Erector spinae muscles
The erector spinae muscles are a group of muscles located in the intermediate layer of the intrinsic back muscles. The muscles are expansive; extending the entirety of the vertebral column from the pelvis inferiorly, to the base of the cranium superiorly. Erector spinae differ in size and composition at different levels along the vertebral column.
They are composed of fascicles that attach to different parts of the skull, cervical, thoracic and lumbar vertebrae, sacrum and ilium. Individual muscles are defined by the attachment of these fascicicles and the regions that they extend across. There are three columns of muscle: Iliocostalis, Longissimus, and Spinalis. Each column is then divided regionally according to its superior attachments.
The erector spinae muscles are located in a groove on either side of the vertebral column between the spinous processes and the angles of the ribs. Erector spinae are covered by the thoracolumbar fascia in the thoracic and lumbar regions. They lie deep to the serratus posterior inferiorly and rhomboids and splenius muscles superiorly. The erector spinae group is the main extensor of the vertebral column.
|Origin||Posterior iliac crest; posterior surface of sacrum; sacroiliac ligaments; spinous processes of sacral and lower lumbar vertebrae; supraspinous ligament|
|Insertion||Iliocostalis: angles of the lower ribs; cervical transverse processes
Longissimus: ribs between tubercle and angle; thoracic and cervical transverse processes; mastoid process
Spinalis: upper thoracic spinous processes; cranium
|Innervation||Posterior rami of cervical, thoracic and lumbar spinal nerves|
|Function||Bilaterally: extension of the vertebral column and head; controlling flexion of the vertebral column by gradually lengthening
Unilaterally: Lateral flexion of vertebral column
This article will discuss the anatomy and function of the erector spinae muscles.
Iliocostalis is the most lateral portion of the erector spinae muscles. It is divided regionally into iliocostalis cervicis, iliocostalis thoracis, and iliocostalis lumborum.
Fibres of the iliocostalis cervicis originate from the posterior tubercles of C4, C5, and C6. They insert onto the angle of ribs 3-6. Iliocostalis thoracis arises from the transverse process of C7 and the superior borders of the angle of the upper six ribs. Its fibres are located more lateral to iliocostalis cervicis, inserting at the upper borders of the angles of the lower six ribs. Iliocostalis lumborum has both lumbar and thoracic parts.
Iliocostalis thoracis arises from the angle of the lower eight or nine ribs. Their tendons join to form a dorsal aponeurosis that covers the lumbar part of iliocostalis lumborum, attaching to the iliac crest. Iliocostalis lumborum originates from the transverse processes of L1, L2, L3, and L4. It passes inferiorly to insert onto the iliac crest.
Longissimus is the central component of erector spinae. Longissimus capitis is a narrow, flat band of muscle attached to the mastoid process, deep to the splenius capitis and sternocleidomastoid. It descends across semispinalis capitis and inserts into the transverse processes of the lower cervical and upper thoracic vertebrae.
Longissimus cervicis is a long thin muscle that originates from the transverse processes of C2-C6 vertebrae. It descends into the thoracic region between the tendons of longissimus capitis and longissimus thoracis to attach to the transverse processes of T1-T5.
Longissimus thoracis is the largest part of erector spinae. It consists of fascicles with small, fusiform muscle bellies that have short rostral tendons and long caudal tendons. The upper fascicles arise from the tips of the transverse processes of T1-T4. The remaining fascicles arise as bifid tendons from the transverse processes and adjacent ribs of the lower eighth thoracic segments.
The long caudal tendons of longissimus thoracis converge to form an aponeurosis, allowing the tendons to assume a variety of insertions. The tendons of the uppermost fascicles insert onto the lumbar spinous processes; the fascicles for the 7th to 9th thoracic segments insert onto the median sacral crest; and the fascicles from the 10th and 11th thoracic segments insert transversely across the posterior surface of the sacrum. The fascicle from the 12th thoracic segment inserts onto the sacrum and the dorsal aspect of the iliac crest.
Longissimus lumborum is formed by fleshy bundles that arise from the accessory process and medial half of the posterior surface of the transverse process of the lumbar vertebrae. The fascicles pass inferiorly and laterally. The first four fascicles converge on a common flat tendon that covers the lateral aspect of the muscle, separating it from the iliocostalis lumborum muscle fibres. This aponeurosis attaches to the medial surface of the ilium, just dorsal to the ala of the sacrum. The 5th fascicle attaches deep to the aponeurosis, onto the anteromedial aspect of the ilium, and the upper part of the dorsal sacroiliac ligament.
Spinalis is the most medial portion of erector spinae. Spinalis thoracis consists of fascicles that arise from the spinous processes of T11-L2. The fascicles overlap each other, varying in length. The shortest fascicles have the lowest origin and the highest insertion; the longest fascicles have the highest origin and the lowest insertion. Laterally, the muscles blend with the longissimus thoracis.
Spinalis is irregular and poorly developed in other regions along the vertebral column. When present, spinalis cervicis arises variably from the spinous process of C2-C4, and inserts onto the lower part of the ligamentum nuchae. Spinalis capitis consists of occasional fibres of semispinalis capitis that insert onto the spines of C7 and T1.
The aponeuroses of the thoracic fibres of longissimus and iliocostalis lumborum form a wide sheet of parallel tendons known as the erector spinae aponeurosis. The aponeurosis is attached to the lumbar spinous processes and supraspinous ligament, the median sacral crest, and the dorsal iliac crest, covering multifidus and the lumbar parts of longissimus and iliocostalis. It is formed exclusively by the tendons of the thoracic fibres of longissimus and iliocostalis lumborum; the lumbar portions of these muscles are attached separately to the ilium.
The thoracic and lumbar components of erector spinae are powerful extensors of the vertebral column. Acting concentrically and bilaterally, they extend the trunk. Unilateral action of the muscle produces lateral flexion of the trunk. However, the action of erector spinae is usually eccentric. From the upright position, the trunk can flex forward, initiated by the flexor muscles, largely rectus abdominis, and influenced by gravity. The erector spinae muscles control the descent of the thorax under gravity. Similarly, lateral flexion of the trunk is controlled by eccentric contraction of the contralateral erector spinae muscles, assisted by the oblique abdominal muscles.
The function of the cervical and capital components of erector spinae is less clear, they have very little force capacity and poor orientation to exercise tension or control flexion. Their extensor function is overshadowed by the action of the semispinalis and splenius muscles.
Innervation & blood supply
The arterial supply to erector spinae is by a large number of arteries that supply at different intervals along the length of the vertebral column. These include: the vertebral artery, the deep cervical artery, superficial and deep descending branches of the occipital artery, the transverse cervical artery, dorsal branches of the superior intercostal, posterior intercostal and subcostal arteries, and dorsal branches of the lateral sacral and median sacral arteries. The dorsal branches form an arterial trunk which runs in a groove between the erector spinae and multifidus muscles, giving off branches to supply the muscle tissue.
Venous drainage mirrors the arterial supply and is by the vertebral vein, deep cervical vein, the occipital vein, the transverse cervical vein, the superior intercostal veins, posterior intercostal and subcostal veins, and lateral sacral and median sacral veins.
Back pain is an extremely common issue. Back pain is one of the leading reasons why people visit their doctors and second only to headaches for cause of people missing work. Localised lower back pain is usually muscular, related to spasming of the muscle as a result of guarding, or contraction of the muscles in anticipation of pain. Common causes of back pain are sprains and strains.
Strains consist of a degree of tearing or stretching of the muscle fibres. Erector spinae muscles are most commonly affected by back strains. Back strains are usually the result of incorrect balancing of a load on the vertebral column, placing strain on the muscles. Using the back as a lever when lifting objects places immense strain on the back muscle, which is why lifting should usually be focused at the knees. The muscles spasm as a protective mechanism after injury.
Spasms are involuntary contractions of muscles which present as cramps, pain, and decreased function. Adequate warm up and stretching, as well as exercises to increase the muscle tone of the back muscles, such as erector spinae, are the main mechanisms for preventing back strains and back pain by stabilizing the vertebral column.