Elbow flexion is a fundamental human movement involving bending the arm at the elbow joint. Anatomically, it decreases the angle between the forearm and the upper arm, bringing the hand closer to the shoulder. This action occurs primarily in the sagittal plane. The elbow functions mainly as a hinge joint, though its structure allows for additional rotational movement.
The Joints Involved in Elbow Movement
The structure commonly called the elbow is composed of three distinct articulations, all enclosed within a single joint capsule. These joints connect the humerus of the upper arm to the radius and ulna of the forearm. The primary action of bending and straightening the arm is managed by the humeroulnar joint.
The humeroulnar articulation acts as the true hinge, formed by the trochlea of the humerus fitting into the trochlear notch of the ulna. Its bony configuration guides the forearm through the flexion and extension arcs. The other two joints, the humeroradial joint and the proximal radioulnar joint, primarily facilitate the rotation of the forearm.
The humeroradial joint connects the head of the radius with the capitulum of the humerus. The proximal radioulnar joint is where the radius and ulna articulate near the elbow. While these two joints contribute little to the bending motion, they are essential for pronation (palm-down) and supination (palm-up) rotations of the forearm.
Primary Muscles That Cause Flexion
Elbow flexion is accomplished by three muscles located on the anterior side of the upper arm and forearm. These muscles contract, pulling the forearm bones toward the humerus. The brachialis muscle is considered the primary workhorse because it is a single-joint muscle that attaches to the ulna. It acts as the strongest and most consistent flexor, producing force reliably regardless of the forearm’s rotational position.
The biceps brachii muscle is the most well-known flexor, but its role changes based on forearm position since it attaches to the radius. It functions most powerfully when the forearm is supinated (palm-up), because it is also a powerful supinator. When the biceps contracts, it pulls the radius and forearm toward the upper arm.
The third contributing muscle is the brachioradialis, located on the side of the forearm. This muscle is most active during rapid movements or when the forearm is held in a neutral position (thumb pointing up). It assists the other two muscles, acting as a stabilizer and flexor, particularly during activities requiring high force.
Measuring Normal Elbow Range of Motion
The extent of elbow flexion is assessed by measuring the range of motion (ROM), the total arc of movement possible at the joint. The reference point for a fully straightened arm is 0 degrees of extension. From this starting position, the average healthy elbow can achieve flexion of approximately 140 to 150 degrees.
This measurement is quantified clinically using a goniometer, a protractor-like device that measures joint angles. The goniometer’s axis is aligned with the lateral side of the elbow to determine the angle created between the upper arm and the forearm. Measuring this range helps medical professionals assess joint health after injury or surgery.
For most daily tasks, such as lifting a glass or using a phone, the full range of motion is not strictly necessary. The functional arc required for nearly all activities of daily living falls between 30 and 130 degrees of flexion. Maintaining this functional range is important for comfortable and independent living.