Overview of the back muscles

Superficial layer

Trapezius muscle

The trapezius is a large triangular muscle that consists of three parts with different origin and insertion points:

• Descending part (superior fibers): originates from the medial third of the superior nuchal line and the external occipital protuberance. It courses inferiorly and inserts onto the lateral third of the clavicle.
• Transverse part (medial fibers): originates from the nuchal ligament attached to the spinous processes of C1-C6 vertebrae, spinous processes and supraspinous ligaments of vertebrae C7-T3. It courses medially and inserts to the medial margin of the acromion of the scapula and superior crest of the spine of scapula.
• Ascending part (inferior fibers): originates from the spinous processes and supraspinous ligaments of vertebrae T4-T12 and inserts on the lateral part of the apex of the spine of scapula.

The trapezius muscle is innervated by the accessory nerve (CN XI). The functions of the trapezius muscle are numerous:

1. It stabilizes, elevates, depresses, retracts and rotates the scapula at the scapulothoracic joint.
2. It contributes to the ipsilateral lateral flexion and contralateral rotation of the head during unilateral contraction, as well as extension of the head during bilateral contraction.

Learn everything about the trapezius muscle with our article:

Latissimus dorsi muscle

The latissimus dorsi is the widest muscle of the human body, stretching across almost the entire surface of the back. This muscle has four parts with different originating points:

• Vertebral part: originates from the spinous processes of the 7th to 12th thoracic vertebrae and the thoracolumbar fascia
• Costal part: originates from the 9th to 12th ribs
• Iliac part: originates from the iliac crest
• Scapular part: originates from the inferior angle of the scapula (inconstant)

All the fibers converge superolaterally and insert onto the intertubercular sulcus of humerus. The functions of the latissimus dorsi muscle are to extend the flexed arm, to adduct and internally rotate the arm and to pull the pectoral girdle backward as a whole when the humerus is fixed against the scapula.

Rhomboid major and minor muscles

The rhomboid muscles are the two small quadrangular muscles located between the medial margins of the scapulae.

• The rhomboid major muscle originates from the spinous processes of the vertebrae T2-T5 and inserts into the medial border of scapula, between the inferior angle and root of the spine of scapula.
• The rhomboid minor muscle originates from the nuchal ligament and spinous processes of vertebrae C7 and T1. It inserts into the base of the medial end of the spine of scapula, just above the rhomboid major.

The function of the rhomboid muscles is to stabilize and retract the scapula at the scapulothoracic joint.

Levator scapulae muscle

The levator scapulae is an elongated muscle that extends from the upper cervical vertebrae to the scapula. It originates from the transverse processes of vertebrae C1-C4 and inserts onto the medial margin of the scapula.

The main function of this muscle is to elevate the scapula, just like its name suggests. It also contributes to the ipsilateral lateral flexion of the neck when it contracts unilaterally, as well as the extension of the neck when it contracts bilaterally.

Intermediate layer

Serratus posterior muscles

The serratus posterior superior and inferior are the two paired muscles that span from the vertebrae to the ribs.

• The serratus posterior superior muscle originates from the nuchal ligament and spinous processes of vertebrae C7-T3. It inserts onto the superior borders of ribs 2-5.
• The serratus posterior inferior muscle originates from the spinous processes of vertebrae T11-L2 and inserts on the inferior borders of ribs 9-12.

The function of the serratus posterior muscles is to assist breathing by elevating the ribs (serratus posterior superior) during inspiration, and depressing them (serratus posterior inferior) during exhalation.

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Superficial layer

Splenius muscles

The splenius muscle group gathers two muscles:

• Splenius capitis muscle, which originates from the spinous processes of vertebrae C7-T3 and nuchal ligament, and inserts onto the lateral superior nuchal line of occipital bone and mastoid process of temporal bone.
  
• Splenius cervicis muscle, which originates from the spinous processes of vertebrae T3-T6, and inserts onto the transverse processes of vertebrae C1-C3.

The function of the splenius muscles is to rotate, laterally flex and extend the neck. Learn everything about them here:

Erector spinae muscles

The erector spinae is a massive muscle group composed of three muscular columns that are located on either side of the spine. From medial to lateral, the muscles of the erector spinae group are:

• Spinalis muscle, which is regionally divided into spinalis capitis, spinalis cervicis (colli) and spinalis thoracis. They attach between the spinous processes of the vertebrae of their corresponding regions.
  
• Longissimus muscle is also divided into longissimus capitis, longissimus cervicis (colli) and longissimus thoracis. They attach between the transverse processes of the vertebrae of their corresponding regions.
• Iliocostalis muscle is divided into iliocostalis cervicis (colli), iliocostalis thoracis and iliocostalis lumborum. They span between the angles of ribs and transverse processes of their corresponding regional vertebrae.
  

The function of each part of the erector spinae, and so of the muscle as a whole is to extend (bilateral contraction) and laterally flex the spine (unilateral contraction).

Deep layer

Transversospinal muscles

The transversospinal muscles gather three groups of back muscles:

• The semispinalis muscle, which is topographically divided into the semispinalis capitis, semispinalis cervicis and semispinalis thoracis. They span between the transverse and spinous processes of the regional vertebrae.
• The multifidus muscle, which consists of the multifidus cervicis, multifidus thoracis and multifidus lumborum. They originate from the articular, transverse and mammillary processes of the regional vertebra and insert onto the spinous processes of the vertebrae 2-5 levels above.
• The rotatores muscles are divided into rotatores breves and rotatores longi. The rotatores breves connect the transverse processes of the thoracic vertebrae with the spinous processes of the vertebra one level above, while the rotatores longi span two levels above the origin point. 

The functions of the transversospinal muscles are shown in the table below:

Deepest layer

Interspinales muscles

The interspinales are the short muscles that extend between the adjacent spinous processes of the vertebrae.

They are divided into interspinales cervicis, interspinales thoracis and interspinales lumborum. However, only the cervical and lumbar parts are well developed, while the thoracic can often be absent or rudimentary. The function of the interspinales muscles is to contribute to the extension of the cervical and lumbar spine.

Intertransversarii muscles

The intertransversarii span between the adjacent transverse processes of vertebrae. They are best developed in the cervical and lumbar spine, while in the thoracic region they are often absent.

• The intertransversarii colli are divided into the anterior and posterior groups, with their joint function being the contribution in lateral flexion and stabilization of the cervical spine.
• The intertransversarii lumborum consist of medial and lateral slips and their function is to assist in lateral flexion of the spine.

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