Arm Muscle Anatomy and Function

The muscles of the arms attach to the shoulder blade, upper arm bone (humerus), forearm bones (radius and ulna), wrist, fingers, and thumbs. These muscles control movement at the elbow, forearm, wrist, and fingers. When affected by injury or neuromuscular disorders, everyday tasks that require hand and arm use can be challenging.

Anatomical Structure and Location

There are 24 different muscles that make up each arm, and they control movement of the elbow, forearm, wrist, and fingers. Compared to the five muscles of the upper arm, the lower arm contains 19 different muscles that are divided into anterior (front of the arm) and posterior (back of the arm). They can be superficial (near the skin) or deep (underlying the superficial group).

Flexors are muscle groups that bend and move body parts, such as the arm, hand, or fingers closer to the body, while extensors are muscle groups that extend and move body parts away from the body.

Upper Arm

The five muscles of the upper arm originate from the front portion of the shoulder blade, called the scapula, or from the upper position of the humerus, the long bone that makes up the upper arm.

Muscles of the upper arm that have more than one head, or muscle segment, have a portion that originates from each location. The biceps brachii in the front of the arm has two heads, while the triceps brachii in the back of the arm has three heads. While each head originates at different locations, all heads of the same muscle group conjoin and attach to the same spot.

The biceps brachii attaches to the top of the radius, the forearm bone on the thumb side of the arm, while the brachialis, triceps brachii, and anconeus attach to the top of the ulna, the forearm bone on the pinky finger side of the arm.

The brachialis is the only muscle of the upper arm that does not connect to the forearm bones and attaches to the humerus. All of these muscles are long muscles that span the length of the upper arm, except for the anconeus, a small, short muscle only 9-10 centimeters (cm) in length that crosses at the back of the elbow.

Anterior Lower Arm (Front of the Forearm)

The superficial muscles of the front of the arm are long and span the length of the forearm. The pronator teres, flexor carpi radialis, flexor carpi ulnaris, and palmaris longus all originate from a specific portion of the lower end of the humerus called the medial epicondyle.

The pronator teres attaches to the lower end of the radius, near the wrist, while the flexor carpi radialis and flexor carpi ulnaris attach to the carpal and metacarpal bones of the wrist and hand. The palmaris longus attaches to the palmar fascia, the connective tissue in the palm of the hand. While the flexor digitorum superficialis starts as one muscle, it separates into four separate tendons that attach to the phalange bones of the index, middle, ring, and pinky fingers.

The deep muscles of the front of the arm underlie the superficial muscles. The flexor digitorum profundus originates at the top portion of the ulna, while the flexor pollicis longus originates at the middle portion of the radius. Both muscles meet up with the connective tissue that joins the radius and ulna together in the forearm.

While both muscles are long and span the length of the forearm, the flexor pollicis longus has one long tendon that attaches to the base of the thumb, whereas the flexor digitorum profundus begins as one muscle but separates into four separate tendons that attach to the phalange bones of the index, middle, ring, and pinky fingers.

The pronator quadratus is a small square-shaped muscle approximately 6 cm long that connects the lower end of the ulna horizontally to the lower end of the radius near the wrist.

Posterior Lower Arm (Back of the Forearm)

Like the muscles of the front of the arm, the superficial muscles of the back of the arm are long and span the length of the forearm. The extensor carpi radialis brevis, extensor carpi ulnaris, extensor digitorum, and extensor digiti minimi all start from a specific portion of the lower end of the humerus called the lateral epicondyle. The extensor carpi radialis longus and brachioradialis originate from the area directly above the lateral epicondyle called the supracondylar ridge.

The extensor carpi radialis longus, extensor carpi radialis brevis, and extensor carpi ulnaris all attach to the metacarpal bones of the wrist, whereas the brachioradialis attaches to the end of the radius near the wrist at a bony prominence called the radial styloid process. While the extensor digitorum begins as one muscle, it separates into four separate tendons that attach to the phalange bones of the index, middle, ring, and pinky fingers. The extensor digiti minimi attaches to just the pinky finger.

The deep muscles of the back of the arm underlie the superficial muscles. The supinator attaches to the lateral epicondyle of the humerus like most of the superficial muscles of the back of the forearm, while the abductor pollicis longus, extensor pollicis longus, extensor pollicis brevis, and extensor indicis originate at varying points along the ulna, radius, and the connective tissue that joins the radius and ulna together in the forearm.

The supinator attaches to the outside edge of the radius, while the extensor indicis, a small muscle near the end of the forearm, connects to the index finger. The abductor pollicis longus, extensor pollicis longus, and extensor pollicis brevis all attach to the thumb.

Function

The muscles of the upper arm, the area between the shoulder and elbow, primarily control movement of the elbow. The biceps brachii, coracobrachialis, and brachialis all control flexion at the elbow joint, or bending of the elbow. The brachioradialis of the lower arm also contributes to elbow flexion. The triceps brachii, along with a small contribution from the anconeus, controls the opposite motion of extension of the elbow. The biceps brachii and triceps brachii also contribute to flexion and extension of the shoulder respectively.

The muscles of the lower arm, the area between the elbow and wrist, control wrist, forearm, and finger movement. The flexor carpi radialis, flexor carpi ulnaris, and palmaris longus flex, or bend, the wrist, while their counterparts, the extensor carpi radialis longus, extensor carpi radialis brevis, and extensor carpi ulnaris, extend the wrist.

The pronator teres and pronator quadratus function to pronate the forearm, the motion of turning the forearm so that the palm faces the ground, while the supinator functions to supinate the forearm, the motion of turning the forearm so that the palm faces up. The biceps brachii and brachioradialis also help supinate the forearm.

The flexor digitorum superficialis and flexor digitorum profundus control bending of the fingers, while the extensor digitorum controls extending of the fingers. The index and pinky fingers also receive additional input from the extensor indicis, which extends the index finger, and the extensor digiti minimi, which extends the pinky finger.

The thumb has its own set of muscles that includes the flexor pollicis longus, which bends the thumb, the extensor pollicis longus and extensor pollicis brevis, which extend the thumb, and the abductor pollicis longus, which abducts the thumb, the perpendicular movement of the thumb away from the palm of the hand.

Associated Conditions

Neuromuscular Disorders

Neuromuscular disorders are conditions that affect the nerves that send electrical signals to muscles to control movement. Symptoms of these disorders include muscle weakness, muscle wasting, called atrophy, muscle twitching, cramps, or spasms, muscle pain, numbness and tingling, and decreased coordination and fine motor movements.

Many neuromuscular disorders are inherited, but they also can be acquired due to causes not fully understood, including spontaneous gene mutations and immune system disorders.

Common neuromuscular disorders that can affect the muscles of the arms include:

• Muscular dystrophies
• Myopathies
• Multiple sclerosis (MS)
• Amyotrophic lateral sclerosis (ALS)
• Myasthenia gravis
• Spinal muscular atrophy (SMA)
• Peripheral neuropathies

Injury

Trauma or injury to the arms can cause various issues, including pain, weakness, and difficulty with everyday and work-related tasks.

Common injuries that affect the muscles of the arms include:

• Carpal tunnel syndrome
• Tennis elbow
• Golfer’s elbow
• Muscle tears or strains
• Tendonitis, or inflammation of a muscle’s tendon from repetitive use

Tests

Different tests are used to confirm a diagnosis of an injury or disorder of the muscles of the arm, including:

• Magnetic resonance imaging (MRI): An MRI is a scan that provides a clear image of soft tissue structures, including muscles and tendons, to check for a muscle tear or tendinitis, which is inflammation of a muscle’s tendon.
• Electromyography (EMG): EMG testing can be used to assess the electrical activity of your muscles. Electrical activity is often decreased in neuromuscular disorders.
• Nerve conduction tests: Nerve conduction tests can be used to assess how signals travel from your nerves to your muscles. The signaling may be delayed if there is nerve compression or a neuromuscular disorder.
• Muscle biopsies: Muscle biopsies involve taking a sample of muscle tissue to examine its quality.
• Genetic testing: Many neuromuscular disorders are inherited. Genetic testing can examine specific genes and mutations linked to neuromuscular disorders to determine a diagnosis or risk of developing a condition.

A Word From Verywell

There are 24 different muscles that make up the upper and lower arms. Problems can result from injury, repetitive use, or neuromuscular disorders that cause weakness of the elbow, forearm, wrist, or finger muscles. This weakness can interfere with your ability to perform daily tasks. Allowing injured or overused muscles to rest can help ease the pain and discomfort. Seeing a physical therapist can help you return to your daily activities and exercises without limitations.