Muscles that Move the Shoulder and Upper Limb

Introduction

The shoulder and upper limb are moved by a complex group of muscles that act on the scapula and humerus. These muscles are essential for a wide range of movements and stability, and their organization is more intricate than that of the lower limb due to the unique anatomy and functional demands of the arm.

Comparative Complexity of the Arm vs. Leg

Key Differences

• Brachial Plexus: The arm is innervated by five terminal nerves from the brachial plexus, unlike the leg, which uses a quadrant system for muscle organization.

• Attachment: The upper limb is not firmly attached to the axial skeleton, requiring additional muscles for stabilization.

• Muscle Structure: Large, fan-shaped muscles support and move the scapula, which are not present in the lower limb.

Movements of the Scapula

Overview

The scapula can perform multiple movements depending on which muscles contract. These movements are crucial for positioning the upper limb and facilitating shoulder joint mobility.

• Stabilization: Simultaneous contraction of specific muscles stabilizes the scapula.

• Movement: Coordinated or sequential contraction of specific muscles causes movement of the scapula.

Types of Scapular Movements

• Elevation: Lifting the scapula upwards.

• Depression: Lowering the scapula downwards.

• Protraction: Moving the scapula forward along the rib cage.

• Retraction: Pulling the scapula backward toward the spine.

• Superior (Upward) Rotation: Rotating the scapula so the glenoid cavity moves upward.

• Inferior (Downward) Rotation: Rotating the scapula so the glenoid cavity moves downward.

Muscles that Move the Scapula

Trapezius

• Origin: Occipital bone, spines of C1-7 and T1-12

• Insertion: Acromion, spine of scapula, lateral third of clavicle

• Innervation: CN XI – Spinal Accessory Nerve

• Functions: Extends the neck; elevates, depresses, and retracts scapula; assists in superior rotation of scapula with serratus anterior.

Serratus Anterior

• Origin: Ribs 1-9

• Insertion: Scapula medial border

• Innervation: Long thoracic nerve

• Functions: Protracts and stabilizes the scapula; assists in superior rotation of scapula with trapezius.

Levator Scapulae

• Origin: Transverse processes of C1-4

• Insertion: Superior angle of scapula

• Innervation: Dorsal scapular nerve

• Functions: Elevates and inferiorly rotates the scapula.

Rhomboid Minor and Major

• Origin: Minor: Spinous process of C7 and T1; Major: Spinous process of T2-T5

• Insertion: Medial border of scapula

• Innervation: Dorsal scapular nerve

• Functions: Stabilizes, retracts, and elevates the scapula; assists in inferior rotation with pectoralis minor and levator scapulae.

Pectoralis Minor

• Origin: Ribs 3-5

• Insertion: Coracoid process of scapula

• Innervation: Medial and lateral pectoral nerves

• Functions: Stabilizes, protracts, and depresses the scapula; assists in inferior rotation with rhomboids and levator scapulae.

Sequential Movements of the Scapula

Superior Rotation

• Sequential contraction of upper and lower parts of trapezius and serratus anterior.

Inferior Rotation

• Sequential contraction of rhomboids, levator scapulae, and pectoralis minor.

Muscles that Move the Humerus

Pectoralis Major

• Origin: Sternal end of clavicle, sternum, costal cartilages 1-6

• Insertion: Intertubercular groove and greater tubercle of humerus

• Innervation: Medial and lateral pectoral nerves

• Functions: Adducts humerus; clavicular head flexes/elevates/protracts humerus; sternal head extends/depresses/retracts humerus.

Latissimus Dorsi

• Origin: Spines of T7-T12, lumbar fascia

• Insertion: Intertubercular groove of humerus

• Innervation: Thoracodorsal nerve

• Functions: Extends, medially rotates, and adducts the humerus.

Teres Major

• Origin: Inferior angle of scapula

• Insertion: Intertubercular sulcus of humerus

• Innervation: Lower subscapular nerve

• Functions: Adducts, extends, and medially rotates humerus; not part of the rotator cuff.

Deltoid

• Origin: Acromion, spine of scapula, lateral third of clavicle

• Insertion: Deltoid tuberosity of humerus

• Innervation: Axillary nerve

• Functions: Abducts the humerus when all fibers contract together; anterior fibers flex and medially rotate, posterior fibers extend and laterally rotate.

Rotator Cuff Muscles

Overview

The rotator cuff is a group of four muscles that stabilize the head of the humerus in the glenoid cavity and facilitate rotational movements. These muscles are essential for shoulder joint integrity and function.

• Supraspinatus: Originates from supraspinous fossa; inserts on greater tubercle; innervated by suprascapular nerve; abducts humerus.

• Infraspinatus: Originates from infraspinous fossa; inserts on greater tubercle; innervated by suprascapular nerve; laterally rotates humerus.

• Teres Minor: Originates from lateral border of scapula; inserts on greater tubercle; innervated by axillary nerve; laterally rotates humerus.

• Subscapularis: Originates from subscapular fossa; inserts on lesser tubercle; innervated by subscapular nerve; medially rotates humerus.

Common Attachment: All rotator cuff muscles attach to the tubercles of the humerus.

Function: Simultaneous contraction stabilizes the humerus; sequential contraction causes rotation.

Clinical Note: Repetitive overuse can cause rotator cuff tears, leading to pain and dysfunction.

Synergists and Antagonists of the Shoulder and Brachium

Key Relationships

• Synergists: Latissimus dorsi, pectoralis major, and teres major all adduct the humerus.

• Antagonists: Latissimus dorsi and pectoralis major are antagonists in flexion and extension of the humerus.

• Common Attachment Points: These muscles often attach to the intertubercular groove or crest of the humerus.

Summary Table: Scapular Movements and Muscles

Additional info:

• The brachial plexus is a network of nerves that supplies the upper limb, with five main terminal branches: musculocutaneous, axillary, radial, median, and ulnar nerves.

• Muscles acting on the shoulder and elbow are classified as ventral (flexors) or dorsal (extensors) based on their position and innervation.