Muscles of the Upper Limb
The upper limb contains numerous muscles organized into four regions – the shoulder (pectoral girdle and glenohumeral joint), the arm (brachial) compartment, the forearm (antebrachial) compartment, and the hand (intrinsic muscles). Broadly, extrinsic muscles of the limb originate on the trunk or shoulder girdle and act on the humerus, forearm, or hand, while intrinsic muscles lie entirely within the limb itself (especially the hand) for fine control. Shoulder muscles include both scapular stabilizers (trapezius, serratus anterior, rhomboids, etc.) and glenohumeral movers (deltoid, rotator cuff, pectoralis major, latissimus dorsi, etc.), setting the stage for arm and forearm function. Arm muscles (biceps, triceps, etc.) largely control elbow movement, and forearm muscles (flexors and extensors) move the wrist, hand, and fingers. Hand muscles enable fine manipulation (pinch, grip, opposition of thumb).
Figure: Muscles of the shoulder girdle. Scapular stabilizers (trapezius, serratus anterior, rhomboids, levator scapulae) position the scapula, enabling arm motion.
Shoulder Region Muscles
Shoulder girdle muscles position the scapula; glenohumeral muscles move the arm at the shoulder joint. Key muscles (with origin, insertion, innervation, action) include:
Trapezius (extrinsic back muscle): Originates from occipital bone, nuchal ligament and spinous processes C7–T12; inserts onto spine of scapula, acromion and clavicle. Innervated by cranial accessory nerve (CN XI) (plus C3–C4). Elevates, retracts and depresses the scapula to stabilize the shoulder.
Serratus anterior: O: ribs 1–8; I: medial border of scapula (anterior surface). Inn: long thoracic nerve. Protracts and upwardly rotates scapula (punching muscle).
Rhomboid major/minor: O: T2–T5 (major) and C7–T1 (minor) spinous processes; I: medial scapular border. Inn: dorsal scapular nerve. Retract and downwardly rotate scapula.
Levator scapulae: O: C1–C4 transverse processes; I: superior angle of scapula. Inn: dorsal scapular nerve. Elevates scapula.
Pectoralis minor: O: ribs 3–5; I: coracoid process of scapula. Inn: medial pectoral nerve. Protracts and depresses scapula (scapular stabilizer).
Deltoid: O: lateral third of clavicle, acromion and spine of scapula; I: deltoid tuberosity of humerus. Inn: axillary nerve. Powerful abductor of the shoulder (middle fibers); anterior fibers flex and medially rotate; posterior fibers extend and laterally rotate the shoulder. (Common training: overhead press strengthens deltoid.)
Supraspinatus: O: supraspinous fossa of scapula; I: greater tubercle of humerus. Inn: suprascapular nerve. Initiates shoulder abduction (first ~15°) and stabilizes the joint. (Frequently injured in rotator-cuff tears.)
Infraspinatus: O: infraspinous fossa; I: greater tubercle of humerus. Inn: suprascapular nerve. Externally (laterally) rotates the humerus. Part of the rotator cuff.
Teres Minor: O: lateral border of scapula; I: greater tubercle of humerus. Inn: axillary nerve. Also externally rotates shoulder and stabilizes humeral head.
Subscapularis: O: subscapular fossa (anterior scapula); I: lesser tubercle of humerus. Inn: upper and lower subscapular nerves. Internally (medially) rotates humerus and holds head of humerus in glenoid. (Unique rotator-cuff muscle for medial rotation.)
Teres Major: O: posterior surface of inferior scapular angle and lower lateral border; I: medial lip of intertubercular sulcus of humerus. Inn: lower subscapular nerve. Adducts, medially rotates and extends the humerus. (Works with latissimus dorsi for climbing motions.)
Pectoralis Major: O: clavicular head (medial clavicle), sternocostal head (sternum, ribs 1–6); I: lateral lip of intertubercular groove of humerus. Inn: medial and lateral pectoral nerves. Adducts and medially rotates shoulder; clavicular head flexes humerus and sternocostal head extends it. (Exercises: bench press, push-ups.)
Latissimus Dorsi: O: spinous processes T7–T12, thoracolumbar fascia, iliac crest, lower ribs; I: floor of intertubercular sulcus of humerus. Inn: thoracodorsal nerve. Extends, adducts and medially rotates the arm. (Key “pull” muscle in swimming/climbing.)
Functional note: The rotator cuff muscles (supraspinatus, infraspinatus, teres minor, subscapularis) stabilize the shoulder; tears (especially of supraspinatus) cause pain and weakness, requiring rehabilitation. Deltoid and rotator-cuff strengthening are common in fitness (e.g. overhead presses, external rotation bands). Scapular muscles (trapezius, serratus, etc.) are crucial for posture and powerful arm movements (e.g. punching, overhead reaching).
Arm (Brachial) Muscles
The arm contains anterior (flexor) and posterior (extensor) compartments. Key muscles:
Biceps Brachii: O: short head – coracoid process of scapula; long head – supraglenoid tubercle of scapula; I: radial tuberosity and bicipital aponeurosis to forearm fascia. Inn: musculocutaneous nerve (C5–C6). Actions: flexes elbow and supinates forearm; also assists shoulder flexion. (The “Popeye” biceps; exercised with curls.)
Brachialis: O: anterior distal half of humerus; I: coronoid process and tuberosity of ulna. Inn: musculocutaneous nerve (C5–C6), often part by radial nerve. Flexes the elbow (the true workhorse flexor).
Coracobrachialis: O: coracoid process of scapula; I: anteromedial mid-shaft of humerus. Inn: musculocutaneous nerve (C5–C7). Flexes and adducts the arm at the shoulder. (Often used by the nerve itself, as musculocutaneous pierces it.)
Triceps Brachii: Long head O: infraglenoid tubercle of scapula; lateral head O: posterior humerus above radial groove; medial head O: posterior humerus below radial groove. All insert on the olecranon of the ulna. Inn: radial nerve (C6–C8). Chief extensor of the elbow; long head also assists shoulder extension and adduction. (Training: dips, pushdowns.)
Anconeus: O: lateral epicondyle of humerus; I: lateral olecranon of ulna. Inn: radial nerve (C7–C8). Assists triceps in extending elbow and stabilizes the elbow joint.
Functional note: Biceps strains or tears (long head tendon) can occur in lifting. The brachial and biceps are heavily used in elbow flexion (e.g. rows, pull-ups). Triceps injuries are less common, but contact or overuse can cause tendonitis. Proper conditioning of both biceps and triceps is important for balanced arm strength.
Forearm Muscles
Forearm muscles lie in two compartments. The anterior compartment (mostly flexors/pronators, innervated by the median or ulnar nerve) flex the wrist and fingers or pronate the hand. The posterior compartment (extensors, innervated by the radial nerve) extend the wrist and fingers or supinate the forearm.
Anterior (Flexor) Compartment – Superficial Layer: These share a common origin at the medial epicondyle. Key muscles:
Pronator Teres: Humeral head O: medial epicondyle of humerus; ulnar head O: coronoid process of ulna; I: lateral surface of radius. Inn: median nerve (C6–C7). Pronates forearm and assists elbow flexion.
Flexor Carpi Radialis (FCR): O: medial epicondyle of humerus; I: bases of 2nd–3rd metacarpals. Inn: median nerve (C6–C7). Flexes and radially abducts the wrist (hand); active in grip.
Palmaris Longus: O: medial epicondyle; I: flexor retinaculum and palmar aponeurosis. Inn: median nerve (C7–C8). Weak wrist flexor; tightens palmar aponeurosis. (Absent in ~10% of people.)
Flexor Carpi Ulnaris (FCU): O: medial epicondyle, olecranon and ulna; I: pisiform, hamate and base of 5th metacarpal. Inn: ulnar nerve (C7–T1). Flexes and ulnarly deviates (adducts) the wrist.
Anterior (Flexor) Compartment – Deep Layer: (for completeness) includes Flexor Digitorum Superficialis/Profundus (flex fingers), Flexor Pollicis Longus (flex thumb), and Pronator Quadratus (pronates). (Innervation: median nerve except medial FDP and ulnar half of 4th finger by ulnar nerve.)
Posterior (Extensor) Compartment – Superficial Layer: Key muscles:
Brachioradialis: O: lateral supracondylar ridge of humerus; I: lateral distal radius. Inn: radial nerve (C5–C7). Flexes elbow (especially with forearm in neutral position).
Extensor Carpi Radialis Longus (ECRL): O: lateral supracondylar ridge of humerus; I: base of 2nd metacarpal. Inn: radial nerve (C5–C8). Extends and radially deviates (abducts) wrist.
Extensor Carpi Radialis Brevis (ECRB): O: lateral epicondyle of humerus; I: base of 3rd metacarpal. Inn: deep radial (posterior interosseous) nerve (C5–C6). Extends and assists radial abduction of wrist. (Overuse can cause “tennis elbow”.)
Extensor Digitorum: O: lateral epicondyle; I: extensor expansions of digits 2–5. Inn: posterior interosseous nerve (C7–C8). Extends the four fingers at the metacarpophalangeal and interphalangeal joints; also assists wrist extension.
Extensor Carpi Ulnaris (ECU): O: lateral epicondyle and posterior ulna; I: base of 5th metacarpal. Inn: posterior interosseous nerve (C7–C8). Extends and ulnarly deviates (adducts) the wrist.
Supinator: O: lateral epicondyle, radial collateral and annular ligaments, supinator crest of ulna; I: proximal radius (anterior, lateral, and posterior surfaces of upper 1/3). Inn: posterior interosseous nerve (C7–C8). Supinates the forearm (turns palm up).
Posterior (Extensor) Compartment – Deep Layer: (for completeness) includes abductor pollicis longus, extensor pollicis brevis/longus (move the thumb), and extensor indicis (extends index finger).
Functional note: Injuries: Lateral epicondylitis (“tennis elbow”) involves overuse of ECRB/ECRL. Medial epicondylitis (“golfer’s elbow”) affects forearm flexors/Pronator Teres. Carpal tunnel syndrome (median nerve entrapment under flexor retinaculum) can weaken thenar muscles (see hand). Training: Wrist curls and reverse curls build forearm flexors/extensors; grip exercises develop hand strength. Supination/pronation exercises (like forearm twists) engage biceps (supination) and pronators.
Figure: Major muscles of the arm and forearm (anterior and posterior views). Key flexors (biceps, brachialis) and extensors (triceps, brachioradialis) of the elbow, and wrist flexors/extensors are shown.
Hand (Intrinsic) Muscles
Intrinsic hand muscles (within the palm) enable fine manipulation. They are divided into thenar (thumb side), hypothenar (little finger side), interossei (between metacarpals), and lumbricals. All thenar muscles are innervated by the median nerve (recurrent branch), and most hypothenar/interossei by the ulnar nerve. Key intrinsic muscles:
Thenar Muscles (thumb eminence): Opponens Pollicis, Abductor Pollicis Brevis, Flexor Pollicis Brevis.
Opponens Pollicis: O: tubercle of trapezium and flexor retinaculum; I: lateral side of 1st metacarpal. Inn: median nerve. Opposes the thumb (medially rotates and flexes the first metacarpal).
Abductor Pollicis Brevis: O: tubercles of scaphoid and trapezium, and flexor retinaculum; I: lateral base of proximal phalanx of thumb. Inn: median nerve. Abducts the thumb (moves it away from palm).
Flexor Pollicis Brevis: O: trapezium and flexor retinaculum; I: base of proximal phalanx of thumb. Inn: median nerve (superficial head) and ulnar nerve (deep head). Flexes the thumb at the metacarpophalangeal joint.
Adductor Pollicis: (deep hand) O: oblique head from capitate and bases of 2nd–3rd metacarpals, transverse head from shaft of 3rd metacarpal; I: medial base of proximal phalanx of thumb. Inn: ulnar nerve (deep branch). Adducts the thumb toward the palm. (Important for strong grip/pinch.)
Hypothenar Muscles (little-finger eminence): Opponens Digiti Minimi (O: hook of hamate, retinaculum; I: medial 5th metacarpal; opposes little finger), Abductor Digiti Minimi (O: pisiform; I: base of 5th proximal phalanx; abducts little finger), Flexor Digiti Minimi Brevis (O: hook of hamate, retinaculum; I: base of 5th proximal phalanx; flexes little finger). All innervated by ulnar nerve.
Interossei Muscles: Dorsal interossei (4 muscles) abduct fingers (spread them) and palmar interossei (3 muscles) adduct fingers (bring them together). O: metacarpal shafts; I: extensor expansions and bases of proximal phalanges. Innervation: deep branch of ulnar nerve. Crucial for fine finger control and grip stability.
Lumbricals (4 muscles): O: tendons of flexor digitorum profundus; I: extensor expansions on dorsum of fingers. Innervation: median nerve (lumbricals 1–2 to index/middle fingers) and ulnar nerve (lumbricals 3–4 to ring/little fingers). Flex the metacarpophalangeal joints and extend the interphalangeal joints (fine finger flexion/extension synergy).
Palmaris Brevis (small): wrinkles skin on ulnar palm (innervated by ulnar nerve).
Functional note: Intrinsic muscles enable precision (thumb opposition, delicate finger movements) and grip power. Injuries: Median nerve compression (carpal tunnel) weakens thenar muscles and lateral lumbricals; ulnar nerve injury causes “ulnar claw” deformity (loss of interossei/lumbrical function). Training: Grip strength exercises (pinch, finger push-ups) engage intrinsics and forearm flexors, improving dexterity.
Figure: Intrinsic hand muscles (thenar and interossei). These small muscles produce thumb opposition, finger ab-/adduction, and control fine movements.
Overall, the coordinated actions of these muscles allow complex upper limb movements – from powerful arm swings to delicate finger coordination. Strength training (e.g. presses, rows, curls, wrist exercises) and flexibility work help maintain shoulder stability and forearm function, while ergonomics and proper technique can prevent common injuries (rotator cuff strains, epicondylitis, carpal tunnel).
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