The arm serves as the primary tool for interacting with the world, enabling a vast range of motions. Anatomically, the term “arm” refers to the entire upper limb, extending from the shoulder joint to the wrist. This structure is composed of an upper arm and a forearm, which work together to position the hand for pushing, pulling, lifting, and manipulating objects.
The Humerus in the Upper Arm
The upper arm contains a single, long bone called the humerus, the longest bone in the upper limb. At its top end, the humerus has a large, rounded head that fits into a socket in the shoulder blade. This connection creates the highly mobile ball-and-socket shoulder joint, which permits the arm to move in a wide circular arc.
The lower end of the humerus flattens and connects to the forearm bones, forming the upper part of the elbow’s hinge joint. It articulates with both the radius and ulna to allow the elbow to bend and straighten. The shaft, or the long middle portion of the humerus, provides attachment points for 13 different muscles that control movements at both the shoulder and elbow.
The Radius and Ulna in the Forearm
The forearm, located between the elbow and the wrist, contains two bones: the radius and the ulna. These long bones run parallel, with the ulna on the side of the pinky finger and the radius on the thumb side. This arrangement is fundamental to the forearm’s rotational capabilities. The bones are connected along their length by a flexible sheet of connective tissue called the interosseous membrane.
The ulna is larger at its upper end, where it forms the main, stabilizing part of the elbow joint. A bony process on the ulna, the olecranon, creates the pointed part of the elbow and prevents the joint from hyperextending. In contrast, the radius is smaller at the elbow but widens at its lower end to form the primary connection with the wrist bones.
How the Arm Bones Create Movement
The shapes and connections of the arm bones facilitate movement at the elbow and within the forearm. The most straightforward motion is the bending and straightening of the elbow, known as flexion and extension. This hinge-like motion involves the ulna and radius moving against the humerus. Muscles like the biceps and triceps contract to produce this movement.
A more complex motion is the rotation of the forearm, which allows the palm to turn upwards (supination) or downwards (pronation). This action originates from the relationship between the radius and ulna, not the elbow hinge. During rotation, the head of the radius pivots where it meets the humerus and ulna, while its wider, lower end sweeps across the stationary ulna. This crossing-over motion turns the wrist and hand without requiring the entire arm to rotate from the shoulder.
Common Arm Bone Fractures
Breaks in the arm bones are frequent injuries, often resulting from direct impacts or falling onto an outstretched hand. A fall transmits force up the limb, which can cause a fracture at the wrist, forearm, or humerus. Fractures of the humerus can occur near the shoulder, along the central shaft, or near the elbow.
Forearm fractures often involve both the radius and ulna breaking together. A “broken wrist” is a fracture of the distal radius, the part of the bone nearest the wrist joint. Elbow fractures can be complicated, as they may involve the lower end of the humerus, the upper end of the ulna, and the head of the radius. An isolated break of the ulna shaft, known as a “nightstick fracture,” results from a direct blow to the forearm.