The human arm contains three long bones: the humerus in the upper arm, and the radius and ulna in the forearm. These three bones work together across two major joints to give your arm its remarkable range of motion, from throwing a ball to turning a doorknob.
The Humerus: Your Upper Arm Bone
The humerus is the single bone that runs from your shoulder to your elbow. At the top, its rounded head fits into a shallow socket on the shoulder blade, forming a ball-and-socket joint. That socket is about four times smaller than the humeral head, which is why the shoulder is so mobile but also why it’s prone to dislocation.
Just below the head are two bony bumps called the greater and lesser tubercles. These serve as anchor points for the rotator cuff muscles that stabilize your shoulder. The narrow zone just below these bumps, called the surgical neck, is one of the most common fracture sites in the upper arm.
Partway down the shaft, a rough patch on the outer surface is where the deltoid muscle attaches, the large triangular muscle that gives your shoulder its rounded shape and lifts your arm sideways. At the bottom, the humerus widens into two bony knobs you can feel on either side of your elbow. These are the medial and lateral epicondyles. The inner one (medial) is your “funny bone,” though the tingling you feel when you bump it actually comes from the nerve running behind it, not the bone itself.
The Radius and Ulna: Your Forearm Bones
Two bones run side by side from your elbow to your wrist. When your palm faces forward, the radius sits on the thumb side (the outer edge) and the ulna sits on the pinky side (the inner edge). The ulna is the longer of the two.
The ulna is the bone you’re most aware of at the elbow. Its hook-shaped upper end wraps around the bottom of the humerus like a wrench gripping a bolt, creating the hinge that lets you bend and straighten your arm. The bony point you feel at the tip of your elbow is the top of the ulna.
The radius plays a smaller role at the elbow but becomes the dominant bone at the wrist. Its wide, flat lower end forms the main connection to the wrist bones, while the ulna tapers to a narrow knob on the pinky side.
How the Forearm Bones Enable Rotation
One of the cleverest features of the forearm is how the radius and ulna let you rotate your hand. When you turn your palm face-down (like pouring out a glass of water), the radius swings over the ulna, crossing it to form an X shape. When you turn your palm face-up, the radius swings back to sit parallel alongside the ulna. The ulna stays nearly fixed during this movement. This rotation happens at joints where the two bones meet near both the elbow and the wrist.
This is why fractures to either forearm bone can significantly limit your ability to rotate your hand, even after healing. The bones need to maintain their precise shape and alignment for the radius to glide smoothly around the ulna.
How Muscles Attach to These Bones
The three arm bones are covered in attachment sites where muscles anchor via tendons. Your biceps, the muscle on the front of your upper arm, originates at the shoulder blade and inserts into a bump on the upper radius. That’s why flexing your biceps bends your elbow and also helps rotate your forearm palm-up.
The brachialis, a lesser-known muscle that sits underneath the biceps, runs from the front of the humerus to the upper ulna. It’s actually the primary muscle responsible for bending your elbow, doing more of the heavy lifting than the biceps in many positions.
On the back of the arm, the triceps converges into a single tendon that attaches to the tip of the elbow (the top of the ulna). Contracting the triceps straightens the arm. The two bony knobs at the bottom of the humerus also serve as anchor points for the forearm muscles that control your wrist and fingers.
Where the Wrist Bones Begin
At the far end of the forearm, the radius and ulna meet a cluster of eight small carpal bones arranged in two rows. The row closest to the forearm contains four bones (scaphoid, lunate, triquetrum, and pisiform), and the row closest to the fingers contains four more (trapezium, trapezoid, capitate, and hamate). These small bones collectively form the wrist, bridging the forearm to the hand. Most people consider the wrist separate from “the arm,” but the transition point matters because it’s where the most common arm fracture occurs.
Common Fracture Sites
Distal radius fractures, breaks near the wrist end of the radius, are the single most common fracture in adults. They account for roughly 17.5% of all fractures and typically happen when someone falls and catches themselves with an outstretched hand. The force travels up through the wrist and snaps the radius where it’s widest and most exposed.
In the upper arm, fractures most often occur at the surgical neck of the humerus, the narrow zone just below the shoulder joint. These are especially common in older adults after a fall. Fractures through the middle of the humerus shaft are less frequent but can damage the nerve that runs along a groove in the bone, sometimes causing temporary wrist drop (inability to extend the hand upward).
How Arm Bones Develop
In children, arm bones grow from specialized growth plates near each end. The humerus begins forming its secondary growth centers as early as four months of age, with the process of bone hardening continuing through childhood. By around age 13, the bone structure is largely complete, though the growth plates at the top of the humerus typically remain open until age 14 to 16. By 17, these plates are fully closed in most people, marking the end of arm bone growth. This timeline matters for young athletes: injuries near growth plates in children can potentially affect bone development in ways that adult fractures cannot.