Forearm Muscles: What to Know

Reviewed by Poonam Sachdev on October 07, 2022

Forearm Muscles Anatomy 

Many muscles make up the forearm, extending from your elbow joint to your hand. The ulna and radius bones form a rotational joint that allows your forearm to turn the palm of your hand either up or down. Two large arteries, also known as the ulna and radius, run the length of the forearm and branch into smaller arteries that service your forearm's musculature. 

The bones in your forearm are prone to being broken because people often instinctually extend their forearm trying to break a fall or protect their face, which could lead to a fracture. The muscles in your forearm that allow you to bring about different movements can be categorized as anterior and posterior. 

These are the muscles that can be found in your forearm:

The Anterior Compartment 

The anterior superficial layer contains four muscles that originate from the medial epicondyle. The pronator teres muscle attaches to the shaft of the radius and is the most medial of the muscles in this layer. Its primary action is the pronation of the forearm. The flexor carpi radialis contributes to abduction and attaches to the base of metacarpals II and III.

Connecting to the flexor retinaculum and acting to flex at the wrist, the palmaris longus allows you to wave at a friend or say goodbye to a loved one. About 15% of the population does not have this muscle, though. 

The flexor carpi ulnar is the most lateral of the muscles in the superficial layer, responsible for flexion and abduction at the wrist. It attaches the hand to the pisiform bone and base of the 5th metacarpal. This muscle allows you to move your wrist back and forth. 

The Intermediate Layer 

Only one muscle makes up the intermediate layer, which originates from the medial condyle of the humerus and the radius. The flexor digitorum superficialis (FDS) lies between the deep and superficial muscle layers and splits into four tendons that attach to the middle phalanx of a finger. At the proximal interphalangeal joints (PIPJs) and the metacarpophalangeal joints (MCPJs), the FDS flexes the fingers and contributes to wrist flexion. 

The Deep Layer 

The flexor digitorum profundus (FDP) splits into four tendons and originates at the ulna. This muscle attaches to the distal phalanx of each finger and allows flexion of the metacarpophalangeal joints and distal interphalangeal joints. Bending your ring, middle, index, and pinkie fingers is possible because of this muscle. The flexor pollicis longus extends laterally to the flexor digitorum profundus muscle.

This muscle attaches to the distal phalanx of the thumb and originates from the radius. It stretches laterally to the FDP and allows you to bend your thumb. A square-shaped muscle found in the FDL and FDP, the pronator quadratus attaches to the radius and originates from the ulna. The pronator quadratus allows you to pronate your forearm and is innervated by the median nerve. 

Posterior Compartment 

The radial nerve innervates all the muscles in this compartment; it contains more muscles than the anterior compartment and can be split into the superficial and deep layers. Eight muscles are in the superficial layer, and five are in the deep compartment. Four of the five muscles in the deep layer act on your thumb and index finger.  

The Superficial Layer 

The brachioradialis muscle attaches to the distal radius and originates from the lateral humerus. It allows you to flex your elbow and lift a glass of water to your mouth. Contributing to wrist abduction, the extensor carpi radialis longus muscle acts to extend the wrist. It attaches to the metacarpal III and originates from the lateral aspect of the humerus. The extensor digitorum splits into four tendons and connects to the distal phalanx of each of your fingers. Originating from the lateral epicondyle of the humerus, this muscle acts to extend your fingers at the PIJs and MCPJs.

The extensor digiti minimi originates from the lateral epicondyle of the humerus and acts to extend the little finger. The extensor carpi ulnaris attaches to metacarpal V and originates from the lateral epicondyle of the humerus. It contributes to wrist abduction and acts to extend at the wrist. The anconeus is situated superior to the other muscles in the superficial layer. The anconeus attaches to the olecranon and originates from the lateral epicondyle. It acts to extend at your elbow joint. 

The Deep Layer

The supinator muscle can be found in the deep layer and originates from the lateral epicondylitis of the humerus and the ulna. The deep radial nerve innervates this muscle. 

The extensor pollicis brevis allows you to make a thumbs-up signal. It attaches to the proximal phalanx of the thumb and originates from the posterior radius. Your extensor pollicis longus muscle acts to extend the thumb and is attached to the distal phalanx of the thumb. The extensor indicis attaches to the distal phalanx of your index finger and also acts to extend it; this muscle originates from the interosseous membrane and the ulna. 

Your abductor pollicis longus muscle attaches to metacarpal I and abducts the thumb. It sits between the radius and ulna and originates from the interosseous membrane. 

What Do Forearm Muscles Do?

From taking a heavy box up a flight of stairs to playing basketball, your forearm muscles are used in your daily life, and strengthening these muscles can also help increase your grip strength. A firm grip can help you in many ways in your everyday life. 

Many of the exercises you do in the gym can be improved by strengthening your forearms. Exercises like rows, kettlebell swings, deadlifts, and bicep curls may be challenging if you lack forearm strength. 

How to Strengthen Arm Muscles 

Here are some exercises you can try to strengthen your arm muscles: 

Find weighted objects like tires to carry and walk for as long as possible. Set it down and then pick it back up. Repeat this until you get tired. If you have access to a sturdy bar, try pull-ups or chin-ups. Other activities like gardening are an excellent way to keep your body in motion and strengthen your arms and hands. Pulling weeds and hammering nails can also help you achieve stronger forearms. 

If you go to the gym, grab a couple of dumbbells, and hold them out in front of you until you are exhausted. Finger curls, dumbbell reverse curls, barbell reverse bicep curls, dumbbell wrist extensions, and hammer curls are great exercises you can try to strengthen your forearms.  

Show Sources

Cleveland Clinic: “Arm Muscles.”
National Library of Medicine: “Anatomy, Shoulder and Upper Limb, Forearm Muscles.”
National Cancer Institute: “Muscles of the Upper Extremity.”
American Journal of Roentgenology: “The Forearm Anatomy of Muscle Compartments and Nerves.”

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