Logan Steele
The forearm is the section of the upper limb between the elbow and the wrist, and it contains twenty muscles that act on the elbow and wrist joints. These muscles are divided into two compartments: the anterior (flexor) compartment and the posterior (extensor) compartment. The triceps brachii is a large muscle found in the posterior (extensor) compartment of the forearm, and it is the prime extensor of the forearm at the elbow joint. The anconeus is a small muscle located in the same compartment, and it assists the triceps brachii in elbow extension. The abductor pollicis longus, extensor pollicis longus, and extensor indicis are three muscles that originate from the ulna and extend into the dorsum of the hand.
| Characteristics | Values |
|---|---|
| Muscles | Triceps Brachii, Anconeus, Extensor Carpi Radialis, Extensor Digitorum, Extensor Digiti Minimi, Extensor Carpi Ulnaris, Abductor Pollicis Longus, Extensor Pollicis Brevis, Supinator, Brachialis, Biceps Brachii, Coracobrachialis |
| Location | Posterior Compartment of the Upper Arm, Elbow Joint, Lateral Aspect of the Posterior Forearm, Posterior Forearm, Posterior Aspect of the Elbow, Medial Aspect of the Posterior Forearm, Deep Layer of the Posterior Forearm, Hand, Anterior Compartment of the Arm |
| Function | Extension of the Forearm, Extension of the Elbow Joint, Extension and Abduction of the Wrist, Extension of the Fingers, Extension of the Little Finger, Extension and Adduction of the Wrist, Extension of the Thumb, Extension of the Elbow, Supination of the Forearm, Flexion of the Forearm, Flexion of the Arm |
| Innervation | Radial Nerve, Musculocutaneous Nerve, Median Nerve |
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Triceps Brachii
The triceps brachii is a three-headed muscle in the arm. It is the only constituent of the posterior muscle group of the arm, spanning almost the entire length of the humerus. The triceps brachii consists of a long, medial, and lateral head, which originate from their respective attachments on the humerus and scapula and insert via a common tendon on the ulna. The main function of the triceps brachii is the extension of the forearm at the elbow joint.
The triceps brachii is innervated by the radial nerve, which gives off a separate branch for each head. The C6 root value of the radial nerve innervates the lateral head, the C7 root value innervates the long head, and the C8 root value supplies the medial head. However, more recent studies have found that in around 14% of individuals, the long head of the triceps brachii is innervated by the axillary nerve, and in 3% of people, it receives dual innervation from both the radial nerve and axillary nerve. The triceps brachii muscle is mainly supplied by the deep brachial artery and the superior ulnar collateral artery, which arises from the brachial artery.
The triceps brachii can be trained in a variety of ways, including isolation and compound elbow extension movements. Isolation exercises include lying triceps extensions, behind-the-back arm extensions, cable push-downs, and standing triceps "kickbacks." Compound exercises include pressing movements like push-ups, bench presses, military presses, and tricep dips. The triceps can also be contracted statically to keep the arm straight against resistance.
The triceps brachii also plays a role in creating anatomical spaces that are traversed by neurovascular structures, making it an important surgical landmark. The upper triangular space is bounded by the teres minor and subscapularis superiorly, the teres major inferiorly, and the long head of the triceps laterally. The medial bicipital groove, formed by the space between the medial head of the triceps and the biceps brachii, provides a passageway for the brachial artery and the median and ulnar nerves.
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Anconeus
The anconeus is a small, triangular muscle located at the elbow, attaching the humerus and ulna. It is situated in the superficial region of the posterior forearm compartment, which contains five muscles. The anconeus moves the ulna during pronation and extends the forearm at the elbow. It is also believed that the anconeus helps stabilize the ulna, especially during pronation movements of the forearm.
The anconeus is considered a continuation of the triceps brachii muscle, which is the major muscle responsible for elbow extension. The anconeus and triceps brachii are often partially or completely blended together, and they are innervated by the same nerve. The anconeus is supplied by a motor branch of the radial nerve (C6-C8), which arises at the radial sulcus of the humerus. The skin over the anconeus is supplied by the T1 spinal nerve.
The anconeus originates at the dorsal side of the lateral epicondyle of the humerus and inserts at the olecranon of the ulna, along the proximal third of the posterior face of the ulna. Its tendon lies deep to the muscle belly of the extensor carpi radialis longus and is partially attached to the dorsal capsule of the humeroulnar joint. The anconeus tendon spreads out obliquely and medially into a wide muscle belly.
The anconeus can be easily palpated at the lateral side of the forearm near the elbow, especially during pronation and supination movements. Trauma to the nerve supply of the anconeus can result from a shoulder dislocation or fractures of the upper part of the humerus or around the olecranon. Any injury that damages the radial nerve can paralyze the anconeus and other extensors of the elbow and wrist.
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Biceps Brachii
The biceps brachii (BB), commonly known as the biceps, is a large, thick muscle on the front of the upper arm between the shoulder and the elbow. It is composed of a short head and a long head, with the long head located on the lateral side and the short head on the medial side. The biceps brachii is a two-headed muscle, although in some cases it can have three or more heads.
The biceps brachii works across three joints: the glenohumeral, elbow, and radio-ulnar joints. It is involved in various tasks such as lifting, sports involving throwing and racket use, and gesturing. The muscle's main functions are the flexion and supination (outward rotation) of the forearm. This is facilitated by the 90-degree rotation of the muscle as it connects to the radius. The biceps brachii flexes the forearm and works with the supinator to rotate it so that the palm faces upward.
The biceps brachii is an important landmark for locating the brachial artery during physical examinations and ultrasound-guided arterial cannulation. It receives its blood supply from the muscular branches of the brachial artery. The musculocutaneous nerve C5, C6, and C7 provide sensory and motor innervation to the biceps brachii.
Pathologic conditions affecting the biceps brachii tendons are often initially managed non-operatively. Rehabilitation focuses on restoring muscle balance, improving shoulder range of motion, and strengthening the rotator cuff. Conditions such as tendinopathy can occur due to physical trauma or repetitive activity.
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Supinator
The supinator is a broad muscle in the posterior compartment of the forearm. It is a spiral muscle that curves or wraps around the upper third of the radius, connecting it with the ulna. This muscle is innervated by the deep branch of the radial nerve, which becomes the posterior interosseous nerve upon exiting the supinator muscle. The nerve roots are primarily from C6, with some C5 involvement, and possible additional C7 innervation. The supinator is attached to the humerus and the ulna, and together, they attach to the radius.
The supinator's primary function is to supinate the forearm, or to rotate the radius laterally, putting the radius parallel to the ulna. This action brings the hand into the supine position, facing anteriorly with the palm facing upwards. The supinator always acts together with the biceps brachii muscle, except when the elbow joint is extended. When the elbow is flexed at a 90-degree angle, the most powerful supination occurs. However, when the forearm is fully extended, the biceps brachii cannot act as a supinator.
The supinator is located deep in the forearm, underneath the parts of the radius and ulna that it lies upon. It forms the floor of the cubital fossa, along with the brachialis muscle. The extensor carpi radialis brevis and longus cover the muscle's radial surface, while the inferior part of the anconeus muscle overlies it from the ulnar side.
The supinator is associated with a nerve syndrome known as supinator entrapment syndrome. This occurs when the radial nerve divides into deep and superficial branches just proximal to the supinator muscle, leading to entrapment and compression of the deep branch. This can potentially result in selective paralysis of the muscles served by the nerve, including the extensor muscles and the abductor pollicis longus.
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Extensor Carpi Radialis Muscles
The extensor carpi radialis muscle is one of the five main muscles that control movements at the wrist. It is situated on the lateral aspect of the posterior forearm, which is the section of the upper limb from the elbow to the wrist. The muscle is quite long, starting on the lateral side of the humerus and attaching to the base of the second metacarpal bone (metacarpal of the index finger).
The extensor carpi radialis muscle originates from the lateral supracondylar ridge of the humerus, from the lateral intermuscular septum, and by a few fibres from the lateral epicondyle of the humerus. The fibres end at the upper third of the forearm in a flat tendon, which runs along the lateral border of the radius, beneath the abductor pollicis longus and extensor pollicis brevis. The tendon of the extensor carpi radialis longus, in particular, can be felt in the floor of the anatomical snuffbox when wrist extension and abduction movements are performed.
The extensor carpi radialis longus is one of three primary wrist extensors. It is most effective as a wrist extensor when the elbow is extended and when radial deviation is balanced by the primary ulnar deviator- extensor carpi ulnaris. Functionally, the wrist extensors work strongly in the action of gripping, together with the extensor carpi ulnaris. By keeping the wrist in extension, flexion of the wrist by flexors digitorum superficialis and profundus is prevented, with the result that these muscles act on the fingers.
The extensor carpi radialis longus is innervated by the radial nerve, with contributions mainly from spinal nerves C5-C8. The radial nerve courses within a deep groove in the elbow, medially to the extensor carpi radialis longus muscle. The radial nerve is a branch of the brachial plexus. The extensor carpi radialis longus receives blood supply mainly from the radial artery.
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Frequently asked questions
The triceps brachii is the prime extensor of the forearm at the elbow joint.
The triceps brachii is a large, thick muscle on the dorsal part of the upper arm. It often appears in the shape of a horseshoe on the posterior aspect of the arm.
The anconeus is a small muscle located at the posterior aspect of the elbow. It assists the triceps brachii in elbow extension and stabilizes the elbow joint.
