The muscles of the arm and hand

The muscles of the arm and hand are specifically designed to meet the body’s diverse needs of strength, speed, and precision while completing many complex daily tasks. Activities such as lifting weights or heavy boxes require brute strength from the muscles of the arm. Writing, painting, and typing all require speed and precision from the same muscles. Complete athletic activities such as boxing or throwing a ball require arm and hand muscles to be strong, fast, and precise all at the same time.

The muscles of the upper arm are responsible for the flexion and extension of the forearm at the elbow joint. Flexion of the forearm is achieved by a group of three muscles – the brachialis, biceps brachii, and brachioradialis. These flexor muscles are all located on the anterior side of the upper arm and extend from the humerus and scapula to the ulna and radius of the forearm. Additionally, the biceps brachii operates as a supinator of the forearm by rotating the radius and moving the palm of the hand anteriorly. On the posterior side of the upper arm is the triceps brachii, which acts as an extensor of the forearm at the elbow and the humerus at the shoulder. The triceps brachii, as its name indicates, has three heads whose origins are on the scapula and humerus. These three heads merge to insert on the olecranon of the ulna.

Most of the muscles that move the wrist, hand, and fingers are located in the forearm. These thin, strap-like muscles extend from the humerus, ulna and radius and insert into the carpals, metacarpals, and phalanges via long tendons. The muscles on the anterior side of the forearm, such as the flexor carpi radialis and flexor digitorum superficialis, form the flexor group that flexes the hand at the wrist and each of the phalanges. The tendons of these muscles pass through a small corridor in the wrist known as the carpal tunnel. Inflammation of this region caused by repetitive stress or trauma may lead to pain and numbness known as carpal tunnel syndrome.

On the posterior side of the arm the extensor muscles, such as the extensor carpi ulnaris and extensor digitorum, act as antagonists to the flexor muscles by extending the hand and fingers. The extensor muscles run as long, thin straps from the humerus to the metacarpals and phalanges. The extensors are generally somewhat weaker than the flexor muscles that they work against, due to the relative ease in opening a hand compared to gripping something firmly.

Two special motions produced by the muscles of the forearm are the supination (anterior rotation) and pronation (posterior rotation) of the forearm and hand. Supination is produced by the biceps brachii of the upper arm and the supinator muscle of the forearm. Pronation is likewise produced by the pronator teres of the forearm. Both supinator and pronator teres muscles have their origins on the humerus and ulna and insert on opposite sides of the radius to roll the wrist in opposite directions.


The abductor hallucis muscle is one of the hallux muscles. This one serves to move the big toe away from the other toes. The hallux (plural, hallucis) are muscles pertaining to the big toe.



The anconeus muscle is a small, triangular muscle that lies on the elbow joint and appears to be a continuation of the triceps brachii. It assists in extending the forearm and is supplied by a branch of the radial nerve.



The long head of the biceps brachii muscle is the larger of the two muscle bodies that forms the entire biceps brachii muscle. The biceps brachii gets its name from the Latin words for “two-headed” and “arm” which describe its structure and location. The long and short heads of the biceps brachii work together to achieve the same functions.

The long head extends from its origin on the superglenoid tubercle of the scapula and passes over the head of the humerus before merging with the short head. From the merger point, the entire muscle continues beyond the distal end of the humerus and inserts on the radial tuberosity of the radius.

Together with the short head, the long head of the biceps brachii acts as a flexor of the arm at the elbow joint and a supinator of the forearm. The biceps brachii, brachialis, and brachioradialis muscles all act as flexors of the arm at the elbow, with the brachialis acting as the agonist and the biceps brachii and brachioradialis acting as synergists. At the radioulnar joint in the forearm, the biceps brachii acts as a supinator to turn the palm of the hand upwards.



The short head of the biceps brachii is the shorter and medial of the two bodies that form the biceps brachii muscle in the upper arm. Like the long head of the biceps brachii, the short head is a flexor and supinator of the elbow joint. At the shoulder joint, the short head aids in adduction of the humerus.


The biceps brachii muscle gets its name from its two origins, or immovable ends. The long head arises from the supraglenoid tubercle of the scapula, while the short head arises from the coracoid process of the scapula. From its origin, the short head passes anterior to the head and shaft of the humerus and fuses with the long head around the middle of the humerus. The fused biceps brachii muscle crosses the elbow joint along its anterior surface and inserts on the radius at the radial tuberosity.


The short head of the biceps brachii acts upon the bones of the upper limb across both the elbow and shoulder joints. Together with the long head of the biceps brachii, the short head flexes and supinates the forearm at the elbow. The biceps is often incorrectly thought of as the prime flexor at the elbow, when in actuality it is a synergist to the true prime flexor, the brachialis muscle. The biceps does act as the prime supinator of the elbow and is assisted by the supinator muscle of the forearm.

At the shoulder joint, the biceps brachii provides some help to the deltoid to flex the humerus or move it anteriorly. The short head also provides some unique functions that are not provided by the long head. It acts as an adductor to move the humerus toward the body’s midline and pull the arm closer to the trunk. The short head also acts as a fixator to stabilize the shoulder joint.


The biceps brachii is a skeletal muscle, and as such is an organ made mostly of skeletal muscle and connective tissues. Skeletal muscle tissue is made of many elongated cells known as fibers; each fiber is wrapped in a thin fibrous connective tissue sheath known as endomysium. Many fibers are further bundled into groups known as fascicles, which are in turn wrapped with more fibrous connective tissue known as perimysium. Many blood vessels and nerves pass between the fascicles to provide blood flow and communication to the skeletal muscle fibers. The fascicles, nerves, and blood vessels are bundled yet again to form the whole biceps muscle, wrapped in an outer layer of fibrous connective tissue known as epimysium. All of the layers of connective tissue converge at the ends of the biceps brachii to form the tendons that bind it to the scapula and radius. At the proximal end of the short head, the tendon merges with the periosteum of the scapula at the coracoid process to form the origin of the short head. On the opposite end of the biceps, the distal tendon merges with the periosteum of the radius at the radial tuberosity to form the insertion of the biceps.



The brachialis muscle is a large muscle beneath the biceps brachii. It connects the shaft of the humerus (upper arm bone) to the ulna (longest forearm bone) and is the strongest flexor of the elbow.



The brachioradalis muscle connects the humerus (upper arm bone) to the radius (shortest lower arm bone) and aids in flexing the elbow.



The coracobrachialis muscle originates from the coracoid process of the scapula, inserts on the middle, inner border of the humerus and functions to raise and adduct arm.



The deltoid muscle is a rounded, triangular muscle located on the uppermost part of the arm and the top of the shoulder. It is named after the Greek letter delta, which is shaped like an equilateral triangle. The deltoid is attached by tendons to the skeleton at the clavicle (collarbone), scapula (shoulder blade), and humerus (upper arm bone). The deltoid is widest at the top of the shoulder and narrows to its apex as it travels down the arm. Contraction of the deltoid muscle results in a wide range of movement of the arm at the shoulder due to its location and the wide separation of its muscle fibers.

The deltoid has three origins: the lateral end of the clavicle, the acromion of the scapula at the top of the shoulder, and the spine of the scapula. Each origin gives rise to its own band of muscle fibers with the anterior band forming at the clavicle, the lateral fibers forming at the acromion, and the posterior fibers forming at the spine of the scapula. The bands merge together as they approach the insertion point on the deltoid tuberosity of the humerus.

The deltoid has three distinct functions that correspond to the three bands of muscle fibers. Contraction of the anterior fibers flexes and medially rotates the arm by pulling the humerus towards the clavicle. Flexion and medial rotation of the arm moves the arm anteriorly, as in reaching forward or throwing a ball underhand. The lateral fibers abduct the arm by pulling the humerus toward the acromion. Abduction of the arm results in the arm moving away from the body, as in reaching out to the side. Contraction of the posterior fibers extends and laterally rotates the arm by pulling the humerus toward the spine of the scapula. Extension and lateral rotation moves the arm posteriorly, as in reaching backwards or winding up to throw a ball underhand.



The digital fibrous sheaths are compartments through which the tendons of the extensor muscles pass to the wrist and fingers. The retinaculum gives off branches of connective tissue to the underlying wrist bones, creating a series of sheath-like compartments. As in the case of the wrist, the tissue formations in various regions of the ankle are thickened to form retinacula. Toward the front, for example, extensor retinacula connect the tibia and fibula (lower leg bones) as well as the calcaneus (Achilles tendon) and tissue of the sole of the foot. These retinacula form sheaths for tendons crossing the front of the ankle.


The extensor carpi radialis brevis is a companion of the extensor carpi radialis longus and extends toward the middle of it. This muscle runs from the humerus (upper arm bone) to the bones in the hand and functions to extend the wrist. It also assists in abducting the hand.



The extensor carpi radialis brevis tendon is a long aponeurosis from the center of the forearm that inserts the extensor carpi radialis brevis muscle into the base of the 3rd metarcarpal bone. The extensor carpi radialis brevis muscle is a companion of the extensor carpi radialis longus and extends toward the middle of it. This muscle runs from the humerus (upper arm bone) to the bones in the hand and functions to extend the wrist. It also assists in abducting the hand.


The extensor carpi radialis longus muscle runs along the lateral side of the forearm, connecting the humerus (upper arm bone) to the hand. It functions to extend the wrist and assists in abducting the hand.



The extensor carpi ulnaris muscle is located along the outer surface of the ulna (longer forearm bone) and connects the humerus (upper arm bone) to the hand. It acts to extend the wrist and assists in abducting it.



The extensor digiti minimi muscle is a long, thin muscle of the forearm that arises from the humerus bone and inserts with the little finger\'s extensor expansion. It serves to straighten the little or pinkie finger. Muscle extension occurs when the angle between the bones is increased - as when the forearm is aligned with the arm. An extensor, then, is any muscle serving to extend a bodily part (usually a limb) away from the body or when a digit is extended from its base.



The extensor retinaculum of the hand consists of a group of heavy connective fibers in the tissues of the wrist. It connects the lateral margin of the radius (shorter forearm bone) with the inside border of the ulna (longer forearm bone) and with certain bones of the wrist. The retinaculum gives off branches of connective tissue to the underlying wrist bones, creating a series of sheath-like compartments through which the tendons of the extensor muscles pass to the wrist and fingers.



The flexor carpi radialis muscle is an extrinsic muscle of the hand found on the anterior forearm. It arises from the medial epicondyle at the elbow to insert on the second metacarpal bone, on the anterior base. It serves to flex the wrist (move it toward the anterior forearm). Generally, a flexor muscle is one that decreases the angle between two bones, as in bending the arm at the elbow; raising the leg toward the stomach as in kicking a football; or bringing the lower leg up toward the thigh. Another flexor muscle will bring the thumb across the palm of the hand.



The flexor carpi ulnaris muscle is an extrinsic muscle of the hand found on the ulnar side of the forearm. It arises from the medial epicondyle, medial olecranon, and proximal ulnar head at the elbow to insert on several of the carpal bones. It serves to flex the wrist (move it toward the anterior forearm) and adduct the wrist. Generally, a flexor muscle is one that decreases the angle between two bones, as in bending the arm at the elbow; raising the leg toward the stomach as in kicking a football; or bringing the lower leg up toward the thigh. Another flexor muscle will bring the thumb across the palm of the hand.



The flexor digiti minimi brevis muscle of the hand is a short muscle arising from the hamate bone of the wrist and inserting on the phalanx of the little finger, serving to flex that finger. A flexor muscle is one that decreases the angle between two bones, as in bending the arm at the elbow; raising the leg toward the stomach as in kicking a football; or bringing the lower leg up toward the thigh.



The flexor digitorum profundis muscle is a large muscle that connects the ulna (longest forearm bone) to the bones of the fingers. It acts to flex the distal joints of the fingers, as when a fist is made.



The flexor digitorum superficialis muscle is a large muscle that runs along the bottom of the forearm from all the bones at the elbow to the four fingers. It serves to flex or curl the fingers. A flexor muscle is one that decreases the angle between two bones, as in bending the arm at the elbow; raising the leg toward the stomach as in kicking a football; or bringing the lower leg up toward the thigh.



The flexor pollicis brevis muscle is a short muscle that runs from the wrist to the proximal phalanx of the thumb to bend the thumb toward the palm. A flexor muscle is one that decreases the angle between two bones, as in bending the arm at the elbow; raising the leg toward the stomach as in kicking a football; or bringing the lower leg up toward the thigh.



The flexor retinaculum in the wrist and palm of the hand consists of a group of heavy connective fibers in the tissues of the wrist. It connects the lateral margin of the radius (shorter forearm bone) with the inside border of the ulna (longer forearm bone) and with certain bones of the wrist. The retinaculum gives off branches of connective tissue to the underlying wrist bones, creating a series of sheath-like compartments through which the tendons of the flexor muscles pass to the wrist and fingers.



The lumbrical muscles of the hand are four small, fleshy muscles that are associated with tendons between the four fingers of the hand. Each lumbrical muscle flexes the corresponding digital joint to extend the finger in question.



The opponens pollicis muscle flexes and adducts the thumb.



The palmar interosseous muscles are adductors of the digits in the hand.



The palmaris longus muscle is one of the most variable muscles in the body and is sometimes (10% of the time) absent. It may be mostly tendon near the point of attachment and muscular at the far end, or it may be muscular in the center with tendons above and below. It may have two bundles of muscle with a central tendon, or it may be made up of a tendinous band, or the muscle make-up may be doubled. Its function is to flex the hand at the wrist.



The pronator quadratus muscle runs from the far end of the ulna (longest forearm bone) to the far end of the radius (shortest forearm bone). It assists the pronator teres in rotating the arm toward the inside, as when the hand is turned so the palm is facing downward.



The pronator teres muscle is a short muscle connecting the ends of the humerus (upper arm bone) and ulna to the radius (forearm bones). It functions to rotate the arm toward the inside, as when the hand is turned so the palm is facing downward (pronation).



The supinator muscle is a short muscle whose fibers run from the ulna (long forearm bone) and the lateral end of the humerus (upper arm bone) to the radius (short forearm bone). It assists the biceps brachii in rotating the forearm laterally (supination).



The lateral head of the triceps brachii muscle is a muscle of the back of the arm, originating from the back of the humeral shaft and inserting at the elbow. The triceps brachii has three heads (connective immovable muscle) and is the only muscle on the back of the upper arm. It connects the humerus (upper arm bone) and the scapula (shoulder blade) to the ulna (longest of the forearm bones) and is the primary extensor of the elbow. The three heads are the lateral, the medial, and the long head.


The long head of the triceps brachii muscle is a muscle of the back of the arm, originating from the scapula and shoulder to insert at the elbow. The triceps brachii has three heads (connective immovable muscle) and is the only muscle on the back of the upper arm. It connects the humerus (upper arm bone) and the scapula (shoulder blade) to the ulna (longest of the forearm bones) and is the primary extensor of the elbow. The three heads are the lateral, the medial, and the long head. The long head of the triceps brachii muscle, apart from the other triceps muscles, has a role in stabilizing the shoulder joint.


The medial head of the triceps brachii muscle is one head of the triceps muscle and originates from the back of the humerus. The triceps brachii has three such heads (connective immovable muscle) and is the only muscle on the back of the upper arm. It connects the humerus (upper arm bone) and the scapula (shoulder blade) to the ulna (longest of the forearm bones) and is the primary extensor of the elbow. The three heads are the lateral, the medial, and the long head.