Arm Muscles

Extensors

The muscles work together in groups contracting and relaxing to produce the fine control needed for movements such as picking up an object or writing.  When the flexor muscle contract the arm bends at the elbow.  When the extensor muscles contract and the flexor muscles relax the arm straightens.  Similar pairs of opposing muscles flex and extend the wrists and fingers.  The rotator muscles work with the other muscles to turn the arm and wrist.

The extensor carpi radialis brevis is a short, wide, flattened muscle.  It arises from the humerus (upper arm bone) and narrows into a long, flat tendon about two-thirds of the way down the arm.  It lies between the extensor carpi radialis longus and the extensor digitorum along the outer surface of the radius.  This muscle extends and radially deviates the hand at the wrist joint.

The extensor carpi radialis longus is a short, flat muscle that arises from the humerus and extends down the arm.  The muscle belly narrows into a long flat tendon at the upper third of the forearm and continues down to the outer edge of the radius.  This muscle helps to extend and radially deviate hand at the wrist.  It also helps flex (bend) the elbow joint.

The extensor carpi ulnaris is a superficial muscle on the forearm.  It is a narrow, elongated, flattened muscle that extends from the humerus (upper arm bone) to the base of the fifth metacarpal bone of the little finger where it inserts via a flat, narrow tendon.  The tendon begins three-fourths of the way down the forearm.  This muscle helps extend the wrist.

The extensor digiti minimi is a small, slender muscle that originates from the humerus (upper arm bone), and lies between the extensor digitorum and the extensor carpi ulnaris.  At the wrist, the muscle develops a double tendon which inserts into the little finger.  It is the predominant tendon of the little finger.

The extensor digitorum is a wide, lateral muscle group.  It has a flat, fusiform belly that extends from the humerus (upper arm bone) and towards the lower half of the forearm and develops into four tendons that insert into the middle and distal phalanges of the fingers.  It does not insert into the thumb.  This muscle works to extend all the joints of the fingers.  It also extends the wrist.

The extensor indicis is located deep in the forearm, where it originates from the back of the ulna.  At the wrist it develops into a tendon that extends along the back of the hand with the extensor digitorum and inserts in the index finger.  It extends and adducts the index finger.

Combined with the abductor pollicis longus, the extensor pollicis brevis (thumb muscle) creates a narrow, triangular muscle form which wraps around the lower end of the radius (the bone of the forearm on the thumb side).  The extensor pollicis brevis originates from the back side of the radius and inserts in the base of the first phalanx of the thumb.  This muscle extends the thumb and continued action rotates the hand.  The combination of the abductor pollicis longus and the extensor pollicis brevis forms the oblique carpal muscle group.

The abductor pollicis longus lies next to the extensor pollicis brevis forming a narrow, triangular muscle which wraps around the lower end of the radius.  The muscle inserts in the base of the metacarpal bone of the thumb on the back surface near the palm.  This muscle abducts and extends the thumb.  It also assists in bending the hand at the wrist.

The extensor pollicis longus originates deep in the forearm from the shaft of the ulna (the forearm bone on the index finger side), crosses over the tendons of the extensor carpi radiales brevis and longus and descends along the back of the thumb and inserts in the last phalanx of the thumb.  This muscle extends the thumb and helps to extend the hand at the wrist.

The triceps brachii (three-headed muscle) lies at the upper portion of the inside of the arm.  It is the main extensor of the arm and is made up of three teardrop shaped heads: the long head, the lateral head, and the medial head.  When working with other nearby muscles it can also move the shoulder, since its upper ends are attached to the scapula.  The long head, the largest of the three heads, is attached to the scapula (shoulder blade) just below  the rounded socket of the shoulder joint, and extends almost three-fourths of  the way toward the front of the arm.  The lateral head lies on the back and side of the upper arm.  The medial head curves around the back of the humerus (upper arm bone) and is mostly covered by the long head.  The lower end is attached to the flattened end of the ulna.  The triceps brachii extend the forearm at the elbow joint.  It works with the biceps brachii to control the up and down movement of the forearm.

The anconeus is a small triangular muscle that lies just below the elbow joint and extends a fourth of the way down the forearm.  It is located on the outer back corner of the elbow.  It extends and stabilizes the elbow joint.
 

Flexors

The muscles work together in groups contracting and relaxing to produce the fine control needed for movements such as picking up an object or writing.  When the flexor muscle contract the arm bends at the elbow.  When the extensor muscles contract and the flexor muscles relax the arm straightens.  Similar pairs of opposing muscles flex and extend the wrists and fingers.  The rotator muscles work with the other muscles to turn the arm and wrist.

The flexor carpi radialis is located on the upper half of the forearm.  It originates at the humerus (upper arm bone) and extends down the arm developing into a wide, flat tendon about one-third of the way down the arm and inserts in the base of the metacarpal bones of the index and middle fingers deep in the palm.  This muscle bends and turns the hand at the wrist.

The belly of the flexor digitorum profundus lies on the forearm.  It originates from the inner side of the ulna, extends down the forearms and divides into four tendons.  These tendons insert in the base of the last (distal) phalanges of the four fingers.  A tendon does not insert in the thumb.  This muscle is used to flex (bend) the fingers, but can do so only after the flexor digitorum superficialis has contracted.  The muscle also assists in flexing the wrist.

The flexor digitorum superficialis is a large, wide muscle that lies partially deep and partially superficial on the forearm.  It originates from all three arm bones: the humerus, the ulna, and the radius.  The muscle extends down the arm narrowing into a tendon that inserts in the palmar side of the phalanges of the index, middle, ring, and little fingers.  It does not have an attachment to the thumb.  This muscle is used to bend the fingers.

The flexor pollicis longus is a deep forearm muscle.  It arises from the shaft of the radius and the interosseus membrane and extends down the arm where it narrows into a flat tendon and inserts on the palmar side of the base of the distal phalanx of the thumb.  It bends the last phalanx of the thumb.

The palmaris longus is a short, narrow muscle that arises from the humerus (upper arm bone) and converges into a long, slender, slightly flattened tendon that inserts in the flexor retinaculum of the wrist.  This muscle bends the hand at the wrist.
 

Rotators

The muscles work together in groups contracting and relaxing to produce the fine control needed for movements such as picking up an object or writing.  When the flexor muscle contract the arm bends at the elbow.  When the extensor muscles contract and the flexor muscles relax the arm straightens.  Similar pairs of opposing muscles flex and extend the wrists and fingers.  The rotator muscles work with the other muscles to turn the arm and wrist.

The pronator quadratus is a small, flat muscle with a quadrilateral form.  It arises from the ulna and extends across the front of the radius and ulna and inserts in the shaft of the radius.  This muscle helps to rotate the radius upon the ulna and thus assists in rotating the hand.

The pronator teres is a short, round, deep arm muscle.  It arises high on the humerus (upper arm bone), slightly higher than the flexor mass on the fore arm.  The muscle ends in a flat tendon which inserts in the radius.  This muscle pronates the forearm and assists in bending the elbow.

The supinator brevis is a broad cylindrical muscle that arises from the elbow joint and curves around the upper third of the radius.  This muscle works in opposition of the pronator and flexor muscles.  It helps turn the forearm and the wrist.

The supinator longus is commonly known as the brachioradialis muscle.  It originates two-thirds of the way down the humerus (upper arm bone) between the triceps and the brachialis.  The muscle begins wide and flat and twists toward the front of the arm as it descends.  It then widens and flattens again before ending in a flat tendon, which inserts on the thumb side of the radius.  Unlike most of the long tendons of the forearm, the tendon does not cross the wrist joint, but rather ends at the distal end of the radius.  This muscle bends the forearm at the elbow.  It does not assist in supination or pronation (turning the forearm).
 

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