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The vertebral column, or spine, is made up of 33 vertebrae and provides the body with structural support, enabling movement and sensation. The muscles attached to the spine help to maintain posture and balance the body's weight. These muscles are divided into extrinsic and intrinsic groups, with the former further divided into superficial and intermediate layers, and the latter into deep and deepest layers. The superficial muscles include the splenius capitis and splenius cervicis, which are involved in neck flexion, rotation, and extension. The intermediate layer is made of the erector spinae muscles, which include the iliocostalis, longissimus, and spinalis. These muscles are crucial for maintaining posture and extending the vertebral column. The deep layer comprises the transversospinalis muscles, including the semispinalis, multifidus, and rotatores, which stabilize the spine, aid in extension, and allow for controlled rotation and bending.
| Characteristics | Values |
|---|---|
| Function | Flexion, extension, rotation, and side bending of the back |
| Layers | Superficial, intermediate, deep, and deepest |
| Superficial layer | Splenius capitis and splenius cervicis |
| Intermediate layer | Erector spinae, iliocostalis, longissimus, and spinalis |
| Deep layer | Transversospinalis, semispinalis, multifidus, and rotatores |
| Deepest layer | Interspinales and intertransversarii |
| Muscle groups | Superficial, intermediate, and deep |
| Muscle shape | Named according to shape, location, or a combination of both |
| Muscle function | Flexion, extension, or rotation |
| Muscle contraction | Contraction of both sides results in erect posture, while contraction of one side causes movement |
| Muscle innervation | Nervous supply to the intrinsic/deep back arises from dorsal rami of spinal nerves |
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What You'll Learn
The erector spinae muscles
The deep erector spinae muscles include the quadratus lumborum and multifidus muscles, which are located beneath the superficial muscles. These deep muscles contribute to similar functions, such as extension and rotation of the vertebral column. Contraction of both sides of these deep muscles results in an erect posture, while contraction of one side causes lateral flexion. The small size of these deep muscles makes them susceptible to injury, which can lead to lower back pain.
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The semispinalis muscle
Semispinalis capitis is the largest and most superior component of the semispinalis muscle. It arises from the articular processes of vertebrae C4-C7 and the transverse processes of vertebrae T1-T6. The muscle fibres converge into a single muscle belly that runs superiorly across the lateral side of the vertebral column, inserting onto the occipital bone. The outer surface of semispinalis capitis is covered by the splenius capitis muscle, while its inner surface overlies semispinalis cervicis.
Semispinalis cervicis forms the middle division of the semispinalis muscle. It assists in the extension, lateral flexion, and rotation of the head, cervical, and thoracic spine.
Semispinalis thoracis, also known as semispinalis dorsi, is the most inferior division of the semispinalis muscle. It consists of thin, narrow, fleshy fasciculi interposed between long tendons. This muscle arises from the transverse processes of the sixth to tenth thoracic vertebrae and inserts into the spinous processes of the upper four thoracic and lower two cervical vertebrae.
Together, the semispinalis muscles, along with the multifidus and rotatores muscles, comprise the deep transversospinal layer of the back muscles. These muscles are responsible for the extension, lateral flexion, and rotation of the head, cervical, and thoracic spine. The semispinalis capitis muscle, in particular, assists in maintaining the balance of the head on the neck and helps dynamically maintain cervical lordosis during bilateral contraction.
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The iliocostalis muscle
The function of the iliocostalis muscle is to extend the spine when contracting bilaterally and to laterally flex the spine when contracting unilaterally. The muscle acts in synergy with the two other erector spinae components (longissimus and spinalis) to produce movements of the vertebral column. When these muscles contract on one side and relax on the other, lateral flexion of the vertebral column occurs. When they contract together, they extend the vertebral column. The iliocostalis muscle is also involved in respiration, assisting as an accessory muscle of expiration due to its insertion on the ribs.
The iliocostalis cervicis arises from the angles of ribs 3 to 6 and inserts into the transverse processes of vertebrae C4 to C6. The iliocostalis thoracis originates from the angles of ribs 7 to 12 and inserts into the angles of the upper six ribs and the transverse process of vertebra C7. The lumbar part of the iliocostalis lumborum originates from the lateral crest of the sacrum, the medial end of the iliac crest, and the thoracolumbar fascia. It inserts into the transverse processes of vertebrae L1 to L4 and the adjacent part of the middle layer of the thoracolumbar fascia.
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The multifidus muscles
The multifidus muscle is a long, narrow muscle that runs down both sides of the spine. It is part of the core muscle group, which includes the abdominal muscles and oblique muscles on each side of the body. The multifidus is the innermost layer of three layers of muscles in the back, with the other two layers being responsible for the movement of the thoracic cage (rib cage) and shoulders.
The multifidus muscle has several attachment points, which allow each spinal bone (vertebra) to work individually and more efficiently. This helps protect against spinal deterioration and the onset of arthritis. The main function of the multifidus muscle is to stabilise the lumbar spine, but it also helps extend the lower spine whenever reaching or stretching.
The multifidus muscle works together with the transversus abdominis muscle of the stomach and the pelvic floor muscles of the pelvis to stabilise the lumbar spine while standing or moving. This is achieved by providing stability around the entire torso, including the abdomen and lower back. When the oblique abdominal muscles contract to produce trunk rotation, some flexion of the trunk also occurs. The multifidus muscles oppose this trunk flexion, maintaining a pure axial rotation and acting as stabilisers during trunk rotation.
Weak or atrophied multifidus muscles can cause back pain, and dysfunction in the lumbar multifidus muscles is strongly associated with low back pain. Core stabilization programs are suggested to increase multifidus cross-section area and decrease low back pain. Investigators have categorized multifidus fibre types by layers, with the deepest layer contributing more strength and stability to the spine than the superficial layers. This is possibly because the deep layer only spans two vertebral segments, resulting in a shorter "excursion" of the deep layer of the multifidus.
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The transversospinalis muscles
The semispinalis is the most superficial of the transversospinalis muscles. It spans the thoracic and cervical regions of the vertebral column, with attachments to the occipital bone at the base of the skull. The semispinalis thoracis, cervicis, and capitis are the three semispinalis muscles, each spanning 4-6 vertebral segments. Bilateral contraction of the semispinalis capitis pulls the head backward, while unilateral contraction pulls the head backward and rotates the chin to the same side as the contracting muscle.
The multifidus muscle lies deep to the semispinalis and spans the entire length of the vertebral column, with the thickest development in the lumbar region. The multifidus cervicis arises from the superior articular processes of vertebrae C4-C7, while the multifidus thoracis originates from the transverse process of the thoracic vertebra, and the multifidus lumborum arises from the mammillary processes of lumbar vertebrae.
The rotatores muscle lies deep to the multifidus and is most developed in the thoracic region. It consists of two parts: the long (longus) and short (brevis) rotatores. The long rotatores pass from the transverse process medially to the spinous process, crossing two vertebrae, while the rotatores brevis pass from the transverse process and insert at the base of the spinous process of the vertebrae above the origin.
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Frequently asked questions
The deep muscles of the back, or intrinsic back muscles, are divided into three or four layers: superficial, intermediate, deep, and deepest layers. The superficial layer consists of splenius muscles, which include the splenius capitis and splenius cervicis. These muscles are responsible for head and neck movements, including extension, rotation, and lateral flexion.
The intermediate layer includes the erector spinae muscles, which are made up of three long, column-like muscles: the iliocostalis, longissimus, and spinalis. These muscles are crucial for maintaining posture and extending the vertebral column.
The deep layer comprises the transversospinalis muscles, including the semispinalis, multifidus, and rotatores. These muscles stabilize individual vertebrae positions, aiding in controlled rotation and bending of the vertebral column.
The deepest layer consists of minor deep muscles like the interspinales and intertransversarii, which contribute to the stability and localized movements of the spine.
