Anatomy, Back, Muscles

Splenius Capitis

• Origin: Spinous processes of C7 vertebrae and T1-T3 (or T4) vertebrae and supraspinous ligaments
• Insertion: Mastoid process and lateral third of the superior nuchal line
• Action:
• Unilateral: rotation of the head to the ipsilateral side.
• Bilateral: extension of the head.
• Relationships: Deep to the trapezius and superficial to semispinalis capitis and longissimus capitis
• Forms a portion of the floor of the posterior triangle of the neck

Splenius Cervicis

• Origin: T3-T6 spinous processes
• Insertion: transverse processes of atlas and axis, posterior tubercle of the C3 vertebra
• Action:
  • Unilateral: rotation of the upper cervical vertebra
  • Bilateral: extension of the upper cervical spine
• Relationships:
  • Deep to serratus posterior superior, rhomboids and trapezius
  • Superficial to parts of erector spinae and semispinalis muscles

Erector Spinae[4]

• Three muscles that span the entire back comprise the erector spinae. It divides into three regional groups, named for the region they span. 
• Action: It controls the forward flexion of the thorax, which can occur secondary to gravity. The actions of the cervical and capitis groups are unknown. These muscles are small when compared to the larger cervical muscle groups and have little force capacity.
• Relationships: Covered by thoracolumbar fascia, serratus posterior inferior, rhomboids, and splenii muscle groups

Spinalis

• Spinalis thoracis
• Most medial of erector spinae in the thoracic region
• Origin: Spinous processes of T11-L2
• Insertion: Spinous processes of the upper thoracic vertebra
• Blends with longissimus thoracis laterally.
• Spinalis cervicis and capitis are not well defined and are poorly developed. These fibers may be absent in some people.
  • Spinalis cervicis
    • Origin: ligamentum nuchae and C7 spinous process
    • Insertion: spinous processes of the axis and C3-C4
  • Spinalis capitis
    • Usually a few fibers of semispinalis capitis that inserts on the spinous processes of C7 and T1 (rather than the usual insertions on the thoracic transverse processes)

Longissimus

• Longissimus capitis
• Origin: C4-T4 transverse processes
• Insertion: Posterior edge of the mastoid process
• Longissimus cervicis
• Origin: T1-T4 transverse processes
• Insertion: Posterior tubercle of C2-C6 transverse processes
• Longissimus thoracis
  • Contains lumbar and thoracic sections
• Largest of erector spinae group
• Origin: Transverse process at inferior vertebral levels
• Insertion: Transverse process at superior vertebral levels and mastoid process

Iliocostalis[5]

• Most lateral of erector spinae
• Attaches to ribs

• Iliocostalis cervicis
  • Origin: Angle of ribs 3 to 6
  • Insertion: posterior tubercle of the transverse process of C4-C6
• Iliocostalis thoracis
  • narrow, fusiform shape.
  • Origin: Angle of lower six ribs
  • Insertion: Transverse process of C7 and angles of the upper six ribs
• Iliocostalis lumborum
  • Contains lumbar and thoracic sections
  • Origin: Medial end and the dorsal segment of the iliac crest 
  • Insertion: L1-L4 lumbar transverse processes, the angle of ribs 4 to 12 and thoracolumbar fascia

 Transversospinalis Group

• This group consists of muscles between a spinous process and the transverse process of a vertebra below
• Grouped by length and region covered
• Rotatores are deepest and shortest
  • Span 1-2 segments
  • Eleven pairs between T1-T12
  • Rotator brevis connects the transverse process of the lower vertebra to the lateral lamina of the upper vertebra immediately above.
  • Rotator longus connects the transverse process of the lower vertebra to the base of the spinous process of the upper vertebra two levels above.
• Multifidus can span 2 to 4 segments.
  • Covers lamina of vertebrae
  • Origin: Sacrum and ilium, transverse processes of T1-L5 and articular processes of C4-C7
  • Insertion: Spinous processes 2 to 4 segments above the origin
• Semispinalis can span 4 to 6 segments.
  • Origin: Thoracic and cervical transverse processes
  • Insertion: Occipital bone and spinous processes in thoracic and cervical regions 4 to 6 segments above the origin

• The splenius cervicis and splenius capitis may be absent or vary in the vertebral attachment.
• Rotatores thoracis may have one or more pairs absent at the upper or lower end of the thoracic vertebrae.

The segmental innervation of the deep muscle groups makes these muscles vulnerable during surgical procedures. Only one branch of the dorsal ramus innervates the medial multifidus with no intersegmental supply. This lack of intersegmental supply can cause injury and paralysis of these small muscle groups. As the paraspinal muscles depend on the antagonistic relationship between bilateral groups, a weakness in the small muscles can cause an imbalance and impairment for the spine.[6][7]

The chief pathology associated with the back muscles is pain. These muscles can develop spasms that can be debilitating. The lower back muscles are a common cause of low back pain. This entity is often mistaken for spinal stenosis and involves millions of people of all ages and gender. Patients often undergo exhaustive workup, including an MRI.

All individuals with low back pain should perform a straight leg test to differentiate neurologic abnormalities from purely muscular conditions.[8] The treatment of muscular complaints is conservative with a few days of rest, pain control, and physical therapy.

The back muscles in the upper neck may also be associated with pain and mistaken for migraines or cervical spondylopathy. Botulinum toxin has proven to be an effective remedy for these spasms.[9][10]