The arms are the upper limbs of the body. They’re some of the most complex and frequently used body parts.

Each arm consists of four main parts:

Read on to learn more about the bones, muscles, nerves, and vessels of the upper arm and forearm, as well as common arm problems you may encounter.

Anatomy and function of arm bones and joints

Upper arm

The upper arm includes the shoulder as well as the area between the shoulder and elbow joint. The bones of the upper arm include the:

  • Scapula. The scapula is also called the shoulder blade. It’s a triangle-shaped flat bone that’s connected to the body by mostly muscle. It attaches the arm to the torso.
  • Clavicle. The clavicle is also called the collarbone. Like the scapula, it attaches the arm to the torso. It also helps to distribute force from the upper part of the arm to the rest of the skeleton.
  • Humerus. The humerus is a long bone in the upper arm. It’s located between the scapula and the elbow joint. Many muscles and ligaments in the arm are attached to the humerus.

The upper arm also contains several joints, including the:

  • Acromioclavicular joint. The scapula and the clavicle meet at this joint.
  • Glenohumeral joint. This is the joint where the scapula and humerus meet.
  • Sternoclavicular joint. The clavicle connects to the sternum (breastbone) at this joint.


The forearm is the area between the elbow joint and the wrist. Its two major bones are the radius and the ulna:

  • Radius. The radius is located on the side of the forearm closest to the thumb. It twists around the ulna and can change its position depending on how the hand is moved. There are many muscles attached to the radius that aid in movement of the elbow, wrist, and finger joints.
  • Ulna. The ulna runs parallel to the radius. It’s on the side of the forearm that’s closest to the pinky finger. Unlike the radius, the ulna is stationary and doesn’t twist.

Elbow joint

The elbow joint is where the humerus bone of the upper arm connects with the radius and ulna bones in the forearm.

The elbow joint is actually composed of three separate joints:

  • Ulnohumeral joint. This is where the humerus connects to the ulna.
  • Radiocapitellar joint. At this joint, the radius connects to an area of the humerus called the capitellum.
  • Proximal radioulnar joint. This joint connects the radius and ulna, allowing for rotation of the hands.

Anatomy and function of upper arm muscles

The upper arm contains two compartments, known as the anterior compartment and the posterior compartment.

Muscle movement

Before learning about the different muscles, it’s important to understand the four major types of movement they’re involved in:

  • Flexion. This movement brings two body parts closer together, such as the forearm and upper arm.
  • Extension. This movement increases the space between two body parts. An example of this is straightening the elbow.
  • Abduction. This refers to moving a body part away from the center of the body, such as lifting the arm out and away from the body.
  • Adduction. This refers to moving a body part toward the center of the body, such as bringing the arm back in so it rests along the torso.

Anterior compartment

The anterior compartment is located in front of the humerus, the main bone of the upper arms.

The muscles of the anterior compartment include:

  • Biceps brachii. Often referred to as the biceps, this muscle contains two heads that start at the front and back of the shoulder before joining together at the elbow. The end near the elbow flexes the forearm, bringing it toward the upper arm. The two heads near the shoulder help with flexion and adduction of the upper arm.
  • Brachialis. This muscle lies underneath the biceps. It acts as a bridge between the humerus and ulna, one of the main bones of the forearm. It’s involved with the flexing of the forearm.
  • Coracobrachialis. This muscle is located near the shoulder. It allows adduction of the upper arm and flexion of the shoulder. It also helps to stabilize the humerus within the shoulder joint.

Posterior compartment

The posterior compartment is located behind the humerus and consists of two muscles:

  • Triceps brachii. This muscle, usually referred to as the triceps, runs along the humerus and allows for the flexion and extension of the forearm. It also helps to stabilize the shoulder joint.
  • Anconeus. This is a small, triangular muscle that helps to extend the elbow and rotate the forearm. It’s sometimes considered to be an extension of the triceps.

Anatomy and function of forearm muscles

The forearm contains more muscles than the upper arm does. It contains both an anterior and posterior compartment, and each is further divided into layers.

Anterior compartment

The anterior compartment runs along the inside of the forearm. The muscles in this area are mostly involved with flexion of the wrist and fingers, as well as rotation of the forearm.

Superficial layer

  • Flexor carpi ulnaris. This muscle flexes and adducts the wrist.
  • Palmaris longus. This muscle helps with flexion of the wrist, though not everyone has it.
  • Flexor carpi radialis. This muscle allows for flexion of the wrist in addition to abduction of the hand and wrist.
  • Pronator teres. This muscle rotates the forearm, allowing the palm to face the body.

Intermediate layer

  • Flexor digitorum superficialis. This muscle flexes the second, third, fourth, and fifth fingers.

Deep compartment

  • Flexor digitorum profundus. This muscle also helps with flexion of the fingers. In addition, it’s involved with moving the wrist toward the body.
  • Flexor pollicis longus. This muscle flexes the thumb.
  • Pronator quadratura. Similar to the pronator teres, this muscle helps the forearm rotate.

Posterior compartment

The posterior compartment runs along the top of the forearm. The muscles within this compartment allow for extension of the wrist and fingers.

Unlike the anterior compartment, it doesn’t have an intermediate layer.

Superficial layer

  • Brachioradialis. This muscle flexes the forearm at the elbow.
  • Extensor carpi radialis longus. This muscle helps abduct and extend the hand at the wrist joint.
  • Extensor carpi radialis brevis. This muscle is the shorter, wider counterpart to the extensor carpi radialis longus.
  • Extensor digitorum. This muscle allows for the extension of the second, third, fourth, and fifth fingers.
  • Extensor carpi ulnari. This muscle adducts the wrist.

Deep layer

  • Supinator. This muscle allows the forearm to rotate outward so the palm faces up.
  • Abductor pollicis longus. This muscle abducts the thumb, moving it away from the body.
  • Extensor pollicis brevis. This muscle extends the thumb.
  • Extensor pollicis longus. This is the longer counterpart to the extensor pollicis brevis.
  • Extensor indices. This muscle extends the index finger.


Explore the interactive 3-D diagram below to learn more about the arm.

Anatomy and function of arm nerves

Brachial plexus

The brachial plexus refers to a group of nerves that serve the skin and muscles of the arm. It begins in the spine and runs down the arm.

The brachial plexus is divided into five different divisions:

  • Roots. This is the beginning of the brachial plexus. The five roots are formed from the spinal nerves C5, C6, C7, C8, and T1.
  • Trunks. Three trunks form the brachial plexus roots. These include the superior, middle, and inferior trunks. The superior trunk is a combination of the C5 and C6 roots, the middle trunk is a continuation of the C7 root, and the inferior trunk is a combination of the C8 and T1 roots.
  • Divisions. Each of the three trunks contains an anterior and posterior division, meaning there are six divisions in total.
  • Cords. The anterior and posterior divisions of the brachial plexus combine to form three cords, known as the lateral, posterior, and medial cords.
  • Branches. The branches of the brachial plexus go on to form the peripheral nerves that supply the arm.

Peripheral nerves

The peripheral nerves of the arm provide motor and sensory functions to the arm.

The six peripheral nerves of the arm include the:

  • Axillary nerve. The axillary nerve travels between the scapula and humerus. It stimulates the muscles in the shoulder area, including the deltoid, the teres minor, and part of the triceps.
  • Musculocutaneous nerve. This nerve travels in front of the humerus and stimulates the biceps, brachialis, and coracobrachialis muscles. The musculocutaneous nerve also provides sensation to the outside of the forearm.
  • Ulnar nerve. The ulnar nerve is located on the outside of the forearm. It stimulates many muscles in the hand and provides sensation to the pinky finger and part of the ring finger.
  • Radial nerve. The radial nerve travels behind the humerus and along the inside of the forearm. It stimulates the triceps muscle of the upper arm as well as muscles in the wrist and hand. It provides sensation to part of the thumb.
  • Median nerve. The median nerve travels along the inside of the arm. It stimulates most of the muscles in the forearm, wrist, and hand. It also provides sensation for part of the thumb, the forefinger, middle finger, and part of the ring finger.

Function and anatomy of arm blood vessels

Each arm contains several important veins and arteries. Veins carry blood toward the heart, while arteries transport blood from the heart to other areas of the body.

Below are some of the main veins and arteries of the arm.

Upper arm blood vessels

  • Subclavian artery. The subclavian artery supplies blood to the upper arm. It begins close to the heart and travels under the clavicle and to the shoulder.
  • Axillary artery. The axillary artery is a continuation of the subclavian artery. It can be found under the armpit and supplies blood to the shoulder area.
  • Brachial artery. The brachial artery is a continuation of the axillary artery. It travels down the upper arm and splits into the radial and ulnar artery at the elbow joint.
  • Axillary vein. The axillary vein transports blood to the heart from the area of the shoulder and armpit.
  • Cephalic and basilic veins. These veins travel upward through the upper arm. They eventually join the axillary vein.
  • Brachial veins. The brachial veins are large and run parallel to the brachial artery.
  • Radial artery. This is one of two arteries that supply blood to the forearm and hand. It travels along the inner side of the forearm.
  • Ulnar artery. The ulnar artery is the second of the two vessels supplying blood to the forearm and hand. It travels along the outside of the forearm.
  • Radial and ulnar veins. These veins are situated parallel with the radial and ulnar arteries. They join the brachial vein at the elbow joint.

Forearm blood vessels

Common arm problems

As two of the most heavily used body parts, the arms are vulnerable to a variety of health problems. Here’s a look at some of the main ones.

Nerve injuries

The nerves of the arm can be injured in a variety of ways, including stretching, pinching, or a cut. These injuries can occur slowly over time or quickly due to some sort of trauma.

While the specific symptoms of a nerve injury depend on the location and nature of the injury, general symptoms include:

  • pain, which can be at the site of the injury or anywhere along the nerve
  • a sensation of numbness or tingling in the arm or hand
  • weakness in or around the affected area

Some examples of nerve conditions of the arm include carpal tunnel syndrome and medial tunnel syndrome.


A fracture occurs when bone cracks or breaks due to an injury or trauma. Any bone in the upper arm or forearm can be fractured.

Symptoms of a fractured bone in the arm include:

  • pain or tenderness in the arm
  • swelling of the arm
  • bruising at the site of the injury
  • a limited range of arm motion

Joint problems

The joints of the upper arm and forearm, such as the shoulder and elbow, can be affected by a variety of problems. Repeated use, injuries, and inflammation can all cause joint issues.

Some general symptoms of an arm joint problem may include:

  • pain in the affected joint
  • limited range of motion or stiffness in the affected joint
  • inflammation or swelling of the affected joint

Examples of arm joint problems include arthritis, tennis elbow, and bursitis.

Vascular problems

Vascular problems in the arms are less common than they are in the legs.

When they do occur, they can be caused by a variety of conditions, including plaques on the walls of the arteries (atherosclerosis) or blocking of an artery by something like a blood clot.

Symptoms of a vascular issue affecting the arm include:

  • pain, cramping, or discomfort in the affected arm
  • a feeling of weakness in the affected arm
  • a sensation of heaviness in the affected arm