Muscles of the Back: Superficial and Deep

Superficial and Deep Muscles of the Back

Objectives for Study

• Know the muscles of the back, including their innervation, attachment points, and functions.

• Understand the differences between superficial and deep back muscles.

• Identify the muscles forming the suboccipital triangle and its contents.

Surface Anatomy of the Back

Key landmarks mentioned on the posterior view include:

• Trapezius (descending, middle, ascending parts)

• Deltoid (clavicular, middle, spinal parts)

• Latissimus dorsi

• Teres major

• Rhomboids (deep to trapezius)

• Spine of scapula, medial border of scapula

• Iliac crest, site of posterior superior iliac spine

• Triangle of auscultation

Classification of Back Muscles

Superficial Back Muscles (Extrinsic)

• Also known as posterior axio-appendicular muscles.

• Primarily innervated by anterior (ventral) rami.

• Generally involve movement of the upper limb.

Intrinsic (Deep) Back Muscles

• Attach along the axial skeleton (vertebral column and skull).

• Primarily innervated by dorsal rami.

• Responsible for movement and control of the vertebral column and skull.

• Enclosed by the thoracolumbar fascia.

Superficial Back Muscles

Superficial Layer

• Trapezius

  • Named based on shape (trapezoid).

  • Divisions: $3$ divisions based on muscle fiber direction.

    • Superior (descending) part: Action on scapula: elevation, rotation.

    • Middle part: Action on scapula: retraction.

    • Inferior (ascending) part: Action on scapula: depression, rotation.

  • Proximal attachment (origin): Medial $1/3$ of superior nuchal line, external occipital protuberance (EOP), nuchal ligament, spinous processes of $C<em>7-T</em>{12}$.

  • Distal attachment (insertion): Lateral $1/3$ of clavicle, acromion process, and spine of scapula.

  • Innervation:

    • Spinal accessory nerve (motor) ($CN ext{ } XI$).

    • $C<em>3, C</em>4$ spinal nerves (for pain and proprioception).

  • Clinical Relevance: Weakness or $CN ext{ } XI$ injury can lead to weakness in shrugging, impaired abduction/arm elevation, and shoulder drooping (Journal of Medical Case Reports 2010;$4:158$).

• Latissimus Dorsi

  • Expansive muscle that forms the posterior axillary fold.

  • Proximal attachment (origin): Spinous processes of $T<em>7-L</em>5$, thoracolumbar fascia, iliac crest, inferior $3$ or $4$ ribs, inferior angle of the scapula.

  • Distal attachment (insertion): Humerus (floor of intertubercular sulcus).

  • Innervation: Thoracodorsal nerve.

  • Function: Extends, adducts, and medially rotates the humerus. Involved in pull-up type motions.

Intermediate Layer

• Levator Scapulae

  • Proximal attachment: Posterior tubercles of $C<em>1-C</em>4$ transverse processes.

  • Distal attachment: Medial border of the scapula, superior to the spine.

  • Innervation: Dorsal scapular nerve ($C<em>4, C</em>5$) (motor); $C<em>3, C</em>4$ (proprioception).

  • Function: Elevates and rotates the scapula (glenoid rotates inferiorly), retracts the scapula.

• Rhomboids (Minor and Major)

  • Proximal attachment: Nuchal ligament, spinous processes of $C<em>7-T</em>5$.

  • Distal attachment of all three (minor, major): Medial scapular border.

  • Innervation: Dorsal scapular nerve.

  • Function: Retract and stabilize the scapula; rotate the scapula to depress the glenoid cavity.

Deep Layer (of Superficial Back Muscles)

• Serratus Posterior Superior

  • Covered by rhomboids.

  • Origin: Spinous processes of $C<em>7-T</em>3$ and ligamentum nuchae.

  • Insertion: Ribs $2-5$.

  • Innervation: Anterior primary rami of $T<em>2-T</em>5$.

  • Function: Assists forced inspiration.

• Serratus Posterior Inferior

  • Covered by latissimus dorsi.

  • Origin: Spinous processes of $T<em>{11}-L</em>2$.

  • Insertion: Lower $4$ ribs.

  • Innervation: Anterior primary rami of $T<em>9-T</em>{12}$.

  • Function: Assists forced expiration.

Triangle of Auscultation

• Borders:

  1. Trapezius (superomedial border).

  2. Latissimus Dorsi (inferior border).

  3. Medial border of the scapula (lateral border).

Deep Back Muscles (Intrinsic Back Muscles)

• Supply: Dorsal primary rami.

• Extent: From the pelvis to the cranium.

• Enclosure: Enclosed by the thoracolumbar fascia.

• Primary Function: Movement and control of the vertebral column; maintain posture.

Multiple Layers

Superficial Layer

• Splenius capitis and cervicis

  • Proximal attachment (origin): Ligamentum nuchae and spinous processes of $C<em>7-T</em>6$.

  • Distal attachment (insertion):

    • Splenius capitis: Mastoid process and lateral $1/3$ of superior nuchal line of the occipital bone.

    • Splenius cervicis: Transverse processes of $C<em>1-C</em>3$.

  • Innervation: Dorsal rami of cervical nerves.

  • Function: Extension, lateral flexion, and rotation of the head and neck.

Intermediate Layer

• Erector Spinae (Sacrospinalis)

  • Three parallel groups, named by location/attachment.

  • Location: Situated posterolaterally to the spinal column, between the vertebral spinous processes and the costal angle of the ribs.

  • Common Tendinous Origin: Arises from lumbar and lower thoracic vertebrae, scrum, posterior aspect of the iliac crest, and sacroiliac and supraspinous ligaments.

  • Function: Trunk extension, lateral flexion, and stabilization.

  • Groups and Subdivisions:

    1. Iliocostalis (most lateral)

       • Iliocostalis lumborum

       • Iliocostalis thoracis

       • Iliocostalis cervicis

    2. Longissimus (intermediate)

       • Longissimus thoracis

       • Longissimus cervicis

       • Longissimus capitis

    3. Spinalis (most medial)

       • Spinalis thoracis

       • Spinalis cervicis

       • Spinalis capitis

Deep Layer (Transversospinalis Muscles)

• Attachment Pattern: Attach from transverse processes (proximally) to spinous processes (and laminae) (distally).

• Location: Fill the gutter between the spinous process and transverse process.

• Semispinalis

  • Most prominent in the cervical region.

  • Attaches to the back of the occipital bone.

  • Spans $4-6$ segments.

• Multifidus

  • Most prominent in the lumbar region.

  • Also attach to mammillary processes.

  • Spans $2-4$ segments.

• Rotatores

  • Best developed in the thoracic region.

  • Types:

    • Short (Brevis): Spans $1$ segment.

    • Long (Longus): Spans $2$ segments.

Minor Deep Intrinsic Muscles

• Interspinalis: Located between spinous processes.

• Intertransversarii: Located between transverse processes.

• Levatores costarum: Attach from tips of transverse processes to ribs.

• Primary Function: These muscles primarily serve a proprioceptive function due to their high density of muscle spindles, providing sensory feedback for posture and movement control.

Suboccipital Region

Suboccipital Muscles

1. Rectus Capitis Posterior Minor (RCPm)

2. Rectus Capitis Posterior Major (RCPM)

3. Obliquus Capitis Superior (OCS)

4. Obliquus Capitis Inferior (OCI)

Suboccipital Triangle

• Borders:

  • Superomedial: Rectus Capitis Posterior Major (RCPM).

  • Superolateral: Obliquus Capitis Superior (OCS).

  • Inferolateral: Obliquus Capitis Inferior (OCI).

• Floor: Posterior atlanto-occipital membrane.

• Roof: Semispinalis capitis muscle.

• Content (within the triangle):

  • Vertebral artery.

  • Suboccipital nerve (dorsal ramus of $C_1$), which innervates the suboccipital muscles.

Greater Occipital Nerve

• Originates from the dorsal ramus of $C_2$.

• Not a content of the suboccipital triangle.

• Innervates:

  • Semispinalis capitis muscle.

  • Skin of the upper neck and posterior head.

Thoracolumbar Fascia

• A deep fascia that covers and encloses the deep back muscles.

• Layers:

  • Anterior layer

  • Middle layer

  • Posterior layer

• The origin of the internal oblique (2) and transversus abdominis (3) muscles is from the lateral part of the thoracolumbar fascia.