The erector spinae group is the intermediate layer of the intrinsic muscles of the back. This group is made of three subgroups, with the group divisions occurring by location. The iliocostalis group occurs most laterally, followed by the longissimus group, and finally the spinalis as the most medial. The erector spinae muscles are the most powerful extensors of the spine.
Several texts state that the erector spinae muscles have a single, common insertion point of a large broad tendon on the back of the sacrum, the inner side of the iliac crest, sacroiliac ligaments, and lumbosacral spinous processes 2-5. The reference for the following origin and insertion points is Gray's Anatomy (40th edition) 1 unless otherwise referenced.
The spinalis group consists of the spinalis capitis, spinalis cervicis and spinalis thoracis. The spinalis thoracis is the most consistent and prominent muscle of this group. The spinalis capitis and spinalis cervicis are variably developed and variably present.
- not a truly separate muscle - when existent consists of occasional fibres from the semispinalis capitis, but with a different insertion
- origin: (as of semispinalis capitis) medial aspect between the superior and inferior nuchal lines of the occiput
- insertion: spines of C7 and T1 (as opposed to the superior articular processes of C4-T6 or T7 for the semispinalis capitis
- spinalis cervicis
- most medial of the erector spinae muscle group with fascicles varying in length and attachments
- origin: spinous processes T1-4 6
- insertion: spinous processes of T11, T12, L1, and L2
- blends with the longissimus thoracis laterally; the erector spinae is considered in some texts to be a component of that muscle 1
The longissimus group is the predominant component of the erector spinae group and consists of longissimus capitis, longissimus cervicis, and longissimus thoracis.
- longissimus capitis
- origin: tendons from the transverse processes of C2-C6
- insertion: tendons to the transverse processes of T1-T4 or T5
- largest muscle of the erector spinae group, and has a thoracic and lumbar component.
- thoracic component
- multiple short muscle bellies with shorter tendons cephalically and longer tendons caudally
- muscle bellies are organised such that the highest are more medial and the lower are more lateral
- origin: uppermost fascicles start from transverse processes of T1-4. Subsequent fascicles have bifid origins and arise from the transverse processes of T5-12 as well as the adjoining ribs of T5-12.
- insertion: caudal fibres of the thoracic fascicles form a wide aponeurosis with the uppermost fascicles inserting into the L1-5 spinous processes and their supraspinous ligaments. The pattern is as follows: 1st thoracic segment inserts into L1 2; 6th thoracic segment inserts into L5; 7th to 9th thoracic segments insert into the median sacral crest; 10th to 11th thoracic segments insert into posterior surface of the 3rd segment of the sacrum; 12th thoracic segment inserts into the sacrum and the dorsal iliac crest segment at a point inferior to the insertion of the intermuscular aponeurosis of the lumbar fibres of the longissimus thoracis.
- lumbar part
- origin: medial half of the transverse processes of L1-5
- insertion: fascicles from the L1-L4 insertion converge on a single flat tendon. This convergence is known as the lumbar intramuscular aponeurosis and it covers the lateral surface of the muscle. The aponeurosis begins in the mid-lumbar region, tapering inferiorly before inserting on the medial surface of the ilium dorsal to the ala of the sacrum. The lumbar intramuscular aponeurosis is an extension of the erector spinae aponeurosis. L5 fascicles pass deep to the lumbar intramuscular aponeurosis, attaching at both the ventromedial aspect of the ilium and the upper fibres of the dorsal sacroiliac ligament.
The iliocostalis muscles are the most lateral components of the erector spinae group. This subgroup includes the iliocostalis cervicis, iliocostalis thoracis, and iliocostalis lumborum.
- consists of long and slender tendons
- origin: posterior tubercles of C4-6
- insertion: ribs 3-6 at the posterior angles
- narrow, fusiform, lies lateral to the iliocostalis cervicis
- origin: transverse process of C7, ribs 1-6 superior borders of the posterior angle
- insertion: upper borders of the angles of ribs 7-12
- has a thoracic component as well as a lumbar component
- thoracic part
- each fascicle has a ribbon-like tendon attaching to the point of origin, with the longest tendons at the higher level with progressive shortening towards the lower levels. This is accompanied by a uniformly long muscle belly and a caudal tendon.
- caudal tendons form an aponeurosis that covers the lumbar part of the iliocostalis lumborum
- origin: ribs 3 or 4 to 12 at their angles, attaching lateral to the iliocostalis thoracis
- insertion: fascicles form an aponeurosis which inserts linearly to the medial end and dorsal segment of the iliac crest. Upper fascicles insert more medially and inferiorly, while lower fascicles inserting more laterally and superiorly.
- lumbar component
- origin: tips of the transverse processes and the posterior surface of the middle layer of the thoracolumbar fascia of L1-L4
- insertion: medial end and dorsal segment of the iliac crest in a fan-like fashion, with L1 fascicle being most dorsal and medial, and L4 being most ventral and lateral
Erector spinae aponeurosis
- the dorsal aponeurosis of the thoracic fibres of longissimus and the thoracic fibres of iliocostalis lumborum both form tendons, which combine to from the erector spinae aponeurosis
- other contributions come from some superficial fibres of the multifidus
- covers the multifidus and lumbar fibres of both longissimus and iliocostalis
- attached to the lumbar spinous processes and corresponding supraspinous ligaments, median sacral crest, the 3rd sacral segment, dorsal segment of the iliac crest, medial end of the iliac crest
- a portion of the upper fibres of the gluteus maximus arise from the erector aponeurosis
- the lumbar intermuscular aponeurosis is a ventral extension of the erector spinae aponeurosis, separating the lumbar fibres of the longissimus from the iliocostalis
- deep to the thoracolumbar fascia in the lumbar and thoracic spinal regions, serratus posterior inferior in the caudal region, and rhomboid major and minor and splenius capitis and cervicis cephalically
- between the posterior and middle layers of the thoracolumbar fascia in the lumbar regions
- lateral branches of dorsal rami of cervical, thoracic and lumbar spinal nerves
- at the lumbar levels, lateral branches of the dorsal rami innervate iliocostalis and intermediate branches innervate longissimus
- extensors of the back when the thoracic and lumbar components of the erector spinae act bilaterally
- lateral flexion when the thoracic and lumbar components act unilaterally
- control of the descent of the thorax towards gravity when the abdominal muscles contract until abdominal muscles are fully flexed - at that point flexion is limited by passive tension from the components of the back.
- control of the descent of the thorax towards gravity during lateral flexion, aided by the abdominal obliques
- functions of the cervical and capital erector spinae muscles are not clear 1
- 1. Gray's Anatomy. Churchill Livingstone. (2008) ISBN:0443066841. Read it at Google Books - Find it at Amazon
- 2. Sinnatamby CS. Last's Anatomy. Elsevier Health Sciences. (2011) ISBN:0702048399. Read it at Google Books - Find it at Amazon
- 3. Kelley LL, Petersen C. Sectional Anatomy for Imaging Professionals. Elsevier Health Sciences. (2013) ISBN:0323277608. Read it at Google Books - Find it at Amazon
- 4. Benzel EC. Spine Surgery 2-Vol Set. Elsevier Health Sciences. (2012) ISBN:1455723320. Read it at Google Books - Find it at Amazon
- 5. Dalley AF. Grant's Atlas of Anatomy. Lippincott Williams & Wilkins. (2009) ISBN:0781770556. Read it at Google Books - Find it at Amazon
- 6. Clemente CD. Clemente's Anatomy Dissector. LWW. (2010) ISBN:1608313840. Read it at Google Books - Find it at Amazon
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