Latissimus Dorsi Outline

The latissimus dorsi is a large, broad, flat superficial muscle that forms a major part of the visible muscular contour of the lower back. Its expansive origin covers the lower thoracic and lumbar regions, while its fibers converge superiorly and laterally toward the axilla and proximal humerus.

On surface examination, the muscle contributes to the contour of the posterolateral trunk and forms an important component of the posterior axillary fold. Its outline becomes more apparent when the upper limb performs resisted extension, adduction, or medial rotation.

Understanding the surface outline of latissimus dorsi is useful for identifying muscular landmarks of the back, interpreting posterior trunk anatomy, examining muscle function, and planning surgical or reconstructive procedures involving the muscle.

General Surface Outline

The latissimus dorsi occupies a broad triangular area over the lower thoracic, lumbar, and posterolateral trunk.

Its broad inferior and medial attachments narrow progressively as the muscle passes superolaterally toward the axilla.

Surface Position

The muscle lies superficially over much of the lower back and can influence the external contour from the thoracolumbar midline toward the lateral trunk.

Superiorly, part of the muscle is overlapped by the trapezius, while laterally it contributes to the transition between the back and axillary region.

Shape

Latissimus dorsi is approximately triangular or fan-shaped when viewed from behind.

Its broad base occupies the inferior back, while the converging fibers form a narrower muscular and tendinous portion near the humerus.

Major Surface Relationships

Region	Surface Relationship
Lower thoracic back	Broad superficial muscular sheet
Lumbar region	Major contributor to posterolateral contour
Iliac region	Fibers arise near the posterior iliac crest
Inferior scapular region	Passes near the inferior angle of the scapula
Posterior axilla	Contributes to posterior axillary fold

Medial Outline

The medial extent of latissimus dorsi is associated with the lower thoracic and lumbar midline through its attachments to spinous processes and the thoracolumbar fascia.

The exact visible medial border may be difficult to distinguish because of fascial continuity and neighboring muscles.

Inferior Outline

Inferiorly, the muscle extends toward the posterior iliac crest.

This broad attachment helps create the muscular contour between the lower back and upper gluteal region.

Superolateral Course

From its broad origins, the muscle fibers pass superiorly and laterally toward the axilla.

This convergence produces the characteristic narrowing of the muscle as it approaches the proximal humerus.

Relationship to the Scapula

The latissimus dorsi passes near the inferior angle of the scapula.

Depending on individual anatomy, muscular slips may attach to the inferior angle, and contraction can influence the contour of this region.

Posterior Axillary Fold

The posterior axillary fold is one of the most important surface landmarks associated with latissimus dorsi.

It is formed principally by the latissimus dorsi together with the teres major as these muscles pass toward their humeral attachments.

Palpation of the Posterior Axillary Fold

The posterior axillary fold can be palpated along the posterior boundary of the axilla.

It becomes more prominent when the patient adducts or extends the arm against resistance.

Relationship to Trapezius

The superior medial portion of latissimus dorsi is partly overlapped by the trapezius.

The relationship between these two superficial back muscles contributes to the visible contour of the lower scapular and thoracic regions.

Triangle of Auscultation

The triangle of auscultation is a small region of the posterior thoracic wall where the muscular covering is relatively thin.

Its boundaries are associated with the trapezius, latissimus dorsi, and medial border of the scapula.

Boundaries of the Triangle of Auscultation

Boundary	Structure
Medial	Trapezius
Inferior	Latissimus dorsi
Lateral	Medial border of scapula

Clinical Use of the Triangle of Auscultation

The triangle of auscultation provides an area where respiratory sounds may be heard through a relatively thin muscular layer.

Protraction of the scapula can enlarge the accessible region by moving the scapula laterally.

Lumbar Triangle

The lateral border of latissimus dorsi also participates in the surface anatomy of the inferior lumbar triangle, commonly called Petit’s triangle.

This is a small area of relative weakness in the posterolateral abdominal wall.

Inferior Lumbar Triangle

Boundary	Structure
Medial	Latissimus dorsi
Lateral	External oblique
Inferior	Iliac crest
Floor	Internal oblique

Thoracolumbar Fascia

The thoracolumbar fascia is an important deep attachment for latissimus dorsi.

Through this broad fascial attachment, the muscle is connected functionally and anatomically with the lumbar spine, posterior abdominal wall, and other trunk musculature.

Vertebral Attachments

Latissimus dorsi has broad attachments associated with the lower thoracic spinous processes and thoracolumbar fascia.

These medial attachments contribute to the extensive surface area occupied by the muscle over the posterior trunk.

Iliac Crest Attachment

Part of the muscle arises from the posterior iliac crest.

This attachment helps define the inferior extent of the muscular sheet on the lower back.

Costal Attachments

Latissimus dorsi also receives fibers from the lower ribs.

These costal attachments contribute to the lateral portion of the muscle and its relationship with the posterolateral thoracic wall.

Insertion

The fibers converge into a tendon that inserts into the intertubercular sulcus of the humerus.

This humeral insertion allows the muscle to act powerfully on the upper limb despite its extensive origin from the trunk.

Functional Surface Anatomy

The outline of latissimus dorsi becomes easier to appreciate when the muscle contracts.

Movements that recruit the muscle strongly include extension, adduction, and medial rotation of the humerus.

Resisted Arm Extension

When the arm is extended against resistance, latissimus dorsi contracts and the posterolateral muscular contour becomes more prominent.

This maneuver can help identify the muscle during physical examination.

Resisted Adduction

Adduction of the arm against resistance also activates the latissimus dorsi.

The posterior axillary fold may become especially prominent during this movement.

Medial Rotation

Latissimus dorsi contributes to medial rotation of the humerus.

Its combined actions reflect the orientation of its fibers from the posterior trunk toward the anterior aspect of the proximal humerus.

Climbing and Pulling Movements

Latissimus dorsi is strongly recruited during climbing, rowing, pulling, and similar movements in which the upper limb is brought toward the trunk.

In these activities, the muscle may become clearly visible along the posterolateral trunk.

Fixed Upper Limb

When the humerus is fixed, latissimus dorsi can act on the trunk rather than the arm.

This action contributes to movements such as raising the body toward the upper limbs during climbing.

Innervation

Latissimus dorsi is innervated by the thoracodorsal nerve.

The thoracodorsal nerve arises from the posterior cord of the brachial plexus, primarily carrying fibers from cervical spinal nerve levels C6 to C8.

Thoracodorsal Nerve

The thoracodorsal nerve descends along the posterior wall of the axilla to reach the deep surface of latissimus dorsi.

Its relationship to the muscle is particularly important during axillary surgery and reconstructive procedures.

Blood Supply

The principal vascular supply of latissimus dorsi is the thoracodorsal artery, a branch of the subscapular artery.

Additional segmental perforating vessels contribute to the extensive vascular network of the muscle.

Palpation

Latissimus dorsi can be palpated along the posterior axillary fold and posterolateral trunk.

Resisted adduction or extension of the arm helps distinguish its contracting fibers from adjacent structures.

Inspection

Inspection of the back can reveal the general symmetry and contour of the latissimus dorsi muscles.

Differences in muscle bulk may reflect normal dominance, training, previous surgery, injury, or neurological dysfunction.

Relationship to Body Habitus

The visibility of the latissimus dorsi outline varies with subcutaneous fat, muscle development, age, and posture.

In muscular individuals, the lateral border and posterior axillary fold may be particularly distinct.

Clinical Significance

Thoracodorsal Nerve Injury

Injury to the thoracodorsal nerve can weaken latissimus dorsi.

This may reduce strength during extension and adduction of the humerus and during activities such as climbing or pulling.

Axillary Surgery

The thoracodorsal neurovascular bundle is relevant during surgical procedures involving the axilla.

Preservation of this bundle maintains the function and vascular supply of latissimus dorsi when the muscle is to remain intact or be used for reconstruction.

Latissimus Dorsi Flap

The latissimus dorsi can be used as a pedicled or free musculocutaneous flap in reconstructive surgery.

Its broad surface area, reliable vascular pedicle, and accessible location make it useful for reconstruction of defects in several body regions.

Breast Reconstruction

A latissimus dorsi flap may be used in selected forms of breast reconstruction.

The muscle and associated tissues can be transferred while maintaining or reconstructing their vascular supply.

Muscle Strain

Latissimus dorsi strains can occur during forceful pulling, throwing, climbing, or athletic movements.

Pain may be experienced along the posterolateral trunk or near the posterior axillary fold depending on the site of injury.

Muscular Asymmetry

Asymmetry of the latissimus dorsi outline can occur because of differences in muscle development, nerve injury, previous surgery, or structural variation.

Inspection should therefore be combined with functional testing when weakness is suspected.

Lumbar Hernia

The inferior lumbar triangle is a potential site of lumbar herniation.

The latissimus dorsi forms one of its boundaries, making the muscle an important surface landmark when evaluating posterolateral abdominal wall defects.

Latissimus Dorsi Compared with Trapezius

Feature	Latissimus Dorsi	Trapezius
Main surface region	Lower and posterolateral back	Upper back and posterior neck
Upper limb attachment	Humerus	Clavicle and scapula
Major visible landmark	Posterior axillary fold	Upper shoulder and scapular contour
Primary nerve	Thoracodorsal nerve	Accessory nerve for motor supply

Key Surface Anatomy Features

Feature	Key Point
Muscle type	Broad superficial back muscle
General outline	Broad inferiorly, narrowing superolaterally
Main surface region	Lower thoracic, lumbar and posterolateral trunk
Inferior landmark	Posterior iliac crest
Axillary landmark	Posterior axillary fold
Related surface triangle	Triangle of auscultation
Related lumbar landmark	Inferior lumbar triangle
Main actions	Extension, adduction and medial rotation of humerus
Innervation	Thoracodorsal nerve
Main blood supply	Thoracodorsal artery

Anatomical Importance

The outline of the latissimus dorsi provides one of the major muscular landmarks of the lower and lateral back. Its broad attachment to the thoracolumbar region and iliac crest contrasts with its narrow convergence toward the humerus, creating a recognizable fan-shaped surface contour.

Its superolateral fibers contribute prominently to the posterior axillary fold, while its borders participate in the triangle of auscultation and inferior lumbar triangle. These relationships make the muscle useful for orienting both posterior thoracic and posterolateral abdominal surface anatomy.

The latissimus dorsi is also functionally important because its outline becomes more apparent during resisted extension and adduction of the arm. Its predictable thoracodorsal neurovascular supply further gives the muscle major clinical importance in axillary surgery and reconstructive procedures.