muscles of upper limb

4 min read 13-03-2025

The upper limb, encompassing the arm, forearm, and hand, boasts a complex network of muscles responsible for a wide range of movements, from delicate finger manipulations to powerful throws. Understanding the organization and function of these muscles is crucial for anyone studying anatomy, physical therapy, or related fields. This comprehensive guide will explore the key muscle groups of the upper limb, their actions, and innervation.

I. Muscles of the Shoulder Girdle

The shoulder girdle, formed by the scapula (shoulder blade) and clavicle (collarbone), provides the foundation for upper limb movement. Muscles acting on this region stabilize the scapula and facilitate arm elevation and rotation.

A. Muscles Moving the Scapula

Trapezius: This large, superficial muscle elevates, retracts (draws together), and depresses the scapula. It also rotates the scapula upward. Innervation is via the spinal accessory nerve (CN XI).
Levator Scapulae: Elevates the scapula and rotates it downward. It's innervated by the dorsal scapular nerve (C3-C5).
Rhomboids (Major and Minor): Retract and rotate the scapula downward. Innervation is by the dorsal scapular nerve (C4-C5).
Serratus Anterior: Protracts (moves forward) the scapula, rotates it upward, and helps in stabilizing it against the rib cage. It's innervated by the long thoracic nerve (C5-C7). Weakness in this muscle can lead to "winged scapula."

B. Muscles Acting on the Glenohumeral Joint

The glenohumeral joint, the articulation between the humerus (upper arm bone) and the scapula, allows for a wide range of motion. Several muscles directly contribute to movements at this joint.

Deltoid: A powerful shoulder muscle with three parts (anterior, middle, and posterior) responsible for abduction, flexion, extension, and medial and lateral rotation of the humerus. Innervation is from the axillary nerve (C5-C6).
Rotator Cuff Muscles: These four muscles – supraspinatus, infraspinatus, teres minor, and subscapularis – are crucial for shoulder stability and movement. They contribute to abduction, external and internal rotation, and help to keep the humeral head centered within the glenoid fossa. Innervation varies: supraspinatus by the suprascapular nerve, infraspinatus and teres minor by the axillary nerve, and subscapularis by the upper and lower subscapular nerves.

II. Muscles of the Arm

The arm, extending from the shoulder to the elbow, contains two primary muscle compartments: anterior and posterior.

A. Anterior Compartment (Flexors)

The anterior compartment muscles primarily flex the elbow. Key muscles include:

Biceps Brachii: Flexes the elbow and supinates the forearm. It's innervated by the musculocutaneous nerve (C5-C6).
Brachialis: A powerful elbow flexor. Innervation is from the musculocutaneous nerve (C5-C6).
Coracobrachialis: Flexes and adducts the arm at the shoulder. Innervation is from the musculocutaneous nerve (C5-C6).

B. Posterior Compartment (Extensors)

The posterior compartment muscles primarily extend the elbow. The most significant muscle is the:

Triceps Brachii: Extends the elbow. Innervated by the radial nerve (C6-C8).

III. Muscles of the Forearm

The forearm muscles are responsible for intricate hand and wrist movements. They are divided into anterior (flexor) and posterior (extensor) compartments.

A. Anterior Compartment (Flexors and Pronators)

This compartment houses muscles responsible for wrist flexion, finger flexion, and forearm pronation.

Flexor Carpi Radialis: Flexes and abducts the wrist.
Flexor Carpi Ulnaris: Flexes and adducts the wrist.
Palmaris Longus: Weakly flexes the wrist.
Pronator Teres: Pronates the forearm.
Flexor Digitorum Superficialis: Flexes the proximal interphalangeal joints of the fingers.
Flexor Digitorum Profundus: Flexes the distal interphalangeal joints of the fingers.
Flexor Pollicis Longus: Flexes the thumb.

All of these muscles are primarily innervated by the median nerve (C5-T1), except for the flexor carpi ulnaris and the medial half of flexor digitorum profundus, which are innervated by the ulnar nerve (C8-T1).

B. Posterior Compartment (Extensors and Supinators)

This compartment contains muscles responsible for wrist extension, finger extension, and forearm supination.

Extensor Carpi Radialis Longus: Extends and abducts the wrist.
Extensor Carpi Radialis Brevis: Extends and abducts the wrist.
Extensor Carpi Ulnaris: Extends and adducts the wrist.
Extensor Digitorum: Extends the fingers.
Extensor Digiti Minimi: Extends the little finger.
Extensor Indicis: Extends the index finger.
Supinator: Supinates the forearm.
Abductor Pollicis Longus: Abducts the thumb.
Extensor Pollicis Brevis: Extends the thumb.
Extensor Pollicis Longus: Extends the thumb.

These muscles are primarily innervated by the radial nerve (C5-T1).

IV. Muscles of the Hand

The intrinsic muscles of the hand are located within the hand itself. They are responsible for fine motor control and precise movements of the fingers and thumb. These muscles are too numerous to detail individually within this overview, but understanding their general function in relation to the forearm muscles is essential.

V. Clinical Significance

Understanding the muscles of the upper limb is crucial in diagnosing and treating various conditions, including:

Rotator cuff injuries: Tears or inflammation of the rotator cuff muscles can cause significant shoulder pain and dysfunction.
Carpal tunnel syndrome: Compression of the median nerve in the carpal tunnel can lead to numbness, tingling, and weakness in the hand.
Tennis elbow (lateral epicondylitis): Inflammation of the tendons on the outside of the elbow, often affecting the extensor muscles of the wrist.
Golfer's elbow (medial epicondylitis): Inflammation of the tendons on the inside of the elbow, often affecting the flexor muscles of the wrist.

This article provides a general overview. For a complete understanding, detailed anatomical texts and atlases are recommended. Remember to consult with a healthcare professional for any concerns regarding your musculoskeletal system.