Superficial and Intermediate Back Muscles

These are the extrinsic back muscles, developmentally migrated onto the back from the limb bud (superficial group) or body wall (intermediate group). They act predominantly on the shoulder girdle/upper limb or ribs rather than the vertebral column itself.

Critically, they are not innervated by the posterior rami of spinal nerves (unlike the true intrinsic back muscles). Instead they retain the innervation appropriate to their embryological origin (ventral rami/plexus branches, or in trapezius's case, a cranial nerve).

Trapezius:

• Origin: External occipital protuberance, superior nuchal line, ligamentum nuchae, spinous processes of C7-T12.
• Insertion: Lateral third of the clavicle, acromion, spine of scapula.
• Innervation: Accessory nerve (CN XI), motor; proprioception via C3-C4 (cervical plexus).
• Action: Elevates, retracts, and rotates the scapula (upward rotation, essential for full arm abduction above the horizontal); upper fibres elevate, middle fibres retract, lower fibres depress the scapula.
• Important relationships: Forms the most superficial layer of the back musculature; the accessory nerve's relatively superficial course through the posterior triangle of the neck makes it vulnerable during neck surgery/lymph node biopsy, with resulting trapezius weakness/scapular winging (a distinct pattern from serratus anterior-related winging).

Latissimus dorsi:

• Origin: Spinous processes of T7-L5 (via the thoracolumbar fascia), iliac crest, lower 3-4 ribs, inferior angle of scapula (variable slip).
• Insertion: Floor of the intertubercular (bicipital) sulcus of the humerus.
• Innervation: Thoracodorsal nerve (C6-C8, posterior cord of the brachial plexus).
• Action: Extends, adducts, and medially rotates the shoulder, the primary muscle of powerful downward arm movements (e.g., climbing, swimming freestyle stroke, or pushing up from a chair using the arms).
• Important relationships: Forms the posterior axillary fold; its broad origin via the thoracolumbar fascia links it functionally to lumbar spine stability and core force transmission.

Levator scapulae:

• Origin: Transverse processes of C1-C4.
• Insertion: Superior angle and medial border of the scapula (upper part).
• Innervation: Dorsal scapular nerve (C5), with a variable direct contribution from the cervical plexus (C3-C4).
• Action: Elevates the scapula; assists in downward rotation of the scapula.

Rhomboid major:

• Origin: Spinous processes of T2-T5.
• Insertion: Medial border of the scapula, below the spine.
• Innervation: Dorsal scapular nerve (C5).
• Action: Retracts and downwardly rotates the scapula; stabilises the scapula against the thoracic wall.

Rhomboid minor:

• Origin: Ligamentum nuchae, spinous processes of C7-T1.
• Insertion: Medial border of the scapula, at the root of the spine.
• Innervation: Dorsal scapular nerve (C5).
• Action: Retracts and downwardly rotates the scapula, functioning together with rhomboid major as a single muscular unit in practice.

Serratus posterior superior:

• Origin: Ligamentum nuchae, spinous processes of C7-T3.
• Insertion: Upper borders of ribs 2-5 (lateral to their angles).
• Innervation: Anterior rami of the upper thoracic spinal nerves (T2-T5, variable).
• Function: Elevates the upper ribs, assisting inspiration, a relatively minor accessory respiratory muscle in normal quiet breathing, more functionally relevant during deep/forced inspiration.

Serratus posterior inferior:

• Origin: Spinous processes of T11-L2.
• Insertion: Lower borders of ribs 9-12 (lateral to their angles).
• Innervation: Anterior rami of the lower thoracic spinal nerves (T9-T12, variable).
• Function: Depresses the lower ribs, assisting forced expiration by fixing the ribs against the pull of the diaphragm (preventing the lower ribs from being drawn upward during diaphragmatic contraction), a stabilising rather than primary mover function.

Accessory nerve injury:

• The accessory nerve runs superficially through the posterior triangle of the neck, so it is vulnerable to iatrogenic injury during lymph node biopsy or other neck surgery in this region.
• Resulting trapezius paralysis produces a downward and lateral rotation/depression of the scapula at rest, weakness of shoulder shrug, and difficulty with arm abduction beyond the initial range (given trapezius's essential contribution to scapular upward rotation during overhead movement).
• This is a distinct clinical pattern from long thoracic nerve injury (serratus anterior paralysis), though both can produce a form of "winged scapula".
• Careful clinical distinction is needed, based on the specific pattern of scapular malposition and the movements that provoke/worsen it.

Scapular movement abnormalities:

• Beyond isolated trapezius or serratus anterior dysfunction, coordinated scapulothoracic movement depends on the combined, balanced action of all the superficial/intermediate extrinsic back muscles described in this section (trapezius, rhomboids, levator scapulae).
• These muscles work together with serratus anterior. Imbalance or weakness in any single contributor can produce recognisable, characteristic patterns of abnormal scapular resting position or dynamic movement (dyskinesis), a clinically important consideration in shoulder pain/dysfunction assessment.

Shoulder girdle weakness:

• Weakness of the muscles covered in this section (particularly trapezius and the rhomboids) contributes to a range of postural and functional shoulder girdle problems, often presenting clinically as difficulty with overhead activity, a rounded-shoulder resting posture, or visible scapular asymmetry.
• Assessment requires distinguishing weakness originating from these extrinsic back muscles (accessory nerve or dorsal scapular nerve pathology) from weakness of the rotator cuff or other primarily glenohumeral joint-related muscles.