The forearm, or antebrachium, is the section of the upper limb situated between the elbow joint and the wrist joint. It is supported by a pair of long bones arranged in parallel. These two bones work together to provide structural support and the broad range of motion necessary for complex hand movements. The unique configuration of this bony pair allows for exceptional rotational ability.
Defining the Antebrachium: The Radius and Ulna
The two bones that form the forearm are the radius and the ulna. These bones maintain a parallel relationship, separated by a strong, fibrous sheet of connective tissue called the interosseous membrane. The ulna is positioned on the medial side of the forearm, aligning with the little finger when the arm is in the anatomical position. Conversely, the radius is located on the lateral side, corresponding to the thumb side of the hand.
The Ulna: Stabilizer and Elbow Connection
The ulna functions primarily as the stabilizing component of the forearm, establishing a strong connection with the humerus. Its proximal end features a large, C-shaped structure called the trochlear notch, which tightly grips the trochlea of the humerus. This close fit forms the hinge joint of the elbow, permitting the movements of flexion and extension.
A prominent feature of the ulna is the olecranon process, the bony projection felt at the tip of the elbow. This process acts as a lever arm for the triceps brachii muscle, which straightens the elbow joint. Because the ulna’s articulation with the humerus is restrictive, the ulna remains fixed and does not rotate significantly during forearm movements.
The Radius: Mobility and Wrist Connection
The radius is the bone specialized for mobility, allowing the hand to rotate. Unlike the ulna, the distal end of the radius is wide and directly articulates with the first row of wrist bones, specifically the scaphoid and lunate carpal bones. This articulation makes the radius the primary weight-bearing bone at the wrist joint. The head of the radius, at the elbow end, is shaped like a disk, allowing it to spin freely against both the humerus and the ulna.
This rotational capacity enables the forearm movements of pronation and supination. During pronation (turning the palm down), the radius crosses diagonally over the ulna. Supination (turning the palm up) uncrosses the bones and returns them to their parallel anatomical position. The radius pivots around the fixed ulna, creating the unique range of motion necessary for tasks like turning a doorknob.