Comprehensive Notes on the Muscles of the Back

Back Muscles: Overview

  • The back muscles act to extend, rotate, and laterally flex the trunk and neck.
  • They contribute to significant balance and stability of the trunk, alongside the anterior abdominal muscles.
  • They are active for static and dynamic positioning and movement of the trunk (e.g., standing at a mirror during morning hygiene; watering the lawn with a garden hose).
  • They help return the trunk to a neutral position after forward flexion (e.g., bending to tie shoes).
  • Main architectural organization: superficial, intermediate, and deep groups with distinct roles and attachments.
  • Cutaneous innervation of the back is via the dorsal rami of spinal nerves.

Organization of the Back Muscles

  • Superficial Group (Posterior thoracoappendicular muscles)
    • Primarily involved in movements of the upper limb
    • Hypaxial (derived from ventral muscle mass in development)
    • Muscles: Trapezius, Latissimus dorsi, Rhomboideus major, Rhomboideus minor, Levator scapulae
  • Intermediate Group
    • Attach to ribs (may have a respiratory function)
    • Hypaxial
    • Muscles: Serratus posterior superior; Serratus posterior inferior
  • Deep Group (Intrinsic muscles of the back)
    • Primarily involved in movements of the vertebral column and head
    • Epiaxial (deep to thoracolumbar fascia)
    • Innervated by the dorsal rami of spinal nerves
    • Subgroups:
    • Spinotransversalis muscles (superficial of the deep group): Splenius capitis and Splenius cervicis
    • Erector Spinae (intermediate): Spinalis, Longissimus, Iliocostalis
    • Transversospinalis muscles (deep): Multifidus, Rotatores, Semispinalis capitis
    • Other intrinsic deep muscles listed: Quadratus lumborum, Intertransversarii, Interspinales

Superficial Group Muscles

Trapezius
  • Purposeful Activity examples: reaching overhead (upper and lower fibers); rowing (middle fibers).
  • Action:
    • Upper fibers: bilaterally extend head/neck; unilaterally laterally flex head/neck to same side; rotate head/neck to opposite side; elevate the scapula; upwardly rotate the scapula.
    • Middle fibers: adduct the scapula; stabilize the scapula.
    • Lower fibers: depress the scapula; upwardly rotate the scapula.
  • Origin: external occipital protuberance; medial portion of the superior nuchal line; ligamentum nuchae; spinous processes of C7–T12.
  • Insertion: lateral one-third of the clavicle; acromion; spine of the scapula.
  • Innervation: spinal accessory nerve (CN XI).
  • Note: ST = scapulothoracic joint.
Latissimus Dorsi
  • Purposeful Activity: opening a heavy door, rowing, swimming (downstroke).
  • Action: extends, adducts, and medially rotates the humerus.
  • Origin: spinous processes of T7–L5; thoracolumbar fascia; sacrum; iliac crest; ribs 10–12.
  • Insertion: floor of the intertubercular sulcus of the humerus.
  • Innervation: thoracodorsal nerve.
Levator Scapulae
  • Purposeful Activity: shrugging shoulders (nonverbal communication) or carrying a briefcase.
  • Action: unilateral: elevate the scapula; downwardly rotate the scapula; laterally flex the head/neck; rotate the head/neck to the same side. Bilateral: extend the head/neck.
  • Origin: transverse processes of C1–C4.
  • Insertion: medial border of the scapula between the superior angle and the superior portion of the spine.
  • Innervation: cervical nerves C3–C4 and dorsal scapular C4–C5.
Rhomboids (Major and Minor)
  • Purposeful Activity: reaching into a back pocket.
  • Action: adduct the scapula; elevate the scapula; downwardly rotate the scapula.
  • Origin: Major — spinous processes T2–T5; Minor — spinous processes C7–T1.
  • Insertion: medial (vertebral) border of the scapula.
  • Innervation: dorsal scapular nerve (C4–C5).

Intermediate Group

Serratus Posterior Superior
  • Location: upper back on the superior side.
  • Origin: spinous processes of C7–T3.
  • Insertion: upper ribs (adjacent to their respective vertebrae).
  • Function: elevates ribs during respiration.
Serratus Posterior Inferior
  • Location: lower back on the inferior side.
  • Origin: spinous processes of T12–L3.
  • Insertion: lower ribs (adjacent to their respective vertebrae).
  • Function: depresses ribs during respiration.

Deep Group (Intrinsic Muscles of the Back)

Erector Spinae Muscle Group
  • Purposeful Activity: maintaining an upright position.
  • Overall Action: unilateral contraction to laterally flex the vertebral column to the same side; bilateral contraction to extend the vertebral column.
  • Origin: common tendon (thoracolumbar fascia) attaching to the posterior surface of the sacrum; iliac crest; and spinous processes of the lumbar and the last two thoracic vertebrae.
  • Insertion: attachments to posterior ribs, spinous and transverse processes of thoracic and cervical vertebrae, and the mastoid process of the temporal bone.
  • Innervation: dorsal rami of lower cervical spinal nerves.
Iliocostalis (part of Erector Spinae)
  • Subparts: Iliocostalis cervicis, Iliocostalis thoracis, Iliocostalis lumborum.
  • Notes: Branches of the iliocostalis shown on diagrams; contributes to lateral flexion and extension.
Longissimus (part of Erector Spinae)
  • Subparts: Longissimus capitis, Longissimus cervicis, Longissimus thoracis.
  • Notes: Branches of the longissimus shown on diagrams; key for extending and rotating the head/neck.
Spinalis (part of Erector Spinae)
  • Subparts: Spinalis cervicis, Spinalis thoracis.
  • Notes: Branches of the spinalis shown on diagrams.
Transversospinalis Muscles
  • General Role: deeper stabilizers contributing to postural control and intersegmental motion.
  • Muscles: Multifidus, Rotatores, Semispinalis capitis.
  • Action:
    • Unilaterally: rotate the vertebral column to the opposite side.
    • Bilaterally: extend the vertebral column and the head (semispinalis capitis contributes to head extension).
  • Origin/Insertion:
    • Multifidus: origin from sacrum and the transverse processes of lumbar through cervical vertebrae; insert onto the spinous processes two to four vertebrae above.
    • Rotatores: origin from transverse processes of lumbar through cervical vertebrae; insert onto the spinous processes one to two vertebrae above.
    • Semispinalis capitis: originates from transverse processes of C4–T5; inserts between the superior and inferior nuchal lines of the occiput.
  • Innervation: dorsal rami of lower cervical spinal nerves.
  • Note: multifidi span two to four vertebrae; rotatores span one to two vertebrae.
Splenius Group
Splenius Capitis and Splenius Cervicis
  • Purposeful Activity: e.g., working at a computer, looking overhead, social interaction, driving.
  • Action:
    • Unilaterally: rotate the head/neck to the same side; laterally flex the head/neck to the same side.
    • Bilaterally: extend the head and neck.
  • Origin:
    • Capitis: inferior half of the ligamentum nuchae and spinous processes of C7–T4.
    • Cervicis: spinous processes of T3–T6.
  • Insertion:
    • Capitis: mastoid process and the lateral portion of the superior nuchal line.
    • Cervicis: transverse processes of C1–C3.
  • Innervation: dorsal rami of lower cervical spinal nerves.
Quadratus Lumborum
  • Purposeful Activity: involved in lifting and walking; acts as an important contractile link between the pelvis and the lower back.
  • Action:
    • Unilaterally: laterally tilt (elevate) the pelvis; lateral flexion of the vertebral column to the same side; assist to extend the vertebral column.
    • Bilaterally: fix the last rib during forced inhalation and exhalation.
  • Origin: posterior iliac crest.
  • Insertion: last rib and transverse processes of the first through fourth lumbar vertebrae.
  • Innervation: lumbar plexus (T12, L1–L3).
Intertransversarii and Interspinales
  • Intertransversarii: attach to adjacent transverse processes in the lumbar and cervical regions; stabilize and contribute to lateral flexion between vertebrae.
  • Interspinales: connect adjacent spinous processes of the lumbar and cervical vertebrae; contribute to trunk extension.

Cutaneous Innervation of the Back

  • Cutaneous innervation of the back occurs via the dorsal rami of spinal nerves.
  • Dermatomal map (posterior trunk):
    • Cervical: C2, C3, C4, C5, C6, C7, C8
    • Thoracic: Th1, Th2, Th3, Th4, Th5, Th6, Th7, Th8, Th9, Th10, Th11, Th12
    • Lumbar: L1, L2, L3, L4, L5
    • Sacral: S1, S2, S3, S4
  • These dermatomes reflect the skin areas innervated by the dorsal rami corresponding to each spinal level.

Quick Reference: Key Terms and Abbreviations

  • ST joint: scapulothoracic joint (not a true synovial joint; area of scapular motion against thoracic wall).
  • Dorsal rami: posterior primary rami of spinal nerves; provide motor innervation to deep back muscles and cutaneous sensation to the back.
  • Thoracolumbar fascia: a connective tissue layer important for the origin of the Erector Spinae muscles.
  • Hypaxial vs. Epiaxial: terminology describing muscle groups relative to the axis of the back; epiaxial muscles are intrinsic back muscles, while hypaxial muscles lie more anteriorly and include elements of the superficial/back wall.

Connections to Foundational Principles and Real-World Relevance

  • Posture and ergonomics: strong back muscles support upright posture and reduce fatigue during daily activities (sitting, lifting, reaching).
  • Movement synergy: back muscles coordinate with abdominal and pelvic muscles to stabilize the trunk during dynamic activities (lifting, walking, pushing/pulling).
  • Respiratory interactions: Serratus posterior superior/inferior can assist in rib movement during respiration; Quadratus lumborum can influence rib positioning during heavy breathing.
  • Clinical relevance: understanding dorsal rami innervation helps diagnose radiculopathies; dermatomal maps aid in localizing nerve injuries or shingles (herpes zoster) along dermatomes.
  • Ethical/practical implications: ergonomic design in workplaces and schools should consider the load and posture requirements that engage back muscles to prevent strain or repetitive strain injuries.