The humerus is a long bone, the single bone of your upper arm that runs from shoulder to elbow. It belongs to the same category as the femur (thigh bone), tibia (shin bone), and radius and ulna (forearm bones). Long bones are defined by their shape: they are longer than they are wide, with a cylindrical shaft and two broader ends. The humerus is also the largest bone in the upper limb.
What Makes the Humerus a Long Bone
Bones in the human body fall into five categories: long, short, flat, irregular, and sesamoid. Long bones share a specific structure. They have a hard, dense outer layer that forms a thick-walled shaft, and their interior contains a central cavity filled with bone marrow. The two ends of a long bone are wider and capped with a thin layer of smooth cartilage where they meet other bones at joints.
The humerus fits this pattern perfectly. Its shaft is roughly cylindrical and makes up most of the bone’s length. The upper end is rounded into a ball shape that fits into the shoulder socket, while the lower end flares out into a complex shape that forms the top half of the elbow joint. The shaft is made of compact (dense) bone tissue, while the broader ends contain spongy bone tissue, a lighter, honeycomb-like structure that absorbs shock and houses red bone marrow where blood cells are produced.
How the Humerus Connects to Other Bones
The upper end of the humerus connects to the shoulder blade at the glenohumeral joint, which is a ball-and-socket joint. This design allows the widest range of motion of any joint in the body: you can rotate your arm, swing it forward and backward, and move it out to the side. The ball-shaped head of the humerus sits in a shallow cup on the shoulder blade, held in place mainly by muscles and ligaments rather than by the shape of the bones themselves.
The lower end meets two forearm bones, the radius and ulna, at the elbow. This creates a hinge-type joint that lets you bend and straighten your arm, along with a pivot joint that allows your forearm to rotate so you can turn your palm up or down. Between the shoulder and elbow, the humerus serves as an anchor point for more than 20 muscles that control movements of the arm, forearm, and shoulder.
Key Landmarks on the Humerus
Like most long bones, the humerus has raised bumps, grooves, and ridges where muscles and ligaments attach. Near the top, two bony projections called the greater and lesser tuberosities serve as attachment points for the rotator cuff muscles that stabilize your shoulder. A groove between these two bumps channels the tendon of the biceps muscle.
About halfway down the shaft, a roughened area called the deltoid tuberosity marks where the deltoid muscle (the large muscle capping the shoulder) attaches. A shallow, spiraling groove on the back of the shaft is where the radial nerve travels. This nerve controls extension of the wrist and fingers, and its close contact with the bone is the reason a mid-shaft humerus fracture can sometimes cause wrist drop, a temporary inability to lift the hand.
How the Humerus Grows
Long bones grow in length from specialized zones of cartilage near each end called growth plates. The humerus develops from multiple growth centers that appear and fuse at different ages. At the upper end, the first growth center appears around six months of age, with additional centers at the greater tuberosity (around age 3) and lesser tuberosity (around age 5). These centers gradually merge between ages 5 and 13, and the upper growth plate fully fuses between ages 14 and 17 in females and 16 and 18 in males.
Until those growth plates close, the ends of the humerus are structurally weaker than in adults. This is why children and teenagers sometimes fracture through a growth plate rather than through the solid bone itself. Once the plates fuse, the humerus reaches its adult length and the bone becomes uniformly strong along its full length.
Where the Humerus Is Most Likely to Break
Fractures of the humerus are more common near the top, where the bone meets the shoulder, than at the bottom near the elbow. Proximal humerus fractures often happen from falls onto an outstretched hand or directly onto the shoulder, and they are especially common in older adults with lower bone density. Mid-shaft fractures typically result from higher-energy impacts like car accidents or sports injuries, while fractures near the elbow are less frequent overall but more common in children.
Specific fracture locations on the humerus are described by their anatomical landmarks. The “surgical neck,” a slightly narrower zone just below the ball-shaped head, is one of the most common fracture sites because it’s a natural weak point where the wider upper end transitions into the narrower shaft. These labels help doctors pinpoint exactly where the break occurred and plan treatment accordingly.
How the Humerus Compares to Other Bone Types
Short bones, like those in the wrist and ankle, are roughly cube-shaped and built for stability rather than leverage. Flat bones, such as the skull, ribs, and shoulder blade, are thin and broad, protecting organs or providing large surfaces for muscle attachment. Irregular bones, like the vertebrae, have complex shapes that don’t fit neatly into other categories. Sesamoid bones, the most familiar being the kneecap, are small bones embedded within tendons.
The humerus shares none of these shapes. Its elongated shaft acts as a lever, amplifying the force produced by muscles so you can throw, lift, push, and pull with your arm. This lever function is the defining role of all long bones, and it’s the reason the limbs of the body are built almost entirely from them.