Arm muscles: Functions and tips for health

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All the muscles in the body have different functions. So, when a person wants to move their arm or do something else, one or more of the muscles engage to perform the action.

Muscles usually work in pairs and move in various ways: The basic functional movements include:

• Flexion: This occurs when two muscles move closer together, such as when moving the hand up toward the shoulder.
• Extension: This occurs when increasing the space between two body parts, such as when moving the hand down to straighten the elbow.
• Adduction: This occurs when moving a body part toward the centerline of the body. An example of adduction includes moving the right hand when it is palm side up toward the left or the center of the body.
• Abduction: This occurs when moving a body part away from the body’s centerline, such as moving the right hand when it is palm side up toward the right or away from the body.
• Pronation: This involves turning the forearm so that both the palm and forearm face downward.
• Supination: A person supinates the arm when turning the forearm so that the palm and forearm face upward.

Understanding this terminology might help a person visualize how the muscles work.

There are also terms for the location of a muscle within the body.

• Anterior muscles are toward the front of the body.
• Posterior muscles are toward the back of the body.
• Lateral muscles are toward the side of the body or away from the midline of a given area.
• Medial muscles are toward the middle of the body or midline of a given area.

Click on the body map above to explore the anatomical model.

The upper arm is the area between the shoulder and elbow joints. There are two major compartments in the upper arm, each containing muscles.

Anterior compartment

The anterior compartment of the upper arm refers to the part of the arm that faces forward (to the front of the body) when a person’s arms are by their sides. It includes the bicep muscles that sit in front of the humerus, which is the main bone of the upper arm.

There are three muscles in the anterior compartment:

• Biceps brachii: The biceps brachii, or simply bicep, is a two-headed muscle sitting on the front side of the humerus. It does not connect to the humerus itself. The two heads of the muscle begin at the front and back of the scapula (shoulder blade) and join together at the front of the elbow. The biceps allows for flexion, adduction, and abduction of the upper arm. It also allows for supination of the forearm.
• Brachialis: The brachialis lies underneath the biceps. It starts at the humerus and attaches to the ulna, one of two long bones in the forearm. It helps anchor and flex the forearm.
• Coracobrachialis: This muscle is on the upper anterior part of the arm near the shoulder. It starts in the scapula and extends to the shaft of the humerus. The coracobrachialis allows flexion at the shoulder joint and adduction in the upper arm. It also helps stabilize the humerus.

Posterior compartment

The posterior compartment refers to the part of the upper arm that faces away from the body when a person’s arms are by their sides. It includes the triceps muscle that sits behind the humerus.

• Triceps brachii: Commonly called the triceps, this muscle runs on the posterior side of the humerus. It is a large thick muscle that may look like a horseshoe and includes three heads: long, lateral, and medial. These heads extend down the bone and meet in one tendon at the forearm. The triceps muscle allows the arm to extend at the elbow. It also plays a role in shoulder stabilization.

The forearm is the lower part of the arm, from the elbow to the wrist.

Similar to the upper arm, the forearm contains an anterior and posterior compartment. There are more muscles in the forearm, and anatomists further organize muscles in each compartment into layers.

Anterior compartment

Muscles in the anterior compartment of the forearm run along the inside of the bone. Anatomists can further divide them into three layers based on the depth of the muscles.

Superficial layer

All muscles in the superficial layer originate from the front side of the humerus, just above the elbow joint:

• Pronator teres: This muscle helps rotate the forearm. It attaches to the middle part of the radius, the other long bone in the forearm.
• Flexor carpi radialis: This muscle flexes and abducts the hand and wrist. It attaches to the second metacarpal. Metacarpals are the five bones in the palm of the hand.
• Flexor carpi ulnaris: This muscle flexes and adducts the wrist and hand. It attaches to the fifth metacarpal.
• Palmaris longus: This is an accessory muscle that helps flex the wrist. It runs from the humerus into the hand. It may be completely absent in some people.

Intermediate layer

• Flexor digitorum superficialis: This is the largest muscle in the anterior compartment and the only muscle in this layer. It attaches to the base of the hand and allows people to flex their fingers. Some people classify this muscle as part of the superficial layer, while others classify this muscle as an independent middle, or intermediate layer.

Deep layer

• Flexor digitorum profundus: This muscle helps flex the fingers. It starts at the ulna, running through the carpal tunnel and attaching to the base of the hand. The carpal tunnel is a narrow passageway in the wrist.
• Flexor pollicis longus: This muscle helps flex the thumb. It starts on the front of the radius and attaches to the base of the thumb.
• Pronator quadratus: This muscle allows for pronation of the forearm. It starts on the ulna and runs to the outer side of the radius. Its structure helps bind the two bones together.

Posterior compartment

The posterior compartment muscles run on top of the forearm, and most function to allow the wrist and fingers to extend.

Superficial layer

• Brachioradialis: This muscle allows for flexing of the forearm at the elbow. It attaches to the wrist.
• Extensor carpi radialis longus: This muscle helps extend and abduct the hand and wrist. It attaches to bones in the hand.
• Extensor carpi radialis brevis: This muscle helps extend and abduct the hand and wrist. It attaches to bones in the hand.
• Extensor digitorum: This muscle allows the outer fingers to extend. It splits and attaches to the bones at the base of the fingers.
• Extensor carpi ulnaris: This muscle extends and adducts the hand at the wrist joint. It attaches at the base of the fifth finger.
• Extensor digiti minimi: This muscle allows for extension of the outer fingers. It attaches at the base of the fifth finger.
• Anconeus: The anconeus is a small muscle that some people consider to be an extension of the triceps and, therefore, an upper arm muscle rather than a forearm muscle. It helps extend the elbow joint and keep the elbow stable.

Deep layer

• Supinator: This muscle allows the forearm to supinate. It starts on the humerus and wraps around the arm to connect to the radius.
• Adductor pollicis longus: This muscle abducts the thumb. It begins in the ulna and attaches to the first finger and wrist.
• Extensor pollicis longus: This muscle extends the thumb. It begins in the ulna and attaches to the phalanx bone of the thumb.
• Extensor pollicis brevis: This muscle begins in the radius and attaches to and extends the thumb.
• Extensor indicis: This muscle begins in the ulna and attaches to and extends the index finger.

Injuries and other conditions may affect arm muscles. These can include:

• Strains: Strained muscles refer to muscles that a person has overworked or stretched too far. Strains may occur due to overuse or injury in the area.
• Tears: People may view tears as more severe versions of sprains and may occur due to injury that tears muscle tissue.
• Nerve compression: Compression in the area due to pressure from other structures in the arm may cause a pinched nerve, which can cause a range of symptoms, such as pain and tingling.
• Injuries in other areas: Injuries in areas connected to the arm, such as the shoulder, may cause pain to spread throughout the arm.

It can be challenging for doctors to identify symptoms of conditions that may affect arm muscles, as other issues may be causing similar symptoms. Common symptoms may include:

• pain
• swelling
• cramping
• weakness in the muscle
• limited range of motion
• stiffness
• muscle spasms

Keeping the arm muscles healthy may help to prevent possible injuries. General tips for muscle health can include:

• warming up slowly with gentle stretches
• regularly exercising the muscles to keep them strong
• eating a balanced diet, which provides the muscles with the nutrients they need to rebuild and stay strong
• maintaining hydration to allow the muscles to work optimally
• frequently resting during and after activity to allow the muscles to recover

The arms contain many muscles, each serving a function in the arm. This complex system works together to help the arm carry out the most basic actions.

Several issues, such as injury or overuse, may cause the muscles to stop working properly. Regular exercise and healthy lifestyle patterns may help prevent damage to the muscles and keep them strong.