The humerus is the single long bone of your upper arm, running from the shoulder to the elbow. It serves as the structural link between your torso and your forearm, anchoring more than 20 muscles that control movements of the shoulder, elbow, wrist, and hand. It’s also one of the most commonly fractured bones in older adults, accounting for 5% to 6% of all fractures.
Shape and Key Landmarks
The humerus has a distinctive shape: a rounded top, a relatively narrow shaft, and a wide, flared bottom. Each section has landmarks that matter because they’re where muscles attach, joints form, or injuries tend to occur.
At the very top sits the head of the humerus, a smooth, ball-shaped surface that fits into a shallow socket on the shoulder blade to form the shoulder joint. Just below the head are two bony bumps called the greater and lesser tubercles, separated by a vertical groove. These tubercles are anchor points for the rotator cuff muscles. Below the tubercles is an area called the surgical neck, named not for any procedure but because it’s one of the most fracture-prone spots on the bone.
The shaft makes up the long middle portion. It starts roughly cylindrical and gradually flattens as it descends. About halfway down, a roughened patch called the deltoid tuberosity marks where the deltoid muscle (the cap of muscle on your outer shoulder) attaches. A shallow spiral groove winds around the back of the shaft, and the radial nerve runs through it, which is why mid-shaft fractures can cause nerve damage.
At the bottom, the bone widens into two prominent bumps on either side: the medial and lateral epicondyles. You can feel the medial epicondyle as the bony point on the inner side of your elbow. The epicondyles serve as attachment sites for the muscles that flex and extend your wrist and fingers.
Joints at the Shoulder and Elbow
The humerus participates in two major joints. At the top, the ball-shaped head articulates with the glenoid cavity of the shoulder blade to form the glenohumeral joint, a ball-and-socket design that gives the shoulder its wide range of motion. This joint allows you to reach overhead, behind your back, and across your body.
At the bottom, the humerus connects with both the ulna and the radius to form the elbow. The primary hinge is between the humerus and the ulna, which handles bending and straightening. A secondary connection between the humerus and the radius helps with rotation of the forearm, the motion you use when turning a doorknob or a screwdriver. Together, these three articulations at the elbow permit flexion, extension, and the twisting motions called pronation and supination.
Muscles That Attach to the Humerus
More than 20 muscles either originate from or insert onto the humerus, making it one of the most muscle-dense bones in the body. These fall into a few functional groups.
- Shoulder movers: The deltoid inserts onto the shaft, powering the ability to raise your arm. The pectoralis major and latissimus dorsi insert near the top of the humerus and handle pushing, pulling, and rotating the arm inward. The four rotator cuff muscles (supraspinatus, infraspinatus, teres minor, and subscapularis) all insert onto the tubercles and stabilize the shoulder during movement.
- Elbow benders: The brachialis originates from the front of the lower shaft and is the primary muscle for bending the elbow. The triceps originates from the back of the humerus and straightens it.
- Forearm and wrist muscles: A large cluster of flexor muscles originates from the medial epicondyle (the inner bump at the elbow), while a corresponding group of extensor muscles originates from the lateral epicondyle. These control wrist flexion, extension, and finger movement. Overuse of these attachment points is what causes conditions commonly known as golfer’s elbow and tennis elbow.
Nerves Running Along the Bone
Three important nerves travel in close contact with the humerus, which is why fractures at different locations cause very different problems.
The axillary nerve wraps around the surgical neck near the top of the bone. A fracture there can damage this nerve, weakening your ability to lift your arm away from your body and causing numbness over the outer shoulder. The radial nerve spirals along the groove on the back of the shaft. A mid-shaft fracture can injure it, leading to “wrist drop,” where you lose the ability to extend your wrist and fingers. A fracture at the lateral epicondyle can also damage the radial nerve, affecting the extensors of the forearm and hand.
The ulnar nerve passes behind the medial epicondyle. That’s the “funny bone” spot. Hitting it sends a tingling shock down to your ring and little fingers, and fractures or swelling at the inner elbow can compress it more seriously.
How the Humerus Grows
In a developing child, the humerus doesn’t start as solid bone. It forms from a primary center of cartilage that gradually hardens, with secondary growth centers appearing at specific ages. In the proximal humerus, a center in the humeral head appears around 4 months of age and another at the greater tuberosity around 10 months. These centers begin fusing together by age 3 and progressively take on the final adult shape of the bone. Ossification of the proximal humerus is typically complete by age 13. Understanding these growth timelines matters because in children, what looks like a fracture on an X-ray can sometimes be a normal growth plate, and vice versa.
Common Fracture Patterns
Humerus fractures account for 5% to 6% of all fractures across all ages, but they spike in two populations: young people involved in high-energy trauma like car accidents or sports injuries, and older adults who fall from standing height. In people over 65, proximal humerus fractures alone account for 10% of all fractures, largely driven by osteoporosis. The higher incidence in older women reflects their greater risk of bone density loss.
Proximal fractures near the shoulder are classified by how many segments of bone are displaced. A one-part fracture means the bone is cracked but the pieces haven’t shifted significantly. A two-part fracture involves one displaced segment, which could be a tuberosity or the surgical neck. Three- and four-part fractures involve progressively more displaced segments and are more complex injuries. The greater the displacement, the more likely surgery becomes necessary.
Mid-shaft fractures typically result from a direct blow or a fall on an outstretched hand. They carry a particular risk of radial nerve injury. Distal fractures near the elbow are less common in adults but can be complicated by their proximity to the elbow joint, potentially limiting range of motion during healing.
Healing and Recovery Timeline
Most humerus fractures heal without surgery. A splint is typically worn for three to five weeks, while a cast, if needed, stays on for six to eight weeks. Symptoms generally begin improving within the first few weeks, and most people can start moving the arm again shortly after immobilization ends. Full recovery takes a few months for most fractures, though complex injuries requiring surgery may take longer.
Physical therapy plays a significant role in recovery because the shoulder and elbow can stiffen quickly when immobilized. Early gentle movement, guided by a therapist, helps restore range of motion. Strength rebuilding follows once the bone has healed enough to tolerate resistance. For older adults, recovery also involves addressing the underlying bone density loss that made the fracture possible in the first place.