It bends the spine such that the head moves posteriorly while the chest protrudes anteriorly. It is also known as sacrospinalis in older texts. It is sometimes referred to as extensor spinae although this isn’t as commonly used.
The erector spinæ is not just one muscle, but a bundle of muscles and tendons. It is paired and runs more or less vertically. It extends throughout the lumbar, thoracic and cervical regions, and lies in the groove to the side of the vertebral column. Erector spinæ is covered in the lumbar and thoracic regions by the thoracolumbar fascia, and in the cervical region by the nuchal ligament.
This large muscular and tendinous mass varies in size and structure at different parts of the vertebral column. In the sacral region, it is narrow and pointed, and at its origin is chiefly tendinous in structure. In the lumbar region, it is larger, and forms a thick fleshy mass. Further up, it is subdivided into three columns. These gradually diminish in size as they ascend to be inserted into the vertebræ and ribs.
The erector spinæ arises from the anterior surface of a broad and thick tendon. It is attached to the medial crest of the sacrum, to the spinous processes of the lumbar and the eleventh and twelfth thoracic vertebræ and the supraspinous ligament, to the back part of the inner lip of the iliac crests, and to the lateral crests of the sacrum, where it blends with the sacrotuberous and posterior sacroiliac ligaments.
Some of its fibres are continuous with the fibres of origin of the gluteus maximus.
This is the major extensor of the trunk. When all 3 muscle columns on both sides act together, extension of the lumbar, thoracic and cervical spine occurs. Contraction of all 3 muscular columns on one side will result in combined lateral flexion and rotation to the same side. The main mass of muscle is situated in the lumbar region (iliocostalis lumborum, spinalis dorsi, longissimus dorsi). The erector spinae functions most effectively when the pelvis is held up at the front. This pulls the anterior superior iliac spine (ASIS) up and posterior superior iliac spine (PSIS) down into a neutral position.
This lowers the origin of the muscle, improving function while keeping the spine straight. In this position the ribs are raised and consequently the abdominals are more effective in holding neutral and flattening the abdominal wall. When the spine has been flexed beyond 90° when standing, the erector spinae reaches a critical point in which it becomes electrically silent. Extension through the initial 2-10° is through the elastic recoil theory. Following this, the erector spinae will kick in as it cannot stabilise through segmental innervation and will stabilise globally.