Humerus

The humerus is a single long bone in the upper arm. Its rounded upper head fits into a shallow socket in the shoulder blade (scapula) to form the glenohumeral joint (shoulder joint). Its lower end meets the radius and ulna of the forearm to form the elbow joint. It serves as a lever connecting the shoulder and elbow.

Following the bone from top to bottom, there are several features worth knowing.

• Head — The ball-shaped end that fits into the shoulder joint. This rounded head allows the shoulder to rotate through a wide range of angles.
• Greater and lesser tubercles — Two projections just below the head. The muscles of the rotator cuff attach to different sites on these projections.
• Surgical neck — A slightly narrowed region below the head, often discussed in relation to fractures.
• Deltoid tuberosity — A prominent area on the outer side of the shaft where the deltoid muscle, which covers the shoulder, attaches.
• Radial groove (radial nerve groove) — A shallow groove running diagonally across the back of the bone. The radial nerve travels along this groove, lying very close to the bone.
• Epicondyles — Projections on the inner and outer sides of the lower end. Forearm muscles originate here. These are the bony prominences you can feel on the inside and outside of the elbow.

Many muscles attach to the humerus, transmitting force between the shoulder and elbow through this bone. Prominent examples include the deltoid muscle, which covers the shoulder; the brachialis and biceps brachii, which bend the elbow; and the triceps brachii, which straightens the elbow. The lateral and medial heads of the triceps brachii originate on the back of the humerus, above and below the radial groove, respectively.

In Korean, many anatomical names contain “sangwan,” meaning “upper arm,” which can make them confusing. The humerus is the name of a bone, while the brachialis and triceps brachii are muscles that attach to this bone. The elbow is not a single bone but a joint where the humerus, radius, and ulna meet. The forearm contains two bones. The radius is on the thumb side, and the ulna is on the little-finger side. When you turn your palm over, the radius rotates around the ulna. This movement takes place between the two forearm bones, not in the upper arm.

When you raise your arm to take something off a shelf, carry a heavy bag, hold a child, or throw a ball, the humerus transmits force between the shoulder and elbow. When you sleep on your side and feel pressure on your shoulder, the aching in the upper part of your arm occurs because the muscles and tendons around this bone are working together. The name of the bone alone cannot explain what you are feeling; considering the surrounding muscles and joint movements together can help you understand it.

• Move your shoulder in different directions. Slowly raising your arm forward, out to the side, and overhead, and rotating it, helps the muscles and joint around the upper end of the humerus move smoothly together.
• Keep heavy objects close to your body when lifting. Lifting with your arm stretched far out increases the force on your shoulder and upper arm.
• Check whether your shoulder is under pressure before going to sleep. Sleeping only on one side can put pressure on your shoulder. Try placing a pillow under your arm or changing position.
• Be mindful of the arm you use to brace yourself during a fall. The humerus is one of the bones that can fracture easily in a fall. Take care on slippery floors and hold the handrail on stairs.