Muscles of the Back

Terminology & Foundational Concepts

• Origin vs. Insertion - Origin = less-movable attachment; Insertion = more-movable attachment.

  • Caveat: some large muscles (e.g.latissimus dorsi) can move both ends, blurring the distinction.

• Tendons - Cord-like (e.g. long head of biceps) vs. broad/aponeurotic (e.g. thoracolumbar fascia of latissimus dorsi).

Gross Organization of Back Muscles

• Extrinsic (appendicular) – Lie on the back but move the upper limb. - Superficial layer: trapezius, latissimus dorsi, teres major.

  • Intermediate/deep layer: rhomboid minor & major, levator scapulae.

• Intrinsic (axial) – Act primarily on the vertebral column. - Superficial intrinsic: erector spinae (iliocostalis, longissimus, spinalis).

  • Deep intrinsic: multifidus (plus other transverso-spinal muscles not covered here).

• Accessory respiratory muscles – serratus posterior superior & inferior.

Superficial Extrinsic Muscles

Trapezius

• Attachments - Origin: midline $<br>ightarrow$ external occipital protuberance, nuchal ligament, spinous processes $C7 \text{–} T12.$

  • Insertion: lateral third of clavicle, acromion, spine of scapula.

• Fiber orientation - Upper fibers – descend; Middle – horizontal; Lower – ascend.

• Actions on scapula - Elevate (upper fibers).

  • Retract/adduct (middle).

  • Superior rotation for arm elevation (upper + lower acting as a force couple).

• Clinical/functional note: Crucial for overhead reach; weakness $\&amp;$ limited abduction > $\,90^\circ$ due to loss of superior support and impaired upward rotation.

• Innervation: Spinal accessory nerve (CN XI).

Latissimus Dorsi

• Attachments - Origin: spinous processes $T7 \text{– sacrum}$ via thoracolumbar fascia, posterior iliac crest, lower 3–4 ribs.

  • Insertion: floor of intertubercular (bicipital) groove of humerus (wraps anteriorly).

• Actions on humerus - Extension, adduction, medial rotation ("swimmer’s/power-stroke muscle").

• Tendon type: broad, aponeurotic origin $\equiv$ thoracolumbar fascia.

• Innervation: Thoracodorsal nerve.

Teres Major

• Attachments - Origin: inferior angle of scapula.

  • Insertion: medial lip of intertubercular groove (merges with latissimus tendon).

• Actions – Identical to latissimus dorsi.

• Study tip: Remember the duo as the "lat–teres tunnel" delivering the same movement set.

• Innervation: Lower subscapular nerve.

Deep Extrinsic Layer

Rhomboid Minor & Major

• Attachments - Minor: spinous $C7–T1$ $<br>ightarrow$ medial scapular border (root of spine).

  • Major: spinous $T2–T5$ $<br>ightarrow$ medial scapular border (below spine).

• Combined actions - Scapular retraction/adduction.

  • Inferior (downward) rotation – glenoid tips down, assists lowering the arm.

• Innervation: Dorsal scapular nerve.

Levator Scapulae

• Attachments - Origin: transverse processes $C1–C4.$

  • Insertion: superior angle & upper medial border of scapula.

• Actions - Elevates scapula (shrug).

  • Assists rhomboids in inferior rotation.

• Innervation: Dorsal scapular nerve (C3-C4).

Scapulothoracic Rhythm

• Coordinated 2:1 ratio $\bigl(\text{glenohumeral} : \text{scapular}\bigr)$ during total arm elevation of $180^\circ$.

• Film demo: - Upward phase – deltoid/supraspinatus abduct humerus; trapezius + serratus anterior rotate scapula superiorly.

  • Downward (e.g., chin-up descent) – rhomboids & levator scapulae contract eccentrically $<br>ightarrow$ controlled inferior rotation against resistance.

• Clinical: Dysfunction in either component limits overhead reach or produces shoulder pain (e.g., scapular dyskinesis).

Primary vs. Accessory Respiration

• Primary muscle – Diaphragm - Contracts $\downarrow$, increases thoracic volume $<br>ightarrow$ inspiration.

  • Relaxes $\uparrow$, passive recoil $<br>ightarrow$ expiration.

• When demand ↑ (exercise, pathology), rib-moving muscles join.

Serratus Posterior Superior (SPS)

• Origin: spinous $C7–T3.$

• Insertion: ribs $2–5.$

• Action: elevates upper ribs $<br>ightarrow$ accessory inspiration.

• Innervation: Intercostal nerves (T2-T5).

Serratus Posterior Inferior (SPI)

• Origin: spinous $T11–L2.$

• Insertion: ribs $9–12.$

• Action: depresses lower ribs $<br>ightarrow$ accessory expiration.

• Innervation: Intercostal nerves (T9-T12).

Intrinsic (Deep) Back Muscles

Erector Spinae (I Like Spaghetti mnemonic: Iliocostalis, Longissimus, Spinalis)

• Common origin: broad tendon from sacrum, posterior iliac crest & lumbar spinous processes.

• Individual insertions - Iliocostalis – angles of ribs $\&amp;$ cervical transverse processes.

  • Longissimus – transverse processes, mastoid process.

  • Spinalis – spinous processes (thoracic $<br>ightarrow$ cervical).

• Bilateral action: extend vertebral column & head; maintain upright posture.

• Unilateral action: lateral flexion (ipsilateral).

• Innervation: Dorsal rami of spinal nerves.

Multifidus (Deepest highlighted)

• Segmented bundles spanning 2–4 vertebrae. - Predominant origin: transverse processes (plus sacrum/ilium inferiorly).

  • Insertion: superior spinous processes.

• Actions - Bilateral – assist extension/stability.

  • Unilateral – contralateral rotation (e.g., left multifidus $\Rightarrow$ right rotation).

• Importance: Key stabilizer of lumbar spine; atrophies in chronic low-back pain.

• Innervation: Dorsal rami of spinal nerves.

Clinical & Functional Pearls

• Thoracolumbar fascia serves as a tension-bearing sheet, integrating latissimus with intrinsic back muscles; dysfunction $<br>ightarrow$ load-transfer issues and often contributing to core instability.

• Scapular stabilizers (trapezius, rhomboids, levator) are essential for efficient shoulder mechanics; weakness $\Rightarrow$ impingement syndromes and altered scapular kinematics.

• Erector spinae endurance over pure strength predicts lower-back injury risk.

• Accessory respiratory muscles hypertrophy in COPD patients due to chronic load.

Summary Cheat Sheet

• Elevate scapula: trapezius (upper), levator.

• Depress/Down-rotate: rhomboids, levator (synergistic).

• Retract scapula: trapezius (middle), rhomboids.

• Superior rotation (overhead): trapezius (upper+lower).

• Extend/adduct/med-rotate arm: latissimus, teres major.

• Extend spine: erector spinae, bilateral multifidus.

• Contralateral spine rotation: unilateral multifidus.

• Inspiration assist: diaphragm $\bigl(\text{primary}\bigr)$ + SPS.

• Expiration assist: SPI (active)/diaphragm relaxation (passive).