The muscles of the shoulder region help move and stabilize the upper limb. 

They also give the shoulder its shape. These muscles are divided into two main groups:

• Extrinsic muscles – start from the torso and attach to the shoulder bones (the clavicle, scapula, or humerus).

• Intrinsic muscles – start from the scapula or clavicle and attach to the humerus.

There are also the rotator cuff muscles, which stabilize the shoulder joint.

Together, these muscles make the shoulder one of the most flexible joints in the body, allowing a wide range of arm movements like lifting, rotating, and reaching.

Superficial Extrinsic Muscles

The trapezius is a large, flat, triangular muscle on the upper back forming a trapezoid shape. Trapezius plays a major role in overhead movements, such as throwing or reaching up. Its control of scapular rotation is essential: without it, the arm cannot fully abduct.

The upper fibers raise the scapula and rotate it upward.

The middle fibers pull the scapula backward.

The lower fibers lower the scapula.

• Origin: Skull, nuchal ligament, and spinous processes of C7–T12.

• Insertion: Clavicle, acromion, and spine of the scapula.

• Innervation: Accessory nerve; sensory fibers from C3 and C4.

The latissimus dorsi is the largest back muscle and a major power generator for movements such as swimming, rowing, and climbing. It also helps stabilize the torso when lifting heavy objects. It works with teres major to perform internal rotation and extension of the humerus.

• Origin: Spinous processes of T7–T12, iliac crest, thoracolumbar fascia, and lower ribs.

• Insertion: Intertubercular groove of the humerus.

• Innervation: Thoracodorsal nerve.

Deep Extrinsic Muscles

The rhomboid major and rhomboid minor pull the scapula toward the spine and help rotate it. Strong rhomboids are important for good posture and help counteract forward rounding of the shoulders.

• Origin: Spinous processes of T2–T5; the rhomboid minor originates from spinous processes of C7–T1.

• Insertion: Medial border of the scapula.

• Innervation: Dorsal scapular nerve.

The levator scapulae lifts the scapula and aids downward rotation. It also helps stabilize the neck when the shoulder girdle is fixed.

• Origin: The transverse processes of C1–C4.

• Insertion: Medial border of the scapula.

• Innervation: Dorsal scapular nerve.

Deltoid

The deltoid is a thick, triangular muscle covering the shoulder.

The anterior fibers flex and rotate the arm inward.

The middle fibers abduct the arm.

The posterior fibers extend and rotate the arm outward.

• Origin: Lateral clavicle, acromion, and scapular spine.

• Insertion: Deltoid tuberosity of the humerus.

• Innervation: Axillary nerve.

Teres Major

The teres major extends, adducts, and rotates the arm inward and stabilizes the shoulder.

• Origin: Lower part of the scapula.

• Insertion: Medial lip of the humerus groove.

• Innervation: Lower subscapular nerve.

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