Anatomy of the Back Muscles

Muscles of the Back

• Divided into extrinsic and intrinsic back muscles
• Intrinsic muscles are located deep to the extrinsic muscles, separated by the thoracolumbar fascia

Extrinsic Muscles of the Back

• Superficial extrinsic muscles:
  • Trapezius
  • Latissimus dorsi
  • Rhomboid major
  • Rhomboid minor
  • Levator scapulae
• Intermediate extrinsic muscles:
  • Serratus posterior superior
  • Serratus posterior inferior
• Serratus posterior muscles:
  • Span from vertebrae to ribs
  • Serratus posterior superior:
    • Origin: nuchal ligament and spines C7-T3
    • Insertion: ribs 2-5
    • Nerve supply: intercostal nerves
    • Action: assists breathing by elevating ribs during inspiration
  • Serratus posterior inferior:
    • Origin: spines of vertebrae T11-L2
    • Insertion: ribs 9-12
    • Nerve supply: intercostal nerves
    • Action: assists breathing by depressing ribs during expiration

Thoracolumbar Fascia

• A large, diamond-shaped area of connective tissue
• Constituted by the thoracic and lumbar parts of the deep fascia
• Encloses the intrinsic back muscles
• Serves as an attachment site for various muscles of the back

Intrinsic Muscles of the Back

• Classified into:
  • Superficial intrinsic muscles
  • Intermediate intrinsic muscles
  • Deep intrinsic muscles

Superficial Intrinsic Muscles

• Splenius capitis:
  • Origin: nuchal ligament and spines C7-T3
  • Insertion: mastoid process and occipital bone of the skull
  • Nerve supply: posterior rami of cervical spinal nerves
  • Actions:
    • Bilaterally: extends the head and neck
    • Unilaterally: lateral flexion and rotation of the neck
• Splenius cervicis:
  • Origin: spines of T3-T6 vertebrae
  • Insertion: transverse processes of C1-3
  • Nerve supply: posterior rami of the cervical spinal nerves
  • Actions:
    • Bilaterally: extends the head and neck
    • Unilaterally: lateral flexion and rotation of the neck

Intermediate Intrinsic Muscles

• Iliocostalis, longissimus, and spinalis (I Like Standing)
• Form a column known as the erector spinae
• Situated between the vertebral spinous processes and the costal angle of the ribs
• Common tendinous origin:
  • Lumbar and lower thoracic vertebrae
  • Sacrum
  • Posterior aspect of iliac crest
  • Sacroiliac and supraspinous ligaments
• Nerve supply: posterior rami of the spinal nerves
• Actions:
  • Bilaterally: extends the vertebral column and head
  • Unilaterally: laterally flexes the vertebral column
• Iliocostalis:
  • Located laterally within the erector spinae
  • Associated with the ribs
  • Divided into three parts: lumborum, thoracis, and cervicis
  • Origin: common tendinous origin
  • Insertion: attaches to the costal angle of the ribs and the cervical transverse processes (C3-7)
• Longissimus:
  • Situated between the iliocostalis and spinalis
  • Largest of the three columns
  • Divided into three parts: thoracic, cervicis, and capitis
  • Origin: common tendinous origin
  • Insertion: lower ribs, the transverse processes of C2-T12, and the mastoid process of the skull
• Spinalis:
  • Located medially within the erector spinae
  • Smallest of the three muscle columns
  • Divided into three parts: thoracis, cervicis, and capitis (although the cervicis part is absent in some individuals)
  • Origin: common tendinous origin
  • Insertion: spines of C2-T8 and the occipital bone of the skull

Deep Intrinsic Muscles

• Located underneath the erector spinae
• Known collectively as the transversospinales
• A group of short muscles, associated with the transverse and spinous processes of the vertebral column
• Three major muscles in this group:
  • Semispinalis
  • Multifidus
  • Rotators
• Semispinalis:
  • Most superficial of the deep intrinsic muscles
  • Divided by its superior attachments into thoracic, cervicis, and capitis
  • Origin: transverse processes of C4-T10
  • Insertion: spines of 5-6 above its origin, and to the occipital bone of the skull
  • Nerve supply: posterior rami of the spinal nerves
  • Actions:
    • Extends and rotates the head and vertebral column
• Multifidus:
  • Located underneath the semispinalis muscle
  • Best developed in the lumbar area
  • Origin:
    • Transverse processes of vertebrae
    • Posterior aspect of sacrum
    • Posterior superior iliac spine (PSIS) of ilium
    • Posterior sacroiliac ligament
  • Insertion: spines of the vertebrae 3-4 levels above origin
  • Nerve supply: posterior rami of the spinal nerves
  • Actions:
    • Stabilizes the vertebral column
    • Bilateral contraction: extension of spine
    • Unilateral contraction: lateral flexion and rotation of vertebral column
• Rotators:
  • Most prominent in the thoracic region
  • Origin: from the vertebral transverse processes
  • Insertion: to the spine of the immediately superior vertebrae
  • Nerve supply: posterior rami of the spinal nerves
  • Action: stabilizes the vertebral column, extension and rotation of vertebral column

Minor Deep Intrinsic Muscles

• Interspinalis: spans between adjacent spinous processes, acts to stabilize the vertebral column
• Intertransversari: spans between adjacent transverse processes, acts to stabilize the vertebral column
• Levators costarum: originates from the transverse processes of vertebrae and attaches to the rib immediately below, acts to elevate the ribs

This quiz covers the classification of back muscles into extrinsic and intrinsic groups, and identifies their origin, insertion, nerve supply, and action.