The Muscles of the Hand

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The hand is an intricately complex structure whose muscles have evolved to permit an unequalled array of movements. More than 30 individual muscles in the hand and forearm work together to achieve these diverse movements. These muscles provide the hands with unsurpassed flexibility, extremely precise control, and gripping strength that are necessary for activities ranging from writing and typing to producing music and gripping a ball in sports.

Six flexor muscles are found in the anterior or palmar side of the forearm. These long, thin muscles extend through the wrist via tendons to insert into the bones of the wrist, palm, and fingers. The flexor carpus radialis, flexor carpus ulnaris, and palmaris longus muscles all have their origins on the humerus of the upper arm and insert into the carpals and metacarpals on the palmar side of the hand. Working together these muscles flex the hand at the wrist. The flexor carpus radialis also abducts the hand toward the thumb side while the flexor carpus ulnaris adducts the hand toward the little finger side. The other three flexor muscles - flexor digitorum superficialis, flexor digitorum profundus, and flexor pollicis longus - extend from the bones of the arm and forearm and insert into the phalanges of the hand to flex the fingers and thumb, respectively. The tendons of the flexor muscles and the median nerve pass through a bony passage in the wrist known as the carpal tunnel. Repetitive motion of the flexor tendons can cause them to become inflamed and impinge the median nerve, leading to pain, numbness and tingling known as carpal tunnel syndrome.

Nine extensor muscles found in the posterior side of the forearm extend the hand and fingers. Just like the flexor muscles of the forearm that these muscles work against, each extensor muscle is long and thin and extends into the hand via long tendons. The extensor carpi radialis longus, extensor carpi radialis brevis, and extensor carpi ulnaris muscles all extend the hand at the wrist, with the radialis muscles abducting the hand and the ulnaris adducting it. Inserting into the phalanges of the fingers, the extensor pollicis brevis (thumb), extensor pollicis longus (thumb), extensor indicis (index finger), extensor digitorum (middle and ring fingers), and extensor digiti minimi (little finger) muscles extend the digits to open the hand. The abductor pollicis longus muscle has the dual role of both abducting the thumb and assisting with the extension of the thumb.

Several muscles in the forearm control the pivoting of the radius around the ulna that rotates the wrist and hand. The supinator muscle inserts on the radius and supinates the hand by turning the palm upwards or toward the front of the body. Working as antagonists to the supinator, the pronator teres and pronator quadratus muscles pronate the hand by turning it posteriorly or palm side down. The pronator muscles both insert on the opposite side of the radius from the supinator so that each set of muscles can rotate the radius in opposite directions.

The muscles of the hand can be broken down into three main regions: the thenar (lateral or thumb side of the palm), hypothenar (medial or little finger side of the palm) and intermediate (middle of the hand) muscles.

• The thenar muscles, which form the bulge of muscles evident at the base of the thumb, are essential to the hand's flexibility and gripping ability. One of these muscles, the opponens pollicis, moves the thumb across the hand to oppose the other fingers, allowing us to pinch a small object between the thumb and finger to pick it up. The abductor pollicis brevis and adductor pollicis work as antagonists to abduct and adduct the thumb respectively. Working with the flexor pollicis longus of the forearm, the flexor pollicis brevis flexes the thumb to grip objects or make a fist. 
• The three hypothenar muscles form a small bulge of muscles on the medial side of the palm opposite from the thenar muscles. These muscles work together to provide a wide range of motion to the little finger. The abductor digiti minimi abducts the little finger as in spreading the fingers apart, while the flexor digiti minimi flexes the little finger. The opponens digiti minimi rotates the fifth metacarpal and pulls it anteriorly during opposition with the thumb or while cupping the palm of the hand.
• Found in the middle of the palm between the metacarpals, the intermediate or midpalmar muscles collectively move the second through fifth metacarpals and the second through fifth phalanges in a variety of ways. The four lumbrical muscles, which get their names from their worm-like shapes, attach to the tendons the flexor digitorum profundus and the extensors of the phalanges to flex the base of the digits at the metacarpophalangeal joints while extending the fingers at the interphalangeal joints. Each lumbrical muscle connects to just one of the digits and causes the digit to flex at its base while remaining straight throughout its length. The four palmar interossei muscles extend from the metacarpals and insert on each of the phalanges to adduct the fingers and pull them together. Working as antagonists to these muscles are the four dorsal interossei muscles that abduct, or spread apart, the fingers.

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