The forearm, the section of the arm between the elbow and the wrist, is structurally supported by two long, parallel bones: the ulna and the radius. These bones work together to provide strength and facilitate movement. Understanding the unique roles and positions of these two bones is helpful for comprehending how the lower arm functions.
The Position of the Ulna and Radius
In the standard anatomical position (palms facing forward), the ulna is situated on the medial side of the forearm, meaning it aligns with the pinky finger. Conversely, the radius is located on the lateral side of the forearm, aligning with the thumb. This placement is consistent regardless of the arm’s movement.
The two bones run parallel for much of their length, but they articulate with one another at both the elbow and the wrist. This connection forms the radioulnar joints, which are critical for rotation. When the palm is turned downward (pronation), the radius crosses over the ulna, which remains relatively stationary. The ulna is typically longer than the radius, extending from the elbow down toward the wrist.
The Ulna’s Primary Role in Movement
The ulna’s major contribution to the arm’s mechanics is providing stability and forming the hinge joint at the elbow. The proximal end of the ulna, the part nearest the elbow, is robust and features a C-shaped structure called the trochlear notch. This notch securely fits around the trochlea of the humerus, the upper arm bone, creating a stable connection for flexion and extension.
A prominent feature of the ulna is the olecranon, which is the bony projection felt as the “point” of the elbow. This process serves as an attachment site for muscles, including the triceps brachii, which extends the forearm. Because of its strong, interlocking connection at the elbow, the ulna acts as the stabilizing bone, while the radius is more involved in the rotational movements that twist the hand and wrist.
Remembering the Forearm Bones
A helpful way to remember the location of the ulna is to associate it with the smaller finger. Thinking of the ulna as being on the pinky-side helps distinguish it from the radius.
To recall the radius, consider the common gesture of giving a “thumbs-up”. The bone aligning with the thumb is the radius, and the letter “R” in radius can be associated with the “ready” position of the thumb or the “rotation” it facilitates.