Back Muscles; Types, Classification, Functions

Back Muscles are separated into extrinsic and intrinsic components, which are based on their function in movement and embryological origin. The extrinsic muscles include the trapezius, latissimus dorsi, rhomboid major and minor, levator scapulae and the serratus posterior superior and inferior muscles. The principal function of the extrinsic muscles of the back is to move the upper extremity by controlling the movement of the scapula and humerus.

Types of Back Muscles

There are seven extrinsic muscles of the back –  the trapezius, the latissimus dorsi, the levator scapulae, the rhomboids (major and minor) and the serratus posterior (superior and inferior)

The trapezius

• It is the most superficial muscle of the back is made up of two triangles that together make the shape of a trapezoid, thus its name.
• This muscle originates from the center of the spinal column and skull; the fibers connect to the middle portion of the superior nuchal line, the external occipital protuberance, the ligament nuchae, and the apices of the spinous processes of C7-T12.
• The fibers run laterally toward the shoulder where they attach. The superior fibers descend to connect to the posterior border of the distal 1/3 of the clavicle; the middle fibers run horizontally to connect to the superior portion of the scapular spine; the inferior fibers ascend toward the apex of the scapula.[rx]

Latissimus dorsi

• It is the largest extrinsic back muscle, spanning across the entire lateral side of the back. It originates from the lateral sides of the spinous processes of T7 – T12, the thoracodorsal fascia, which connects to the spinous processes of the lumbar and sacral vertebrae, the posterior portion of the iliac crest, and proximal 1/3 of ribs 9 – 12.
• The fibers ascend in a superolateral fashion towards its insertion point, the floor of the bicipital groove of the humerus.[rx]

 Levator scapulae

• It is a thin muscle that descends from the neck to the scapula. It originates from the transverse processes of C1 (atlas), C2 (axis), C3, and C4.
• The fibers run diagonally from these points to the medial scapular border just above where the spine of the scapula connects. Origination points of this muscle can vary in the neck and posterior skull.[rx]

The rhomboids

• It has minor and major, these two muscles originate from the spines and supraspinous ligaments of the cervical and thoracic vertebrae.
• Rhomboideus minor is the smaller of the two muscles and located superiorly. It originates from the seventh cervical vertebrae and first thoracic vertebrae and attaches to the medial border of the scapula at the point where the spine connects.
• Rhomboid major takes its origin from the second to the fifth thoracic vertebrae and attaches just below the rhomboid minor on the medial border of the scapula.

The serratus posterior muscles

• Separated into superior and inferior are attached to the spinous processes of the vertebrae and the ribs.
• The superior muscle originates from the spinous processes of C7-T3 and attaches to ribs two through five.
• The inferior muscle originates from T11-L1 and attaches to ribs 9-12.

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
• 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.

• Unilateral: rotation of the head to the ipsilateral side.
• Bilateral: extension of the head.

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

• 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 capital 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 spine in the thoracic region
• Origin – Spinous processes of T11-L2
• Insertion – Spinous processes of the upper thoracic vertebra
• Blends with longissimus thoracis laterally

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

• Most lateral of erector spinae
• Attaches to ribs

• Iliocostalis cervicis

  • Origin: Angle of ribs 3-6
  • Insertion: posterior tubercle of the transverse process of C4-C6
• Iliocostalis thoracic
  • narrow, fusiform shape.
  • Origin: Angle of lower six ribs
  • Insertion: Transverse process of C7 and angles of upper six ribs

• Iliocostalis lumborum

  • Contains lumbar and thoracic sections
  • Origin: Medial end and dorsal segment of the iliac crest
  • Insertion: L1-L4 lumbar transverse processes, the angle of ribs 4-12 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 upper vertebra two levels above.

• Multifidus can span 2-4 segments.

  • Covers lamina of vertebra
  • Origin: Sacrum & ilium, transverse processes of T1-L5 and articular processes of C4-C7
  • Insertion: Spinous processes 2-4 segments above origin

• Semispinalis can span 4-6 segments.

  • Origin: Thoracic & cervical transverse processes
  • Insertion: Occipital bone and spinous processes in thoracic and cervical regions 4-6 segments above origin
• The splenius cervicis and splenius capitis – may be absent or vary in the vertebral attachment.
• Rotatores thoracic – may have one or more pairs absent at the upper or lower end of the thoracic vertebrae.

Nerves

The posterior, or dorsal, primary rami of the spinal nerves innervates only the intrinsic, or true, back muscles.

• Splenius capitis – The lateral branches of the C2-C3 dorsal rami
• Splenius cervicis – lateral branches of lower cervical dorsal rami
• Erector spinae – At the lumbar level, the lateral branches of dorsal rami of the corresponding vertebrae level innervate the iliocostalis muscle.  The intermediate branches of the dorsal rami innervate the longissimus muscle.
• Transversospinalis group: dorsal primary rami of C1-L5

Ventral rami of the spinal nerves innervate the extrinsic muscles (trapezius, latissimus dorsi, levator scapulae, and rhomboid muscles).

Blood Supply and Lymphatics

The deep cervical, posterior intercostal, subcostal or lumbar arteries provide the blood supply for all the muscle groups of the back.  Arterial supply will vary person-to-person.

References