The bones of the arm provide the strength, stability, and flexibility needed for everyday activities, from lifting groceries to writing, reaching overhead, and gripping a tool. The arm’s skeletal structure includes three main bones between the shoulder and elbow and two bones in the forearm between the elbow and wrist. Together with the bones of the shoulder girdle, wrist, and hand, they form a connected system that supports a wide range of movements.
The three main bones of the upper limb are the humerus, radius, and ulna. The humerus is the long bone of the upper arm, while the radius and ulna run parallel to each other in the forearm. Each has a distinct position and function, and their coordinated movements allow the arm to bend, straighten, rotate, and position the hand with precision.
What are the main bones of the arm?
In everyday language, the arm often refers to the entire upper limb, from the shoulder to the fingertips. In anatomical terminology, however, the arm is the section between the shoulder and elbow. The forearm extends from the elbow to the wrist.
This distinction helps explain the arrangement of the bones:
- Humerus: The single long bone of the upper arm, extending from the shoulder to the elbow.
- Radius: One of the two forearm bones, located on the thumb side when the body is in the standard anatomical position.
- Ulna: The other forearm bone, located on the little-finger side in the same position.
The shoulder girdle, made up of the clavicle (collarbone) and scapula (shoulder blade), connects the upper limb to the trunk. Below the forearm, eight carpal bones form the wrist, five metacarpal bones form the palm, and 14 phalanges form the fingers and thumb.
These additional bones are important to the movement of the upper limb, but the humerus, radius, and ulna are its three principal long bones.
The humerus: The bone of the upper arm
The humerus is the longest and largest bone in the upper arm. Its upper end meets the scapula to form the shoulder joint, while its lower end meets the radius and ulna at the elbow.
At the shoulder, the rounded head of the humerus fits into a shallow socket in the scapula called the glenoid cavity. This ball-and-socket joint allows the arm to move forward, backward, sideways, and in a circle. It also permits rotation, making it possible to reach across the body or raise the hand above the head.
The shoulder joint offers considerable mobility, but that flexibility comes with a need for muscular and ligament support. The socket is relatively shallow, so muscles, tendons, and other soft tissues help keep the humeral head properly positioned during movement.
The shaft of the humerus provides attachment points for muscles that move the shoulder and elbow. Its lower end widens and forms several important structures that articulate with the forearm bones. These include the rounded capitulum, which meets the radius, and the spool-shaped trochlea, which meets the ulna.
The humerus helps transfer forces between the shoulder and elbow. When a person pushes against a heavy object, lifts a bag, or throws a ball, forces pass through this bone as muscles contract and joints respond to the load.
Because it supports both shoulder and elbow movement, the humerus is essential for positioning the hand in space. It does not act alone: its function depends on coordinated activity involving the shoulder girdle, muscles, tendons, and forearm.
The radius: The forearm bone on the thumb side
The radius runs along the thumb side of the forearm when the palm faces forward. It extends from the elbow to the wrist and plays a particularly important role in wrist movement and forearm rotation.
Unlike the ulna, the radius is relatively narrow near the elbow and broader near the wrist. Its upper end includes a disc-shaped head that rotates against the humerus and articulates with the ulna. Its lower end contributes substantially to the wrist joint, where it meets the carpal bones, especially the scaphoid and lunate.
One of the radius’s most important functions is to allow the forearm and hand to rotate. When a person turns the palm upward to hold a bowl or turns it downward to place an object on a table, the radius moves around the ulna. These movements are called supination and pronation, respectively.
During supination, the radius and ulna lie roughly parallel. During pronation, the radius crosses over the ulna. This arrangement allows the hand to change orientation without requiring the entire upper arm to rotate.
The radius also helps distribute forces from the hand to the elbow. When someone lands on an outstretched hand, for example, substantial force may travel through the wrist and radius. This is one reason fractures near the wrist are common after falls.
The bone provides attachment points for muscles that move the wrist, fingers, and forearm. These muscles help control both large movements, such as turning a doorknob, and precise actions, such as positioning the fingers to hold a pen.
The ulna: The forearm bone on the little-finger side
The ulna runs along the little-finger side of the forearm in the anatomical position. It extends from the elbow toward the wrist and provides important stability to the elbow joint.
Its upper end has a prominent, curved structure called the trochlear notch. This notch fits around the trochlea of the humerus, forming the main hinge-like articulation of the elbow. The olecranon, the large projection at the back of the elbow, forms the bony point commonly felt beneath the skin.
When a person straightens the arm, the olecranon helps limit further extension. It also serves as an attachment point for the triceps muscle, which produces the force needed to extend the elbow. This action is important for pushing open a heavy door, rising from a chair using the arms, or throwing an object.
The ulna works with the radius to support forearm rotation. At the elbow, its relationship with the humerus provides much of the joint’s stability during bending and straightening. At the wrist, the ulna does not directly articulate with the carpal bones in the same way the radius does. A fibrocartilaginous structure called the triangular fibrocartilage complex helps connect and stabilize the area between the distal ulna and the wrist.
The ulna also provides attachment sites for muscles involved in moving the fingers, wrist, and forearm. Together with the radius, it forms a stable but mobile framework for controlling the hand.
Although the radius and ulna work as a pair, they are not interchangeable. The ulna is particularly important for the elbow’s hinge-like stability, while the radius plays a larger role in transmitting loads from the wrist and rotating the hand.
How the arm bones work together
The humerus, radius, and ulna function as a connected mechanical system. Their shapes and joint surfaces allow different types of movement while maintaining enough stability to support lifting, pushing, pulling, and precise hand control.
The elbow is primarily a hinge joint, allowing the forearm to bend and straighten relative to the upper arm. The humerus articulates with both forearm bones, but the ulna’s trochlear notch and the humerus’s trochlea form the main hinge mechanism. The radius also articulates with the humerus and rotates as the forearm changes position.
Two additional joints, located near the elbow and wrist, enable pronation and supination. These are the proximal and distal radioulnar joints, where the radius and ulna meet. Together, they allow the radius to rotate around the ulna, changing the direction in which the palm faces.
The shoulder and elbow work together to position the hand. For example, reaching for a high shelf involves movement at the shoulder, while bending the elbow brings the hand closer to the body. Rotating the forearm then helps orient the palm to grasp an object.
This coordination is central to the versatility of the upper limb. A bone may provide a surface for a joint, a lever for a muscle, or a pathway for transferring force, but useful movement depends on all these roles working together.
How muscles move the arm bones
Bones cannot move themselves. Movement occurs when skeletal muscles contract and pull on bones through tendons, which are strong bands of connective tissue.
At the elbow, the biceps brachii and brachialis are major muscles involved in flexion, or bending the joint. The triceps brachii performs the opposite action, extending or straightening the elbow. These muscles work in coordinated opposition: as one group produces a movement, the opposing group adjusts its activity to control the motion.
The biceps also contributes to supination, turning the palm upward. Other muscles, including the supinator and pronator teres, help rotate the forearm. The pronator quadratus is another important muscle for pronation.
Muscles in the upper arm and forearm attach to different points on the humerus, radius, and ulna. When a muscle contracts, it pulls on its attachment points, creating torque—a turning effect around a joint. The location of the attachment and the angle at which the muscle pulls influence the amount of force and movement produced.
This arrangement explains why the arm can generate both strength and precision. Large muscle groups can move heavy objects, while smaller muscles and finely controlled contractions help guide the hand during tasks such as typing or threading a needle.
The bones provide the rigid framework, the joints define possible movements, and the muscles supply the force. Ligaments, which connect bones to other bones, help stabilize the joints, while cartilage reduces friction and supports smooth motion at joint surfaces.
The role of arm bones in strength and everyday movement
The arm bones serve several functions beyond providing structural support. They act as levers, protect surrounding tissues, store minerals, and contain marrow involved in blood cell production.
As levers, the bones allow muscles to produce movement around joints. The elbow, for instance, acts as a pivot when the forearm bends. The position of a weight relative to the elbow affects how much muscular force is needed to hold it. Holding a heavy object with the elbow bent can place substantial demands on the muscles, depending on the object’s distance from the joint.
The bones also help transfer force. When the hand pushes against a surface, force travels through the wrist and forearm toward the elbow and upper arm. When the hand grips an object, muscles in the hand and forearm create forces that are transmitted through the bones and joints.
Bone tissue is living tissue. Its mineral content, particularly calcium and phosphate, provides much of its stiffness, while its collagen-rich framework contributes to toughness. Bone is continually remodeled as specialized cells remove old tissue and build new tissue. This process helps maintain bone strength and allows the skeleton to adapt to changing physical demands.
Inside many bones is bone marrow. Red marrow produces red blood cells, white blood cells, and platelets. In adults, much of the marrow within the shafts of long bones is yellow marrow, which contains substantial fat. The distribution of red and yellow marrow varies by age and location.
The arm bones also provide attachment and support for surrounding soft tissues. Although they do not protect the brain or chest organs in the way the skull and rib cage do, their structure helps shield and support nearby nerves and blood vessels as they pass through the upper limb.
Common injuries involving the arm bones
Because the arm is used constantly and often bears the force of falls, collisions, and lifting, its bones can sustain several types of injury.
A humerus fracture is a break in the upper-arm bone. It may occur near the shoulder, along the shaft, or close to the elbow. The location affects the symptoms and treatment. Some fractures can be managed with immobilization, while others require surgery to restore alignment or stability.
A radius fracture often occurs near the wrist, especially after a person falls onto an outstretched hand. Such fractures can cause pain, swelling, bruising, and difficulty moving the wrist. Depending on the pattern and severity, treatment may involve a splint, cast, or surgery.
An ulna fracture may occur alone or alongside a radius fracture. A direct blow to the forearm can damage the ulna, while higher-energy injuries may affect both forearm bones. Because the radius and ulna must maintain a precise relationship to support rotation, treatment often focuses on restoring their alignment and length.
Some injuries affect the joints rather than breaking a bone. An elbow dislocation, for example, occurs when the bones forming the joint move out of their normal alignment. Such injuries may also damage ligaments, cartilage, nerves, or blood vessels.
Children’s bones differ from adults’ bones because they are still growing. Their bones may bend or break in patterns associated with more flexible bone tissue, and injuries involving growth plates—the regions where bones lengthen—require particular attention.
Persistent pain, visible deformity, major swelling, numbness, or an inability to use the arm after an injury warrants prompt medical evaluation. A clinician may use X-rays or other imaging to determine whether a fracture or joint injury is present.
How the arm bones differ from one another
The humerus, radius, and ulna are all long bones, but each is adapted to a different role.
The humerus connects the shoulder and elbow, providing a long lever for upper-arm muscles and allowing the arm to move through a wide range of positions. The radius connects the elbow region to the wrist and contributes strongly to wrist articulation and forearm rotation. The ulna provides a stable hinge relationship with the humerus and works with the radius to control the orientation of the hand.
Their differences become especially clear during common tasks. Bending the elbow relies on the humerus and forearm bones moving around the elbow joint. Turning a key or using a screwdriver depends heavily on rotation between the radius and ulna. Supporting weight through the hand requires coordinated action across the wrist, forearm, elbow, and shoulder.
No single bone performs these tasks independently. The upper limb works because its bones, joints, muscles, tendons, ligaments, and nervous system operate as an integrated whole.
Understanding the names and locations of the arm bones provides a foundation for understanding how the upper limb moves, why particular injuries affect certain functions, and how the skeleton combines strength with an exceptional range of motion.