Muscles of the Back

The back is composed of several muscles of varying sizes and functions, which are grouped into intrinsic (or primary) back muscles and extrinsic (or secondary) back muscles. This division is based on the functionality and embryologic origin of these muscle groups. The extrinsic muscles comprise the superficial and intermediate muscle groups, while the intrinsic muscles comprise the deep muscles. The deep muscles are further subdivided into superficial, intermediate, and deep muscle layers.

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Overview

Extrinsic back muscles:

  • Superficial and intermediate muscles
  • Connect the axial skeleton with the appendicular skeleton
  • Innervated by ventral rami of spinal nerves

Intrinsic back muscles:

  • Deep muscles (subdivided further into superficial, intermediate, and deep layers)
  • Confined to the axial skeleton
  • Innervated by dorsal rami of spinal nerves
Table: Muscle groups of the back
Group Layer Function
Extrinsic muscles of the back Superficial layer:
  • Trapezius
  • Latissimus dorsi
  • Rhomboid major
  • Rhomboid minor
  • Levator scapulae
  • Trapezius: movements of the scapula, stabilizes the spine
  • Latissimus dorsi: adducts and internally rotates the arm
  • Rhomboids: retract the scapula
  • Levator scapula: elevates and rotates the scapula
Intermediate layer:
  • Serratus posterior superior
  • Serratus posterior inferior
Synergistic muscles of respiration
Intrinsic or deep (or primary) muscles of the back Superficial or spinotransversales group:
  • Splenius capitis
  • Splenius cervicis
Extensors of the cervical spine
Intermediate layer or erector spinae group:
  • Spinalis
  • Longissimus
  • Iliocostalis
  • Unilaterally: laterally flexes the vertebral column
  • Bilaterally: extends the vertebral column
Deep group:
  • Semispinalis capitis, cervicis, and thoracis
  • Multifidus
  • Rotatores longus and brevis
  • Interspinales
  • Intertransversarii
  • Levatores costarum
Rotation and extension of the vertebral column

Extrinsic Back Muscles: Superficial Muscles

Superficial extrinsic back muscles are primarily associated with the movement of the shoulder/arm.

Table: Superficial extrinsic back muscles
Muscle Origin Insertion Innervation Action
Trapezius
  • Descending part: external occipital protuberance, superior nuchal line, and ligament and spinous processes of C2‒C3
  • Transverse part: spinous processes of C4‒T1
  • Ascending part: spinous processes of T5‒T12
  • Descending part: lateral ⅓ of clavicle and acromion
  • Transverse part: scapular spine and acromion
  • Ascending part: scapular spine
Accessory nerve (CN XI) and the cervical plexus (C2–C4)
  • Descending part: extension and rotation of the head and cervical spine
  • Transverse part: moves scapula medially
  • Ascending part: moves scapula medially and caudally
Latissimus dorsi Spinous processes of T7–L5, dorsal sacrum, medial ⅓ of iliac crest, and 9th‒12th ribs Crest of lesser tubercle of the humerus Thoracodorsal nerve (C6–C8)
  • Extensor, adductor, and internal rotator of the humerus
  • Auxiliary respiratory muscle
Rhomboid major Spinous processes of T2–T5 Medial border of the scapula Dorsal scapular nerve (C5) Fixes and moves the scapula cranially and medially
Rhomboid minor Spinous processes of C7‒T1 Medial border of the scapula
Levator scapulae Transverse processes and posterior tubercles of C1–C4 Superior angle of the scapula Dorsal scapular nerve (C5) and ventral rami C3–C4
  • Elevates the scapula
  • Dorsal extension and flexion of the cervical spine

Extrinsic Back Muscles: Intermediate Muscles

Intermediate extrinsic back muscles are primarily involved with rib/thoracic cage motion.

Table: Intermediate extrinsic back muscles
Muscle Origin Insertion Innervation Action
Serratus posterior superior Spinous processes of C6–T2 Costal angle of ribs 2–5 Ventral rami of spinal nerves T1–T4 (intercostal nerves)
  • Synergist muscle of inspiration
  • Elevates the ribs
Serratus posterior inferior Spinous processes T11–L2 Lower edge of ribs 9–12 Ventral rami of spinal nerves T9–T12 (intercostal nerves)
  • Synergist muscle of expiration
  • Depresses the ribs

Intrinsic Back Muscles: Superficial layer

The intrinsic, or deep, back muscles are grouped into 3 layers: superficial, intermediate, and deep; they are primarily involved in moving the vertebral column. The superficial layer is primarily responsible for the movement of the cervical spine.

Table: Spinotransversales group
Muscle Origin Insertion Innervation Action
Splenius capitis Spinous processes of C7–T3 and the supraspinal ligament Lateral half of superior nuchal line and the mastoid process Dorsal rami of C1–C6 Extensor and lateral bending of the cervical spine and the head
Splenius cervicis Spinous processes of T3–T6 and the supraspinal ligament Posterior tubercle of the transverse processes of C1–C3
The superficial layer of the intrinsic back muscles biodigital

The superficial layer of the intrinsic back muscles

Image by BioDigital, edited by Lecturio

Intrinsic Back Muscles: Intermediate Layer

Intermediate layer (intrinsic muscles): The 3 muscles of the intermediate layer make up the erector spinae group.

Table: Erector spinae group
Muscle Origin Insertion Innervation Action
Spinalis Thoracis: spinous process of upper lumbar and lower thoracic vertebrae Spinous process of upper thoracic vertebrae Dorsal rami of spinal nerves
  • Acts unilaterally to laterally flex the vertebral column
  • Acts bilaterally to extend the vertebral column and head
Cervicis: nuchal ligament and C7 spinous process Spinous process of C2–C7
Capitis: transverse processes of C4–T6 Superior and inferior nuchal lines
Longissimus Thoracic: dorsal sacrum, spinous processes of lumbar/inferior thoracic spine Accessory processes of the lumbar spine, transverse processes of thoracic spine, and costal angle of ribs 2–12 Dorsal rami T3–T5
Cervical: transverse processes of T1–T6 Cervical: posterior tubercle of transverse processes of C2–C5 Cervical: dorsal rami C3–T2
Capitis: transverse processes of C3–T3 Capitis: mastoid process of the temporal bone Capitis: dorsal rami C1–C3
Iliocostalis Lumbar: iliac crest, dorsal sacrum, and the thoracolumbar fascia Costal angle of ribs 7–12 Dorsal rami T9–L1
Thoracic: costal angle of ribs 7–12 Costal angle of ribs 1–6 Dorsal rami T2–T9
Cervical: costal angle of ribs 3–6 Posterior tubercle of transverse processes of C3–C6 Dorsal rami T1–T2

Intrinsic Back Muscles: Deep Layer

The deepest layer of the deep back muscles is attached to the transverse and spinous processes of the vertebral column and facilitates movement of the spine.

Table: Deepest layer of back muscles
Muscle Origin Insertion Innervation Action
Semispinalis Capitis: articular processes of C4–C6 and the transverse processes of C7–T6 Between the superior and inferior nuchal lines Dorsal rami of the spinal nerves Extension and contralateral rotation of the head and vertebral column
Cervicis: posterior surfaces of the transverse processes of T1–T6 Spines of C2–C5
Thoracis: transverse processes of T6–T10 Spines of C6–T4
Multifidus Dorsal sacrum, iliac crest, posterior sacroiliac ligaments, mammillary processes, transverse processes of T1–T12, and the articular processes C4–C7 Spinous processes of C2–L5 Dorsal rami C3–S3 Extend the complete vertebral column
Rotatores Transverse processes of the cervical, thoracic, and lumbar spine Spinous processes of adjoining superior vertebra Dorsal rami of the spinal nerves Stabilize the vertebral column
Interspinales Spinous processes of C3–T1, T2–L1, and L2–S1 Spinous processes of C2–C7, T1–T12, and L1–L5 Extend the complete vertebral column
Intertransversarii Medial lumbar: mammillary and accessory processes of all lumbar vertebrae Adjoining osseous structures of the lumbar spine Lateral flexion of the spine
Thoracic: transverse processes of T10–T12 Transverse processes of T11–12 and at the accessory processes of L1
Posterior cervical: C1–C7, posterior tubercle of the transverse processes Posterior tubercles of the respective adjoining vertebra
Levatores costarum Transverse processes of C7–T11
  • Short fibers into the respective rib 1 segment below
  • Long fibers: 2 segments below
Extension of the thoracic spine

Clinical Relevance

  • Lumbago (low back pain): Low back pain is 1 of the most common reasons for visits to medical providers. The pain is most commonly related to the musculoskeletal system and increases with age. Common causes include improper lifting, poor posture, lack of regular exercise, skeletal fracture, discogenic disease, or degenerative arthritis. Intrinsic back muscle wasting and increased fatigability are common in patients with chronic low back pain. 
  • Triangle of auscultation: The triangle of auscultation is an area over the posterior thorax where the breathing sounds are most audible due to the relative thinning of the musculature. The borders of the triangle are defined by:
    • Superiorly: inferior border of trapezius
    • Inferiorly: latissimus dorsi muscle
    • Laterally: medial border of the scapula
  • Suboccipital triangle: The suboccipital triangle is a pyramid-shaped anatomical area that contains the vertebral artery and suboccipital nerve. The triangle is located at the posterolateral occipital scalp. The suboccipital nerve may play a role in cervicogenic headaches and occipital neuralgia.

References

  1. Drake, R.L., Vogl, A.W., & Mitchell, A.W.M. (2014). Gray’s Anatomy for Students (3rd ed.). Philadelphia, PA:  Churchill Livingstone.
  2. Jenkins D.B. (2009). Chapter 13. Elsevier Health Sciences. Hollinshead’s Functional Anatomy of the Limbs and Back (9th ed.pp 219-228).

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