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The intermediate back muscles are those that are attached to the ribs and originate from the spinous processes. They are involved in moving the ribs and assisting with breathing. The intermediate group contains two muscles: the serratus posterior superior and serratus posterior inferior. These muscles run from the vertebral column to the rib cage, elevating and depressing the ribs. The intrinsic muscles are the deepest layer of back muscles, sitting under the other two layers closest to the spine.
| Characteristics | Values |
|---|---|
| Number of muscles | 2 |
| Names of muscles | Serratus posterior superior, Serratus posterior inferior |
| Location | Above and below the ribcage |
| Function | Elevate and depress the ribs, Stabilise the thorax, Assist with breathing |
| Muscle type | Extrinsic |
| Innervation | Intercostal nerves, Subcostal nerve (T12) |
| Arterial supply | Intercostal arteries |
| Layer | Intermediate |
| Muscle shape | Thin, rectangular |
| Muscle composition | Finger-like projections of a strap muscle |
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What You'll Learn
Serratus posterior superior and inferior
The intermediate muscles are associated with movements of the thoracic cage. They are classified as extrinsic muscles as they do not develop in the back. The intermediate group contains two muscles: the serratus posterior superior and serratus posterior inferior. These muscles run from the vertebral column to the rib cage and assist with elevating and depressing the ribs. They are thought to have a slight respiratory function.
Serratus Posterior Superior
The serratus posterior superior is a thin, rectangular muscle that originates from the C7-T3 vertebrae spinous process and nuchal ligament. It inserts at the upper border of the 2nd-5th ribs. It is innervated by the ventral rami of the intercostal nerves of T1-T5. The serratus posterior superior helps to elevate the upper ribs during inhalation. Its location in the shoulder girdle region means that its pain pattern overlaps with that of other muscles. Deep aches under the shoulder blade, in the back of the shoulder, the point of the elbow, and the pinky side of the wrist and hand are common symptoms of trigger points in this muscle.
Serratus Posterior Inferior
The serratus posterior inferior originates from the T11-L2 vertebrae spinous process and inserts at the lower border of the 9th-12th ribs. It is innervated by the ventral rami of the intercostal nerves of T9-T12. The serratus posterior inferior helps to draw the lower ribs downward and backward during exhalation. Pain in the serratus posterior inferior is localized and does not have a referred pattern.
Bilateral action of the serratus posterior superior and serratus posterior inferior reduces the extension of the thoracic vertebrae. Unilateral action rotates the spine to the opposite side. Both muscles may participate in and cause myofascial pain syndrome, especially the serratus posterior superior.
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Innervation and functions
The intermediate muscles of the back support respiratory function. They are also referred to as the intermediate extrinsic muscles and include the serratus posterior superior and serratus posterior inferior. These muscles run from the vertebral column to attach to the ribs, providing stabilisation to the ribs and promoting forceful expiration. The intercostal nerves of ribs 2-5 supply sensorimotor innervation to the serratus posterior superior.
The iliocostalis muscle forms the lateral column of the erector spinae muscle group. The muscle is innervated by lateral branches of the posterior rami of cervical, thoracic, and lumbar spinal nerves. The iliocostalis cervicis is vascularised by the occipital, deep cervical, and vertebral arteries. The iliocostalis thoracis is supplied by the dorsal branches of the posterior intercostal and subcostal arteries, while the iliocostalis lumborum is supplied by the lumbar and lateral sacral arteries. The function of the iliocostalis muscles is to produce ipsilateral lateral flexion.
The semispinalis capitis is innervated by the greater occipital nerve (posterior ramus of C2 spinal nerve) and spinal nerve C3. The semispinalis cervicis and the semispinalis thoracis are innervated by medial branches of posterior rami of spinal nerves. The semispinalis muscle is vascularised by branches of the occipital, deep cervical, vertebral, and dorsal branches of posterior intercostal arteries. Bilateral contraction of this muscle draws the head posteriorly, extending the neck and thoracic spine. Unilateral contraction, on the other hand, causes ipsilateral flexion of the neck and thoracic spine with contralateral rotation of the head.
The splenius muscles are innervated by the posterior rami of the middle and lower cervical spinal nerves. The blood supply for both muscles comes from the vertebral, occipital, superior intercostal, deep cervical, and transverse cervical arteries. When acting together, both muscles produce extension of the neck. When acting individually, each muscle causes lateral flexion of the neck and rotation of the head to the same side.
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Attachments
The human body has over 600 muscles, and about 700 of these are attached to the bones of the skeletal system. These muscles make up about half of a person's body weight.
Muscle attachments refer to the points where muscles connect to bones, enabling movement and stability. These connections are typically categorized into two types: the origin, which is the fixed attachment point, and the insertion, which is the movable attachment point that facilitates motion. The triceps brachii, for example, has four points of attachment: one insertion on the ulna and three origins on the humerus and scapula.
The intermediate back muscles include the serratus posterior superior and serratus posterior inferior. These muscles originate from the thoracic and lumbar spines (usually T11-L3). The fibres pass in a superolateral direction, attaching to ribs 9-12.
The superficial and intermediate muscles of the back are classified as extrinsic muscles, as they do not develop in the back. The latissimus dorsi, for example, has a broad origin, arising from the spinous processes of T6-T12, thoracolumbar fascia, iliac crest, and the inferior three ribs. The fibres converge into a tendon that attaches to the intertubercular sulcus of the humerus.
The levator scapulae is another example of a superficial back muscle. It is a small strap-like muscle that begins in the neck and descends to attach to the medial border of the scapula. It originates from the transverse processes of the C1-C4 vertebrae.
Tendons are the connective tissues that attach muscles to bones, enabling the transmission of force that facilitates movement. They act as intermediaries, converting the contraction of muscles into joint movement, and providing stability and support to the skeletal structure.
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Intermediate back muscles anatomy
The back muscles are organised into three main groups that run from the neck, down the spine to just above the hips. The three groups are the intrinsic or deep muscles, the superficial muscles, and the intermediate muscles.
The intrinsic or deep muscles are the deepest layer of back muscles, located under the other two layers, closest to the spine. They are considered the true back muscles. These muscles fuse with the vertebral column and help with movements of the vertebral column. They also play a role in proprioception and balance, as well as maintaining posture. The three deep muscles of the back include the semispinalis, multifidus, and rotatores.
The superficial muscles are located posteriorly on the back and help with shoulder, neck, and scapula movements. They include the trapezius, latissimus dorsi, rhomboid major, rhomboid minor, and levator scapulae.
The intermediate back muscles are located just above and below the ribcage. They help with the movement of the thoracic cage, including the movement of the ribs during breathing. There are two intermediate back muscles: the serratus posterior superior and the serratus posterior inferior. The serratus posterior superior is a thin, rectangular muscle located in the space between the shoulder blades (the shoulder girdle), under the rhomboid muscles. The serratus posterior inferior is located at the bottom of the rib cage, under the lats.
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Intermediate layer muscles
The intermediate back muscles are just above and below the ribcage. They help move the ribs, including during breathing. There are two intermediate back muscles: the serratus posterior superior and the serratus posterior inferior. These muscles run from the vertebral column to the ribcage. The serratus posterior superior is a thin, rectangular muscle in the space between the shoulder blades (the shoulder girdle). It is under the rhomboid muscles and assists in elevating the ribs. The serratus posterior inferior is the muscle at the bottom of the ribcage, under the lats, and it depresses the ribs. The fibres of the serratus posterior inferior are finger-like projections of a strap muscle. It functions to stabilise the thorax during passive respiration and depresses ribs 9 to 12 during active inspiration.
The intermediate layer also contains the large erector spinae muscles, which are sometimes called the long muscles of the back. These consist of the iliocostalis, longissimus, and spinalis. The longissimus muscle is vascularized by the vertebral artery, deep cervical artery, superficial and deep descending branches of the occipital artery, and the deep branch of the transverse cervical artery. During bilateral contraction, the longissimus functions as a powerful extensor of the lumbar, thoracic, and cervical spine, as well as an extensor of the head and neck.
The multifidus muscle also belongs to the intermediate layer of the transversospinalis muscle group. This muscle is composed of many short, triangular muscles that span the entire length of the vertebral column but are thickest and most developed in the lumbar region.
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Frequently asked questions
The intermediate muscles are a group of muscles in the back, specifically located just above and below the ribcage. They are involved in moving the ribs during breathing.
The intermediate back muscles are the serratus posterior superior and the serratus posterior inferior.
The serratus posterior superior is the muscle found in the space between the shoulder blades, under the rhomboid muscles. It helps to elevate the ribs.
The serratus posterior inferior is the muscle at the bottom of the ribcage, under the lats. It helps to depress the ribs.
Other intermediate muscles include the semispinalis, multifidus, and rotares, which are part of the transversospinales group. The erector spinae muscles are also considered intermediate muscles.
