower 10 ribs. Longissimus cervicis runs from the transverse processes of Tl-T6, medial to thoracis, to the posterior tubercles of the transverse processes of C2-C6. Longissimus capitis arises from the transverse processes of Tl-T5, in common with longissimus cervicis, and articular processes of C4-C7, and inserts into the posterior aspect of the mastoid process.
The medial relatively insignificant column of erector spinae, again divided into thoracic, cervical and capitis parts. Spinalis thoracis, the most clearly demarcated portion, runs from the spinous processes of Tl 1 to L2 to those of Tl to T6. Spinalis cervicis and capitis are poorly developed, and frequently blend with adjacent muscles.
All parts of the erector spinae are supplied by adjacent posterior primary rami according to their position.
When the three muscle columns of both sides of erector spinae act together, they extend the lumbar, thoracic and cervical spines, as well as the head on the neck. Consequently, it is the major extensor of the trunk. However, it is also important in controlling flexion of the trunk. When the three muscle columns of one side act together they produce combined lateral flexion and rotation to the same side. When standing on one leg, the lower part of erector spinae on the non-weight-bearing side works strongly to prevent the pelvis dropping. During walking, erector spinae contracts alternately steadying the vertebral column on the pelvis.
Because the main mass of muscle is situated in the lumbar region, it (particularly longissimus thoracis) is responsible for maintaining the secondary lumbar curvature during sitting and standing.
Each erector spinae can be felt and seen as a column of muscle either side of the lumbar spine, particularly during extension. Each muscle mass can also be felt contracting when alternately standing on one leg and then the other.