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The forearm is the part of the upper limb between the elbow and the wrist joints. It contains two bones: the ulna and the radius, which provide support to local muscles supplied by an intricate network of nerves and vessels. There are twenty muscles in the forearm, divided between the anterior and posterior compartments. These muscles enable the arm to flex and extend while coordinating the movements of the wrist and hand. The anterior compartment of the forearm consists of muscles that engage in the flexion of the arms and hands, while the posterior compartment houses the extensors and supinators of the forearm. The anterior compartment can be further divided into three layers: the superficial, intermediate, and deep layers. The superficial layer contains five muscles involved in the flexion of the arm, while the deep layer contains three muscles: the flexor digitorum profundus, the flexor pollicis longus, and the pronator quadratus. Learning the anatomy of the forearm muscles can be daunting, but there are tricks to make it easier, such as using your fingers as a guide to learning the superficial layer of the anterior forearm.
| Characteristics | Values |
|---|---|
| Number of Bones in the Forearm | 2 |
| Names of Bones in the Forearm | Ulna and Radius |
| Number of Compartments in the Forearm | 2 |
| Names of Compartments in the Forearm | Anterior and Posterior |
| Number of Muscles in the Forearm | 20 |
| Muscles Involved in Forearm Movement | Pronator Teres, Pronator Quadratus, Anconeus, Brachioradialis, and Supinator |
| Muscles Involved in Flexion and Extension of the Wrist | Extensor Carpi Radialis Longus, Extensor Carpi Radialis Brevis, Flexor Digitorum Profundus, Flexor Carpi Ulnaris, Flexor Radialis Longus, and Palmaris Longus |
| Muscles Involved in Flexion, Adduction, and Extension of the Thumb | Flexor Pollicis Longus, Adductor Pollicis Longus, Extensor Pollicis Longus, and Brevis |
| Superficial Muscles in the Anterior Compartment | Pronator Teres |
| Deep Muscles in the Anterior Compartment | Flexor Digitorum Profundus, Flexor Pollicis Longus, and Pronator Quadratus |
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What You'll Learn
The anatomy of the forearm
The forearm is the part of the upper limb between the elbow and wrist joints. It contains two bones: the ulna and the radius, which provide support to the local muscles. The ulna is located medially and is both longer and larger than the radius, which runs parallel to it laterally. These two bones are held together by the intervening interosseous membrane. The forearm is divided into two muscular compartments, separated by the radius and ulna and the interosseous membrane running between them.
The anterior compartment contains flexors and pronators, and the muscles here perform flexion and pronation at the wrist, and flexion of the fingers. The anterior flexor compartment contains twenty muscles that act on the elbow and wrist joints, as well as carpometacarpal, metacarpophalangeal, and interphalangeal joints of the hand. The muscles in the anterior compartment include the flexor digitorum superficialis, which is the only muscle of the intermediate compartment, although it can sometimes be classed as a superficial muscle. The flexor digitorum superficialis has two heads, which arise from the medial epicondyle of the humerus and the shaft of the radius. The deep anterior forearm contains the flexor digitorum profundus, the flexor pollicis longus, and the pronator quadratus.
The posterior compartment houses the extensors and supinators of the forearm. The posterior extensor compartment contains three wrist extensors: the extensor carpi radialis longus, extensor carpi radialis brevis, and extensor carpi ulnaris. The forearm muscles are essential to the fine motor actions of the upper limb, allowing for complex movements of the arm, wrist, and fingers.
The intrinsic muscles function to move the forearm by pronating and supinating the radius and ulna. The extrinsic muscles flex and extend the digits of the hand. One muscle, the brachioradialis, traverses the elbow joint, running from the arm to the wrist, helping to flex the elbow.
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The anterior compartment
The forearm is the part of the upper limb between the elbow and wrist joints. It is divided into two compartments, separated by the radius and ulna and the interosseous membrane. The anterior compartment contains flexors and pronators, while the posterior compartment houses the extensors and supinators of the forearm.
The flexor digitorum superficialis is the only muscle of the intermediate compartment. It can sometimes be classified as a superficial muscle, but it usually lies between the deep and superficial muscle layers. It has two heads, with one originating from the medial epicondyle of the humerus and the other from the shaft of the radius. The muscle splits into four tendons at the wrist, which travel through the carpal tunnel and attach to the base of the middle phalanx of the four digits.
The three muscles of the deep layer of the anterior compartment are the flexor digitorum profundus, flexor pollicis longus, and pronator quadratus. The flexor digitorum profundus has two parts, medial and lateral, which attach to the proximal three-quarters of the anterior aspect of the ulna and the interosseous membrane. The medial part splits into two tendons to attach to the base of the 4th and 5th distal phalanges, while the lateral part splits into two tendons to attach to the bases of the 2nd and 3rd distal phalanges.
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The posterior compartment
The forearm is divided into two compartments, the anterior and the posterior. The posterior compartment of the forearm, also known as the extensor compartment, contains twelve muscles that are primarily responsible for extending the wrist and digits. These muscles are commonly referred to as the extensor muscles.
The muscles in the posterior compartment are organised into two layers: the deep and superficial layers, which are separated by a layer of fascia. The superficial layer contains seven muscles, four of which (extensor carpi radialis brevis, extensor digitorum, extensor carpi ulnaris, and extensor digiti minimi) share a common tendinous origin at the lateral epicondyle. The extensor digiti minimi originates from the extensor digitorum muscle, and in some people, these two muscles are fused together. The extensor carpi ulnaris is located medially in the posterior forearm, allowing it to produce adduction and extension at the wrist. The extensor carpi radialis longus has a short muscle belly and a longer tendon. The deep layer contains five muscles, all innervated by the posterior interosseous nerve.
The supinator and the anconeus are the two extensor muscles in the posterior compartment that do not pass through the wrist extensor compartments. The supinator has superficial and deep heads that help to supinate the forearm. The abductor pollicis longus contributes to thumb abduction and assists in the abduction of the wrist. The first compartment, the most radial, is occupied by the extensor pollicis brevis and the abductor pollicis longus, which inserts into the thumb. The second compartment is occupied by the two radial wrist extensors, the extensor carpi radialis longus and the extensor carpi radialis brevis. The third compartment exclusively accommodates the extensor pollicis longus, which hooks around Lister's tubercle of the radius and inserts into the thumb.
The muscles in the posterior compartment are innervated by the radial nerve and its branches. The radial nerve arises from the posterior cord of the plexus. The radial nerve innervates all muscles in the extensor compartment of the forearm. If the radial nerve is injured, these muscles become paralysed.
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Flexors and extensors
The forearm is the upper limb's section between the elbow and wrist joints. It contains two bones: the ulna and the radius, which provide support to the local muscles. The forearm is divided into two compartments: the anterior (or flexor) compartment and the posterior (or extensor) compartment.
The anterior compartment contains flexors and pronators, while the posterior compartment houses the extensors and supinators of the forearm. The flexor muscles in the forearm include the flexor digitorum superficialis, flexor digitorum profundus, flexor pollicis longus, and pronator quadratus. The flexor digitorum superficialis is the only muscle in the intermediate compartment and is a good anatomical landmark in the forearm. The flexor digitorum profundus, flexor pollicis longus, and pronator quadratus are three muscles in the deep layer of the anterior compartment.
The flexor muscles in the forearm are responsible for flexion and pronation at the wrist and flexion of the fingers. An example of forearm flexion is a bicep curl. The flexor digitorum profundus, flexor carpi ulnaris, flexor radialis longus, and palmaris longus help to flex the wrist. Wrist flexion occurs when the hand, palm down, bends in the downward direction.
The extensor muscles in the forearm include the extensor carpi radialis longus, extensor carpi radialis brevis, extensor digitorum, and extensor pollicis longus. The extensors are involved in the extension of the wrist and extension of the fingers. Wrist extension occurs when the hand, palm down, is bent in the upward direction.
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Innervation of the forearm muscles
The forearm is the part of the upper limb between the elbow and wrist joints. It contains two bones: the ulna and the radius, which provide support to the local muscles. The nerves of the forearm are ultimately responsible for innervating the muscles of the forearm. The brachial plexus carries the majority of the innervation to the forearm. However, a small amount of sensory innervation comes from the medial antebrachial cutaneous nerve.
The median nerve provides motor innervation to the pronator teres, flexor carpi radialis, palmaris longus, and flexor digitorum superficialis. The only muscle in the anterior compartment not innervated by the median nerve is the flexor digitorum profundus, which receives innervation from the ulnar nerve. The anterior interosseous nerve (AIN) branches off the median nerve and passes under the arch of the flexor digitorum profundus. AIN provides sensation to the volar wrist capsule and motor innervation to the deep flexors in the anterior compartment.
The flexor carpi radialis is a fusiform muscle of the forearm situated medially to the pronator teres muscle. It is innervated by the median nerve. When it contracts, it flexes and abducts the hand at the wrist joint. The pronator teres is the most laterally placed muscle of the superficial anterior forearm muscles. It receives its innervation via the median nerve. The contraction of the pronator teres pronates and flexes the forearm at the elbow.
The radial nerve provides motor innervation to the anconeus, brachioradialis, and extensor carpi radialis longus muscles. After entering the forearm between the brachioradialis and brachialis, the radial nerve divides into the superficial and deep branches. After the deep branch pierces the supinator muscle, it branches into the posterior interosseous nerve (PIN). The PIN provides sensation to the posterior forearm and the dorsal wrist.
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Frequently asked questions
The forearm contains twenty muscles divided between the anterior and the posterior compartments. The anterior compartment contains flexors and pronators, while the posterior compartment houses extensors and supinators. Five muscles specifically move the forearm: the pronator teres, the pronator quadratus, the anconeus, the brachioradialis, and the supinator.
Learning the muscles of the forearm can be daunting, but there are some tricks that can help. One trick is to use your fingers as a guide to learn the superficial muscles of the forearm. Each muscle roughly follows the course of digits II-V as they course down your forearm. Another tip is to break down the name of a muscle into logical components if you get stuck.
The forearm muscles enable the arm to flex and extend while coordinating the movements of the wrist and hand. For example, the flexor carpi radialis flexes and abducts the hand at the wrist joint. The supinator moves the forearm so that the forearm and palms face forward from the body.
