11)_Forearm_Muscle.pdf

# Forearm muscle compartments The forearm, located between the elbow and the wrist, contains muscles organized into three distinct compartments, each with specific functions and primary innervation [15](#page=15) [2](#page=2). ### 1.1 Anterior compartment The anterior compartment of the forearm houses the muscles responsible for wrist and digit flexion, as well as pronation [15](#page=15) [2](#page=2). #### 1.1.1 Primary functions of the anterior compartment The primary actions of the muscles within this compartment include: * Flexion of the wrist and digits [15](#page=15). * Pronation of the forearm [15](#page=15). #### 1.1.2 Innervation of the anterior compartment The muscles of the anterior compartment are innervated by the median and ulnar nerves [15](#page=15). #### 1.1.3 Muscles of the anterior compartment (by layer) * **Superficial layer:** Includes Pronator Teres, Flexor Carpi Radialis (FCR), Palmaris Longus (PL), and Flexor Carpi Ulnaris (FCU) [15](#page=15). * **Intermediate layer:** Consists of the Flexor Digitorum Superficialis (FDS) [15](#page=15). * **Deep layer:** Comprises the Flexor Digitorum Profundus (FDP), Flexor Pollicis Longus (FPL), and Pronator Quadratus [15](#page=15). ### 1.2 Lateral compartment The lateral compartment of the forearm primarily contains muscles involved in wrist extension and supination [15](#page=15) [2](#page=2). #### 1.2.1 Primary functions of the lateral compartment The main functions of the lateral compartment muscles are: * Extension of the wrist [15](#page=15). * Supination of the forearm [15](#page=15). * Flexion of the elbow [15](#page=15). #### 1.2.2 Innervation of the lateral compartment This compartment is innervated by the radial nerve [15](#page=15). #### 1.2.3 Muscles of the lateral compartment (by layer) * **Superficial layer:** Includes Brachioradialis, Extensor Carpi Radialis Longus (ECRL), and Extensor Carpi Radialis Brevis (ECRB) [15](#page=15). * **Deep layer:** Contains the Supinator muscle [15](#page=15). ### 1.3 Posterior compartment The posterior compartment of the forearm is responsible for the extension of the hand and fingers [15](#page=15) [2](#page=2). #### 1.3.1 Primary functions of the posterior compartment The key functions of the posterior compartment muscles are: * Extension of the wrist and digits [15](#page=15). * Adduction and abduction of the wrist [15](#page=15). #### 1.3.2 Innervation of the posterior compartment The muscles in this compartment are innervated by the radial nerve [15](#page=15). #### 1.3.3 Muscles of the posterior compartment (by layer) * **Superficial layer:** Includes Extensor Digitorum, Extensor Digiti Minimi (EDM), Extensor Carpi Ulnaris (ECU), and Anconeus [15](#page=15). * **Deep layer:** Comprises Abductor Pollicis Longus (APL), Extensor Pollicis Brevis (EPB), Extensor Pollicis Longus (EPL), and Extensor Indicis [15](#page=15). > **Tip:** Understanding the primary functions and innervation of each compartment is crucial for predicting how injuries to specific nerves or muscles will manifest. The division into anterior (flexors/pronators), lateral (extensors/supinators), and posterior (extensors) provides a clear organizational framework for forearm musculature. * * * # Anterior compartment muscles This section details the muscles of the anterior forearm compartment, categorized by their superficial, intermediate, and deep layers, outlining their origins, insertions, actions, and innervations. ### 2.1 The superficial layer: The superficial medial epicondylar layer The superficial layer of the anterior forearm compartment originates from the medial epicondyle of the humerus and is innervated by the median nerve, with the exception of the flexor carpi ulnaris, which is innervated by the ulnar nerve [3](#page=3). #### 2.1.1 Pronator teres * **Origin:** Humeral head from the medial epicondyle of the humerus and ulnar head from the coronoid process of the ulna [3](#page=3). * **Insertion:** Middle part of the lateral surface of the radius [3](#page=3). * **Action:** Pronates the forearm and assists in flexing the forearm at the elbow [3](#page=3). * **Innervation:** Median nerve [3](#page=3). #### 2.1.2 Flexor carpi radialis * **Origin:** Medial epicondyle of the humerus [3](#page=3). * **Insertion:** Base of the second metacarpal bone [3](#page=3). * **Action:** Flexes and abducts the hand at the wrist [3](#page=3). * **Innervation:** Median nerve [3](#page=3). #### 2.1.3 Palmaris longus * **Origin:** Medial epicondyle of the humerus [3](#page=3). * **Insertion:** Superficial palmar fascia of the hand [3](#page=3). * **Action:** Tenses the palmar fascia and weakly flexes the hand at the wrist [3](#page=3). * **Innervation:** Median nerve [3](#page=3). #### 2.1.4 Flexor carpi ulnaris * **Origin:** Humeral head from the medial epicondyle of the humerus and ulnar head from the olecranon and posterior border of the ulna [3](#page=3). * **Insertion:** Pisiform bone, which then attaches to the hamate and fifth metacarpal bone via the pisohamate and short-uinarmate ligaments [3](#page=3). * **Action:** Flexes and adducts the hand at the wrist [3](#page=3). * **Innervation:** Ulnar nerve [3](#page=3). ### 2.2 The intermediate layer The intermediate layer contains one primary muscle responsible for significant digital flexion. #### 2.2.1 Flexor digitorum superficialis (FDS) * **Origin:** Two heads: a humeroulnar head originating from the medial epicondyle of the humerus and the coronoid process of the ulna, and a radial head originating from the anterior edge of the radius. These heads form a tendinous arch under which the median nerve and ulnar artery pass [4](#page=4). * **Insertion:** Splits into four tendons, each inserting onto the base of the second phalanx (middle phalanx) of digits 2-5. Each tendon is "perforated" to allow the deep flexor tendon to pass through [4](#page=4). * **Action:** Flexes the second phalanx on the first, the first phalanx on the metacarpal, and the hand on the forearm [4](#page=4). * **Innervation:** Median nerve [4](#page=4). * **Clinical Landmark:** The four tendons of the FDS, along with the deep flexor tendons and the median nerve, traverse the carpal tunnel at the wrist [4](#page=4). ### 2.3 The deep layer The deep layer of the anterior forearm compartment comprises muscles that provide deeper flexion actions and pronation. #### 2.3.1 Flexor digitorum profundus (FDP) * **Origin:** Upper three-quarters of the anterior surface of the ulna [5](#page=5). * **Insertion:** Base of the third phalanx (distal phalanx) of digits 2-5 [5](#page=5). * **Action:** Flexes all phalanges of digits 2-5 and also flexes the wrist. It is nicknamed the "perforating tendon" because its tendons pass through the tendons of the flexor digitorum superficialis [5](#page=5). * **Innervation:** Dual innervation: the lateral two heads receive innervation from the median nerve, and the medial two heads receive innervation from the ulnar nerve [5](#page=5). #### 2.3.2 Flexor pollicis longus (FPL) * **Origin:** Anterior surface of the radius [5](#page=5). * **Insertion:** Base of the distal phalanx of the thumb [5](#page=5). * **Action:** Flexes the distal and proximal phalanges of the thumb [5](#page=5). * **Innervation:** Median nerve [5](#page=5). #### 2.3.3 Pronator quadratus * **Origin:** The distal quarter of the anterior surface of the ulna [5](#page=5). * **Insertion:** The distal quarter of the anterior surface of the radius [5](#page=5). * **Action:** Pronates the forearm and hand [5](#page=5). * **Innervation:** Median nerve [5](#page=5). * * * # Lateral and posterior compartment muscles This section explores the muscles responsible for extending and supinating the forearm and hand, focusing on those located in the lateral and posterior compartments of the forearm. ### 3.1 The lateral compartment: Radial group muscles The lateral compartment of the forearm primarily contains muscles involved in wrist extension and supination. #### 3.1.1 Brachioradialis * **Origin:** Lateral edge of the humerus [6](#page=6). * **Insertion:** Styloid process of the radius [6](#page=6). * **Action:** Primarily flexes the forearm. It can also act as a supinator when the forearm is fully pronated [6](#page=6). * **Innervation:** Radial nerve [6](#page=6). #### 3.1.2 Extensor carpi radialis longus (ECRL) * **Origin:** Lower lateral edge of the humerus [6](#page=6). * **Insertion:** Base of the 2nd metacarpal [6](#page=6). * **Action:** Extends and abducts the hand [6](#page=6). * **Innervation:** Radial nerve [6](#page=6). #### 3.1.3 Extensor carpi radialis brevis (ECRB) * **Origin:** Lateral epicondyle [6](#page=6). * **Insertion:** Base of the 3rd metacarpal [6](#page=6). * **Action:** Extends and abducts the hand [6](#page=6). * **Innervation:** Radial nerve [6](#page=6). ### 3.2 Supinator muscle The supinator muscle is a broad muscle crucial for forearm rotation. * **Architecture:** It wraps around the upper forearm [7](#page=7). * **Origin:** The supinator has two heads: a superficial head originating from the lateral epicondyle and a deep head originating from the supinator fossa of the ulna [7](#page=7). * **Insertion:** Upper part of the lateral surface of the radius [7](#page=7). * **Action:** Supination of the forearm [7](#page=7). * **Innervation:** Radial nerve [7](#page=7). * **Tip:** The deep branch of the radial nerve passes between the superficial and deep heads of the supinator muscle [7](#page=7). ### 3.3 The posterior compartment: Superficial extensors The posterior compartment of the forearm houses muscles primarily responsible for extending the fingers and wrist. #### 3.3.1 Extensor digitorum * **Origin:** Lateral epicondyle [8](#page=8). * **Insertion:** Dorsal surfaces of the phalanges of the last four fingers [8](#page=8). * **Action:** Extends the phalanges, fingers, and metacarpals [8](#page=8). * **Innervation:** Radial nerve [8](#page=8). #### 3.3.2 Extensor digiti minimi * **Origin:** Lateral epicondyle [8](#page=8). * **Insertion:** Joins the extensor tendon for the fifth finger [8](#page=8). * **Action:** Extends the fifth finger [8](#page=8). * **Innervation:** Radial nerve [8](#page=8). #### 3.3.3 Extensor carpi ulnaris (ECU) * **Origin:** It has a humeral head from the lateral epicondyle and an ulnar head from the posterior ulna [8](#page=8). * **Insertion:** Base of the 5th metacarpal [8](#page=8). * **Action:** Extends and adducts the hand [8](#page=8). * **Innervation:** Radial nerve [8](#page=8). ### 3.4 Anconeus muscle The anconeus muscle assists in forearm extension at the elbow. * **Origin:** Posterior surface of the lateral epicondyle of the humerus [9](#page=9). * **Insertion:** Lateral surface of the olecranon and the upper third of the posterior border of the ulna [9](#page=9). * **Action:** Extends the forearm [9](#page=9). * **Innervation:** Radial nerve [9](#page=9). ### 3.5 The posterior compartment: Deep outcropping muscles (Thumb extensors) This group of muscles is crucial for precise movements of the thumb. #### 3.5.1 Abductor pollicis longus (APL) * **Origin:** Posterior ulna and radius [10](#page=10). * **Insertion:** Base of the 1st metacarpal [10](#page=10). * **Action:** Abducts and extends the thumb [10](#page=10). * **Innervation:** Radial nerve [10](#page=10). #### 3.5.2 Extensor pollicis brevis (EPB) * **Origin:** Posterior ulna and radius, located below the APL [10](#page=10). * **Insertion:** Base of the 1st phalanx of the thumb [10](#page=10). * **Action:** Extends the 1st phalanx of the thumb [10](#page=10). * **Innervation:** Radial nerve [10](#page=10). #### 3.5.3 Extensor pollicis longus (EPL) * **Origin:** Posterior ulna [10](#page=10). * **Insertion:** Base of the 2nd phalanx of the thumb [10](#page=10). * **Action:** Extends the 2nd phalanx of the thumb [10](#page=10). * **Innervation:** Radial nerve [10](#page=10). ### 3.6 Extensor indicis This muscle provides specific extension for the index finger. * **Origin:** Posterior surface of the ulna, below the EPL [11](#page=11). * **Insertion:** Joins the tendon of the Extensor Digitorum for the index finger at the metacarpophalangeal joint [11](#page=11). * **Action:** Extends the index finger [11](#page=11). * **Innervation:** Radial nerve [11](#page=11). * * * # Anatomical landmarks and functional integration This section explores key anatomical structures of the wrist and forearm and their critical roles in enabling various hand movements and grips. ### 4.1 The anatomical snuffbox The anatomical snuffbox is a triangular depression located on the posterolateral aspect of the wrist. Its borders are defined by specific tendons [12](#page=12): * **Anterolateral border:** Formed by the tendons of Abductor Pollicis Longus and Extensor Pollicis Brevis [12](#page=12). * **Posteromedial border:** Formed by the tendon of Extensor Pollicis Longus [12](#page=12). * **Floor:** The floor of the anatomical snuffbox contains the radial artery [12](#page=12). This landmark is significant because the tendons of the short extensor (Extensor Pollicis Brevis) and long extensor (Extensor Pollicis Longus) diverge at the wrist, creating this palpable space [12](#page=12). ### 4.2 Retinacula and tendon sheaths: The pulley system Retinacula are specialized osteofibrous structures that function as pulleys for the tendons of the forearm muscles, ensuring smooth and efficient movement during flexion and extension [13](#page=13). #### 4.2.1 The carpal canal A notable example of a retinaculum is the flexor retinaculum, which, along with the carpal bones, forms the carpal canal. This canal transmits the flexor tendons of the digits and the median nerve [13](#page=13). #### 4.2.2 Serous tendon sheaths Within the fibrous sheaths of the retinacula, serous sheaths (also known as peritendinous serous membranes) facilitate the gliding of tendons. These membranes reduce friction, allowing tendons to move freely as the hand and wrist articulate [13](#page=13). ### 4.3 Functional integration of forearm muscles The muscles of the forearm, organized into anterior and posterior compartments, are responsible for a wide range of hand and wrist actions, enabling complex movements like grips and fine motor control [14](#page=14). * **Power grip:** Primarily facilitated by the muscles of the anterior forearm compartment, which are the flexors. These muscles generate the force needed to hold objects firmly [14](#page=14). * **Fine extension:** Achieved through the muscles of the posterior forearm compartment, which are the extensors. These muscles allow for precise control over the extension of digits and the wrist [14](#page=14). * **Rotation:** Movements such as supination and pronation, essential for rotating the forearm, involve the coordinated action of muscles from both the lateral and anterior compartments [14](#page=14). > **Tip:** Understanding the relationship between tendon pathways, retinacula (acting as pulleys), and the muscle compartments is crucial for comprehending how coordinated forearm and hand movements are executed. * * * ## Common mistakes to avoid * Review all topics thoroughly before exams * Pay attention to formulas and key definitions * Practice with examples provided in each section * Don't memorize without understanding the underlying concepts

| Term | Definition | |------|------------| | Forearm | The part of the upper limb between the elbow and the wrist. | | Muscle Compartment | A group of muscles within a limb that are enclosed by a common fascial sheath and often share a common nerve supply and function. | | Anterior Compartment | The muscle group located at the front of the forearm, primarily responsible for flexion of the wrist and fingers, and pronation of the forearm. | | Lateral Compartment | The muscle group located on the outer side of the forearm, involved in wrist extension and supination. | | Posterior Compartment | The muscle group located at the back of the forearm, mainly responsible for extension of the wrist and fingers. | | Pronation | The movement of the forearm that turns the palm downward or backward. | | Supination | The movement of the forearm that turns the palm upward or forward. | | Flexion | A bending movement that decreases the angle between two body parts. | | Extension | A straightening movement that increases the angle between two body parts. | | Abduction | Movement of a limb away from the midline of the body. | | Adduction | Movement of a limb toward the midline of the body. | | Origin | The point of attachment of a muscle that is generally considered to be fixed and immovable during contraction. | | Insertion | The point of attachment of a muscle that is generally considered to be movable during contraction. | | Innervation | The supply of nerves to a part of the body or a particular organ. | | Tendon | A flexible but inelastic cord of fibrous tissue attaching a muscle to a bone. | | Phalanx | One of the bones of the fingers or toes. | | Metacarpal | One of the five bones in the palm of the hand that connect the carpals to the phalanges. | | Humorous | The long bone of the upper arm, extending from the shoulder to the elbow. | | Ulna | The longer of the two bones in the forearm, located on the medial side when in anatomical position. | | Radius | The shorter of the two bones in the forearm, located on the lateral side when in anatomical position. | | Medial Epicondyle | A bony prominence on the inner side of the distal humerus. | | Lateral Epicondyle | A bony prominence on the outer side of the distal humerus. | | Pisiform Bone | A small, pea-shaped bone located in the proximal row of the wrist, forming part of the carpal tunnel. | | Olecranon | The curved bony projection at the upper end of the ulna, forming the point of the elbow. | | Carpal Tunnel | A narrow passageway in the wrist, formed by carpal bones and the flexor retinaculum, through which the flexor tendons and median nerve pass. | | Retinaculum | A band of tissue that helps to hold an organ or tissue in place, often acting as a pulley for tendons. | | Digital Sheath | A serous membrane surrounding the tendons of the fingers, allowing for smooth movement. | | Anatomical Snuffbox | A triangular depression on the posterolateral aspect of the wrist, formed by the tendons of the abductor pollicis longus, extensor pollicis brevis, and extensor pollicis longus. |