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The forearm and wrist are integral to the fine skilled movements of the fingers and thumb. The wrist is a complex joint that moves in all axes and is manipulated by several muscles in the forearm, which control the pivoting of the radius around the ulna. These muscles include the supinator, pronator teres, and pronator quadratus, which work together to twist the forearm into the palm-up or palm-down positions. The supinator muscle is located just below the elbow and provides about half the power of the biceps muscle for supination. The pronator teres and pronator quadratus muscles work together to achieve pronation by pulling on the radius bone of the forearm. The wrist also contains several flexor and extensor muscles that allow for flexion and extension of the hand and fingers.
| Characteristics | Values |
|---|---|
| Muscles responsible for twisting the wrist | Pronator teres, pronator quadratus, supinator, biceps brachii |
| Location of the supinator muscle | Just below the elbow |
| Function of the supinator muscle | Provides half the power of the biceps muscle for supination |
| Function of the pronator quadratus muscle | Allows us to twist our forearm into the palm-down position (pronation) |
| Function of the pronator teres muscle | Allows us to twist our forearm into the palm-down position (pronation) |
| Function of the biceps brachii muscle | Makes all supination movements against resistance |
| Function of the flexor carpi radialis muscle | Abducts the hand toward the thumb side |
| Function of the flexor carpi ulnaris muscle | Adducts the hand toward the little finger side |
| Function of the palmaris longus muscle | Contracts to tense the palmar fascia |
| Function of the flexor digitorum superficialis muscle | Finger flexor |
| Function of the flexor pollicis longus muscle | Flexes the thumb |
| Function of the flexor digitorum profundus muscle | Flexes the fingers |
| Function of the abductor digiti minimi muscle | Abducts the little finger |
| Function of the flexor digiti minimi muscle | Flexes the little finger |
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What You'll Learn
Forearm pronation and supination
The forearm is the part of the upper limb below the elbow joint. It has two bones: the radius and ulna. The forearm is in a neutral position when the thumb is up and the small finger is towards the ground. Pronation and supination are unique movements that are only possible in the forearms and hands, allowing the human body to flip the palm either face up or face down. These movements help us perform various daily activities, such as turning a screwdriver or a key.
Pronation is the movement of the hands that turns the palms posteriorly or inferiorly, depending on the position of the arm. In the anatomical position, with the arms extended to the sides of the trunk and palms facing forward, the arms are already in the supinated position. During pronation, the distal end of the radius rotates almost 180 degrees around the ulna, turning the hand, wrist, and forearm so that the palm faces down. This movement is produced by the forearm muscles pronator teres, pronator quadratus, and flexor carpi radialis. The pronator quadratus is found in the forearm just below the wrist, and it works together with the pronator teres to allow us to twist our forearm into the palm-down position.
Supination is the opposite movement of pronation, where the palms are turned anteriorly or superiorly to the supine (face-up) position. This movement is achieved by a combination of the supinator muscle in the forearm and the biceps brachii in the upper arm. These muscles rotate the radius in the opposite direction of the pronator muscles, moving the distal end of the radius back to its position on the lateral side of the wrist. The supinator provides about half the power of the biceps muscle for supination. The biceps brachii acts primarily as an elbow flexor and secondarily as a supinator. It is able to supinate when the elbow is flexed.
The biceps brachii, brachialis, and brachioradialis flex the elbow. The biceps brachii is also a powerful supinator when the limb is pronated. The brachialis and brachioradialis are large, deep muscles in the front of the arm that help the biceps brachii in bending the elbow. The biceps brachii is prone to injury, especially the tendon of the long head and the distal tendon which inserts into the radius.
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Palmaris longus
The palmaris longus muscle is a variable muscle, and its absence is a common congenital condition. Studies have shown that between 5.5 and 24% of Caucasian populations and 4.6 to 26.6% of Asian populations may lack the palmaris longus tendon. The evolutionary interpretation of this variation suggests that humans inherited the muscle through common descent, and while some primates like orangutans still actively use it, others like chimpanzees and gorillas demonstrate the same variability.
The main function of the palmaris longus appears to be as an anchor for the skin and fascia of the hand, providing stability and resistance to horizontal shearing forces. It also works synergistically with the long flexors of the forearm to bring about flexion at the wrist joint and small joints of the hand. Additionally, it may contribute to thumb abduction movements, which are necessary for opening the hand.
The Schaeffer test is used to detect the presence of the palmaris longus tendon by asking the patient to touch the pads of the thumb and little finger together while flexing the wrist. The congenital absence of the muscle does not affect grip or pinch strength, and it has no significant impact on wrist flexion.
The palmaris longus tendon has been used in various medical procedures, such as the correction of claw-finger deformities, opponensplasty, and radial palsy. Its attributes, including its length, parallel tendon fibres, and bulkiness, make it suitable for these procedures. The muscle can be easily dissected and spared without causing significant deficits.
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Flexor carpi ulnaris
The flexor carpi ulnaris (FCU) is a muscle in the forearm that flexes and adducts at the wrist joint. It is the most medial flexor muscle in the superficial compartment of the forearm. The FCU is the most medial of the tendons that are visible superficially in this region. It is also the most medial of the superficial flexors.
The FCU originates with two heads, a smaller humeral head and a larger ulnar head, which are linked by a tendinous arch. The humeral head originates from the medial epicondyle of the humerus via the common flexor tendon. The ulnar head originates from the olecranon and upper two-thirds of the dorsal/posterior border of the ulna. The ulnar nerve and ulnar artery pass between the two heads. The FCU inserts onto the pisiform, hook of the hamate, and the base of the fifth metacarpal.
The FCU flexes and adducts the hand at the wrist joint. It can adduct and flex the wrist simultaneously. The FCU contracts with the flexor carpi radialis and palmaris longus to produce flexion of the hand at the wrist joint. When it contracts with the extensor carpi ulnaris, their counteracting forces produce adduction of the hand at the wrist, also known as ulnar deviation or ulnar flexion.
The FCU is innervated by the ulnar nerve, with spinal nerves C8 and T1. It is supplied by the ulnar collateral arteries, along with the anterior and posterior ulnar recurrent arteries.
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Flexor digitorum superficialis
The flexor digitorum superficialis is an extrinsic flexor muscle of the fingers at the proximal interphalangeal joints. It is located in the anterior or volar compartment of the forearm. The muscle has two heads: the humeroulnar and the radial. The median nerve and ulnar artery pass between these heads.
The flexor digitorum superficialis is sometimes considered to be the deepest part of the superficial layer of the anterior compartment. However, it is also sometimes considered to be a distinct "intermediate layer". Four long tendons extend from this muscle near the wrist and travel through the carpal tunnel formed by the flexor retinaculum. These tendons attach to the anterior margins on the bases of the intermediate phalanges of the four fingers, excluding the thumb. The primary function of the flexor digitorum superficialis is flexion of the middle phalanges of these four fingers at the proximal interphalangeal joints. However, under continued action, it also flexes the metacarpophalangeal and wrist joints.
To test the flexor digitorum superficialis, one finger is flexed at the proximal interphalangeal joint against resistance while the other three fingers are held fully extended to inactivate the flexor digitorum profundus. A tight fibrous arch of the flexor digitorum superficialis can compress the median nerve, causing anterior interosseous nerve (AIN) syndrome. It is relatively common for the flexor digitorum superficialis to be missing from the little finger, which can cause problems when diagnosing a little finger injury.
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Flexor digitorum profundus
The flexor digitorum profundus, also known as flexor digitorum communis profundus, is a muscle in the forearm that flexes the fingers. It is considered an extrinsic hand muscle because it acts on the hand while its muscle belly is located in the forearm. The muscle is named after the Latin phrase for "deep bender of the fingers".
The actions of the flexor digitorum profundus are important for establishing and maintaining a strong grip of the hand. It acts best when the wrist is extended, and it is the sole flexor of the distal interphalangeal (DIP) joint of the second to fifth digits. The muscle also acts as a flexor of the midcarpal (wrist), metacarpophalangeal, and proximal interphalangeal (PIP) joints of the index, middle, ring, and little fingers, along with the flexor digitorum superficialis.
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Frequently asked questions
The supinator muscle, located just below the elbow, is responsible for twisting the wrist into the palm-up position. The pronator teres and pronator quadratus muscles are responsible for twisting the wrist into the palm-down position.
The supinator muscle provides about half the power of the biceps muscle for supination, or twisting the forearm into the palm-up position.
The pronator teres muscle, along with the pronator quadratus muscle, allows us to twist our forearm into the palm-down position (pronation).
Other muscles in the wrist and forearm region include the flexor carpi radialis, palmaris longus, flexor carpi ulnaris, flexor digitorum superficialis, flexor digitorum profundus, and flexor pollicis longus.
