The humerus is the single bone of your upper arm, and it serves as the structural foundation for nearly everything your arm does. It supports your arm’s weight, anchors 13 different muscles, connects your shoulder to your elbow, and enables the wide range of motion that makes human arms so versatile. It also plays a quieter role you might not expect: producing blood cells.
How the Humerus Moves Your Arm
Your humerus participates in two major joints. At the top, its rounded head fits into a shallow socket on your shoulder blade to form the glenohumeral (shoulder) joint. At the bottom, it connects to the two bones of your forearm to form the elbow joint. Together, these two connections let you lift, rotate, bend, and extend your arm through an impressive range of directions.
The shoulder joint alone accounts for about 120 degrees of the 180 degrees you get when raising your arm overhead. The remaining 60 degrees comes from your shoulder blade sliding along your ribcage. This two-part system is why your arm can reach so far in so many directions, and the humerus sits at the center of it. At the elbow end, the humerus enables bending and straightening as well as the forearm rotation that lets you turn a doorknob or flip your palm up and down.
Anchoring 13 Muscles
The humerus isn’t just a passive lever. Its surface is covered with ridges, bumps, and grooves that serve as attachment points for 13 muscles. These include the four rotator cuff muscles that stabilize your shoulder, the deltoid that caps your shoulder and lifts your arm sideways, the biceps and triceps that bend and straighten your elbow, and several forearm muscles that control your wrist and grip.
The long middle shaft of the humerus is especially important here. It supports the weight of your upper arm, gives your arm its shape, and provides a broad surface for muscles to anchor to. Without a solid attachment point, these muscles couldn’t generate the force needed to throw a ball, carry groceries, or push yourself up from a chair.
Protecting Nerves and Blood Vessels
A groove spiraling around the back of the humerus, called the spiral groove, cradles the radial nerve and a major artery as they travel down your arm. The radial nerve is responsible for extending your wrist and fingers and providing sensation to the back of your hand. Its close relationship with the bone is why a hard hit to your upper arm can cause temporary numbness or tingling, and why humerus fractures sometimes affect nerve function.
This is also why “funny bone” sensations happen near the elbow. A different nerve (the ulnar nerve) passes close to the bony point at the bottom of the humerus, making it vulnerable to bumps.
Producing Blood Cells
Like other long bones, the humerus contains bone marrow. In children, most of this marrow is the red, active type that produces red blood cells, white blood cells, and platelets. As you age, a significant portion converts to yellow marrow (mostly fat storage), but the spongy ends of the humerus near the shoulder and elbow joints continue to house red marrow into adulthood. The humerus isn’t the body’s primary blood cell factory (that distinction goes to flat bones like the pelvis and sternum), but it contributes to the process throughout life.
Growth and Development
In children and teenagers, the humerus has growth plates near each end, areas of cartilage where new bone forms to lengthen the arm. The growth plate near the shoulder is one of the most active in the body and is responsible for most of the humerus’s length. These plates typically fuse between ages 14 and 17 in females and 16 to 18 in males, at which point the bone reaches its adult length. Until they close, growth plates are softer than mature bone, making them more vulnerable to certain injuries in young athletes.
What Happens When the Humerus Breaks
Humerus fractures are almost always caused by trauma: falls (especially landing on an outstretched hand), car accidents, or sports injuries. Breaks near the shoulder (proximal fractures) are more common than breaks near the elbow (distal fractures). In children, a specific type called a supracondylar fracture, occurring just above the elbow, is particularly common and typically results from falling forward onto an outstretched arm.
Because the humerus anchors so many muscles and shelters critical nerves, a fracture can temporarily affect your ability to move your arm, grip objects, or feel sensation in your hand. The location and severity of the break determine whether treatment involves a brace or sling versus surgery, but in either case the bone’s central role in arm function means recovery involves gradually rebuilding strength and range of motion.