Muscles of the Back

Summary

• Functions of the back muscles
• Layers of back muscles
  • Superficial
  • Intermediate
  • Deep

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Function & Structure of Back Muscles

• Back muscles are required to counteract the mass of the body which is mostly in front of the vertebrae

• • Essentially 3 groups of musculature in the back with different functions
    1. Extrinsic superficial  (aka superficial) – control limb movements
    2. Extrinsic intermediate  (aka intermediate) – add to respiratory movements
    3. Intrinsic (aka deep) – act on the vertebral column specifically
       • Superficial (splenius’/spinotransversales )
       • Intermediate (Erector Spinae)
       • Deep (Transversospinales)

• Nerve supply is mainly from dorsal rami of spinal nerves

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Extrinsic superficial muscles

• These act to control the limbs – in particular connecting the upper limbs to the trunk

• These muscles are:
  • Trapezius
  • Levator scapulae
  • Rhomboid minor
  • Rhomboid major
  • Latissimus dorsi

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• Nerve supply is received mainly from the ventral rami of C nerves (except Trapezius)

1. Latissimus dorsi
   • Origin = T6-T12, iliac crest, lower ribs
     • Insertion = humerus (intertubecular groove)
       • Lat dorsi is the Lady in between two majors – From anterior to posterior the Pec major -> Lat dorsi -> Teres major attach to the humerus)
   • Extension, adduction, medial rotation
   • Thoracodorsal nerve supply

     

2. Trapezius 
   • Large triangular muscle covering the posterolateral neck
     • Also part of the pectoral and back muscles
     • Structure – trapezius has 3 parts
       • Upper part
         • Superiorly attaches to medial third of superior nuchal line, Ligamentum nuchae , and Spinous process of C7
         • Inferiorly inserts into lateral 1/3 of clavicle
       • Middle part
         • Attaches to T1-T4 spinous processes
         • Inserts into medial side of acromion
       • Lower part
         • Attaches to T5-T12
         • Inserts into scapular spine

1. • • Ultimately trapezius attaches at the nuchal line, scapula, clavicle, acromion, and C7-T12 vertebrae spinous processes

     • Action

       • Stabilises neck along with levator scapulae and scalenes

       • Involved in shrugging:

         • Superior rotation of scapula

         • Elevation of lateral clavicle

     • Innervated by CNXI

     • Transverse cervical artery (branch of thyrocervical trunk)

2. Levator Scapulae

 

1. • Structure
     • Origin – Transverse processes of C1-C4
     • Insertion – Medial margin of superior scapula
   • Action
     • Lateral flexion of neck
     • Shoulder elevation
   • Supply
     • Dorsal scapular nerve (C5)
     • Ascending cervical artery, transverse cervical artery, dorsal scapular artery

 

• Rhomboids – major and minor
  • Structure
    • Attach from vertebral spinous processes (C7-T5) to medial border of scapula
    • Lie deep to trapezius muscle

1. • Supply
     • Dorsal scapular nerve
     • Similar positions – Rhomboid major is inferior (T2-T5)

 

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Extrinsic intermediate muscles

• These are superficial respiratory muscles that are part of the posterior thoracic wall
• Main examples are:
  • Serratus posterior superior
  • Serratus posterior inferior

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Intrinsic back muscles

• These act specifically on the spine for postural control

• Can be considered to have 3 layers:
  1. Superficial
  2. Intermediate
  3. Deep

• Extend from the pelvis to skull

• These muscles are enclosed by continuous deep fascia in three regions
  1. Deep fascia of the neck (may also be described as the ‘paravertebral layer’  elsewhere)
  2. Deep fascia of the back
  3. Thoracolumbar fascia in the thoracic/lumbar regions

• This fascia attaches to various structures:
  • Medially attaches to nuchal ligament, spinous processes, and supraspinous ligament
  • Laterally attaches to the cervical/lumbar transverse processes and to the ribs
    

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• Superficial layer of intrinsic muscles
  • Mainly the splenius muscles (aka spinotransversales muscles) – there are 2 muscles
    1. Splenius Capitis – attaches to mastoid process and occipital bone
    2. Splenius Cervicis – attaches to C1-C3
  • Both insert into upper thoracic vertebrae
  • Cover neck posteriorly and laterally
  • Cover deeper muscles like a bandage (splenius) and so hold them in position
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• Intermediate layer – made up of the erector spinae – a group of 3 vertical columns of muscle
  • Remember laterality with I Love Spine
  • All three originate from a broad tendon in the lumbar region (attaches to the iliac crest, sacrum, sacroiliac joints, and inferior lumbar spinous processes)
    • Iliocostalis – lateral column
    • Longissimus – middle layer
    • Spinalis– medial layer
  • Each column is itself divided into 3 parts according to their different attachments (but appear to be the same muscle)
    • E.g. Iliocostalis lumborum, thoracis, and cervicis
    • The other two have thoracis, cervicis, and capitis branches
    • Therefore there are a total of 7 structures from 3 muscles
  • Innervation is from the posterior rami of spinal nerves

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    Image showing the three muscles (I, L, S) and their positions. Taken from Moore & Dalley 4th Edition.

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• Deep layer – this is made up of the transversospinal muscle group
  • These originate from transverse processes of inferior vertebrae and attach to spinous processes of more superior vertebrae
    • Thus occupy the gutter between transverse and spinous processes (overlying the lamina)
  • Each is biggest in a certain area
  • 3 groups of muscles arranged obliquely that run all the way down the spine
    • Semispinalis – most superficial layer and has 3 parts – capitis, cervicis, and thoracis
    • Multifidus – deeper  layer and thickest in the lumbar region
    • Rotatores – deepest and biggest in the thoracic region
      • Has 2 separate Longus and Brevis parts