The forearm is the section of your arm between the elbow and the wrist. It contains two bones, over a dozen muscles, three major nerves, and two primary arteries, all working together to give your hand and wrist their remarkable range of motion. In adults, the forearm typically measures between 20 and 31 cm in length, averaging about 25 cm in men and 23.5 cm in women.
The Two Bones: Radius and Ulna
Two long bones run side by side through the forearm. The ulna sits on the pinky side and is the more stable of the two, forming the main hinge joint at the elbow. The radius sits on the thumb side and is the bone that does most of the work when you rotate your hand.
At the elbow, the ulna locks into the bottom of the upper arm bone like a wrench gripping a bolt. This hinge is what allows you to bend and straighten your arm. The radius, meanwhile, has a cylindrical head that spins in place at the elbow, held in position by a ring-shaped ligament. At the wrist, the arrangement reverses: the radius is the main connection to the wrist bones, while the ulna barely touches them at all, separated by a small disc of cartilage.
A tough sheet of connective tissue called the interosseous membrane stretches between the radius and ulna along their entire length. This membrane keeps the two bones properly spaced, transmits forces between them, and provides attachment points for several forearm muscles.
How the Forearm Rotates
One of the forearm’s most distinctive abilities is rotation. When you turn a doorknob or flip your palm face-up, the radius physically crosses over the ulna. This movement has two directions: pronation (turning the palm down) and supination (turning it up).
Your body uses a tiered recruitment strategy for these rotations. For light tasks like turning a key, smaller muscles close to the bones handle the work. A compact muscle near the wrist called the pronator quadratus manages most everyday pronation, while a muscle wrapped around the upper radius called the supinator handles casual palm-up turns. When you need more force, like twisting a stubborn jar lid, larger muscles kick in. The pronator teres, which runs diagonally across the upper forearm, adds power to pronation, and the biceps in your upper arm becomes a major supination force. This layered approach keeps the forearm efficient for light tasks while still capable of generating strong torque when needed.
Muscle Compartments
The forearm’s muscles are organized into two main compartments, separated by a dense layer of tissue called the antebrachial fascia. This fascia wraps tightly around the forearm and creates distinct compartments that isolate groups of muscles along with their nerve and blood supply.
The Flexor Compartment (Front)
The muscles on the palm side of your forearm are arranged in three layers: superficial, intermediate, and deep. These muscles primarily bend the wrist and curl the fingers. The superficial layer includes muscles that flex the wrist and help with pronation. Deeper down, the intermediate and deep layers control individual finger movements. When you grip something, type, or make a fist, these are the muscles doing the heavy lifting. Many of them originate near the inner point of the elbow, which is why gripping activities can sometimes cause pain there.
The Extensor Compartment (Back)
The muscles on the back of the forearm straighten the wrist and open the fingers. They also include specialized muscles for the thumb and index finger, giving those digits independent extension. Several of these muscles originate near the outer point of the elbow, a spot familiar to anyone who has experienced “tennis elbow.” Deep muscles in this compartment also assist with supination and control fine thumb positioning, which is critical for tasks like pinching and gripping tools.
Nerves and Blood Supply
Three major nerves travel through the forearm, each responsible for different muscles and areas of sensation.
- Median nerve: Passes between the two heads of a forearm muscle just below the elbow and controls most of the flexor muscles on the front of the forearm. It continues into the hand through the carpal tunnel.
- Ulnar nerve: Enters the forearm through the cubital tunnel at the inner elbow (the spot you hit when you bump your “funny bone”). It controls the muscle that bends the wrist toward the pinky side and part of the deep finger-flexing muscle. About 5 cm above the wrist, it sends off branches that provide sensation to parts of the hand.
- Radial nerve: Enters the forearm on the outer side and splits into two branches. One provides sensation, while the other dives deep to power all the extensor muscles on the back of the forearm.
Blood reaches the forearm through two main arteries. The brachial artery in the upper arm splits just below the elbow crease into the radial artery (running along the thumb side) and the ulnar artery (running along the pinky side). The radial artery is the one you can feel pulsing at the wrist when checking your pulse. These arteries branch extensively to supply every muscle and bone in the forearm.
Surface Landmarks You Can Feel
Several forearm structures are easy to identify through the skin. At the elbow, the bony point at the back is the tip of the ulna. At the wrist, you can feel two bony bumps: the radial styloid on the thumb side and the ulnar styloid on the pinky side. With your palm facing up, you can often see tendons pop up when you make a fist and flex your wrist. On the back of the wrist, a small bony ridge called Lister’s tubercle is palpable in most people and serves as an important reference point for clinicians. Fully spreading the thumb makes the tendons of the thumb muscles visible along the outer wrist.
Common Forearm Injuries
Fractures are the most frequent serious forearm injury. Breaks near the wrist end of the radius are especially common, often resulting from catching yourself during a fall. These fractures can range from simple cracks that heal in a cast to complex breaks requiring surgical repair.
A more dangerous complication of forearm fractures is compartment syndrome. Because the forearm’s muscles are tightly wrapped in fascia, swelling from a fracture or crush injury can build pressure inside these sealed compartments, cutting off blood flow to the muscles and nerves. This is a surgical emergency. Symptoms include pain that seems out of proportion to the injury, pain that worsens with passive finger stretching, and tightness or swelling in the forearm.
Overuse injuries are also common. Repetitive gripping, typing, or wrist movements can inflame the tendons where they attach near the elbow or where they pass through tight tunnels at the wrist. These conditions develop gradually and tend to respond to rest, activity modification, and physical therapy.
Why the Forearm Is Built the Way It Is
The human forearm reflects millions of years of evolutionary change. As early humans shifted from moving on all fours and climbing trees to walking upright, the upper limbs were freed from weight-bearing duties. This allowed the forearm and hand to specialize for manipulation rather than locomotion. Over time, muscles that were once useful for climbing became less necessary, while the capacity for precise rotation and fine finger control became more valuable.
One key adaptation is that many of the muscles controlling the fingers are located in the forearm rather than in the hand itself. This keeps the hand slim and light, which improves dexterity for tasks like handling tools. The tendons run from the forearm muscles through the wrist and into the fingers like cables in a puppet, allowing powerful grips without bulky muscle tissue in the palm. The fully opposable human thumb, made possible by the unique saddle shape of the joint at its base, works in concert with these forearm-driven tendons to give humans a grip and precision unmatched in the animal kingdom.