The human wrist is a complex joint that allows for movement in multiple directions. Anatomists use standardized terminology relative to the body’s anatomical position to describe these movements. The wrist primarily moves in two planes: flexion and extension (bending the hand forward and backward), and deviation (side-to-side movement). Radial and ulnar deviation describe the lateral movements of the hand relative to the forearm.
Defining Radial Flexion
Radial flexion is more accurately termed radial deviation. This is the movement of the hand laterally toward the thumb side of the forearm. It is named “radial” because the hand moves in the direction of the radius, the bone on the thumb side.
The range of motion for radial deviation is relatively small, typically ranging from 15 to 20 degrees from the neutral position. This limited range is due to the scaphoid bone impacting the styloid process of the radius, creating a bony block that restricts further motion.
The Opposite Movement: Ulnar Deviation
The movement opposite to radial deviation is known as ulnar deviation, sometimes called ulnar flexion. Ulnar deviation involves tilting the hand medially toward the little finger side of the forearm. This direction is named “ulnar” because the movement is toward the ulna, the forearm bone located on the side of the little finger.
The range of motion for ulnar deviation is significantly greater than for radial deviation, often measuring between 30 to 45 degrees. This difference is attributed to anatomical factors, including the shape of the wrist bones and the position of the ulna relative to the wrist joint.
Key Muscles Involved
The movement of radial deviation requires the coordinated effort of muscles on both the front and back of the forearm. The primary muscles responsible for pulling the hand toward the thumb side are the Flexor Carpi Radialis (FCR) and the Extensor Carpi Radialis Longus (ECRL) and Brevis (ECRB). These muscles are positioned on the radial side of the forearm and attach to the wrist and hand bones.
The Flexor Carpi Radialis is a strong flexor muscle that also assists in radial deviation. The Extensor Carpi Radialis Longus and Brevis are primarily wrist extensors, but their lateral line of pull contributes significantly to the radial movement. When these muscles contract simultaneously, their opposing actions of flexion and extension cancel each other out, resulting in a pure side-to-side radial pull on the hand.