The Flexor Carpi Ulnaris (FCU) is a muscle located on the anterior, or palm-side, of the forearm. It is part of the superficial group of flexors and plays a significant role in hand and wrist mobility. The FCU is a major contributor to common daily motions, and its actions are essential for coordinated movement and grip strength in activities ranging from writing to playing sports. Understanding its function helps explain how the wrist moves and how overuse can lead to discomfort or injury.
Where the Muscle is Located
The Flexor Carpi Ulnaris muscle extends from the elbow region down to the wrist, situated along the little finger side of the forearm. It is unique among the superficial flexors for having two distinct points of origin: a humeral head and an ulnar head. The humeral head arises from the medial epicondyle of the humerus, the bony bump on the inside of the elbow.
The ulnar head originates from the olecranon process (the bony tip of the elbow) and the upper part of the ulna bone. These two heads form a gap through which the ulnar nerve passes as it enters the forearm. The muscle fibers converge into a long tendon that travels toward the wrist, positioning it as the most medial tendon in the forearm.
The FCU tendon crosses the wrist and inserts onto three bones in the hand: the pisiform, the hook of the hamate, and the base of the fifth metacarpal bone (connecting to the little finger). The pisiform bone acts like a small pulley, enhancing the muscle’s mechanical advantage for its primary actions. This extensive path, originating near the elbow and inserting into the hand, gives the FCU its leverage to move the wrist.
How the Flexor Carpi Ulnaris Moves the Wrist
The mechanical structure of the FCU dictates its two primary movements at the wrist joint: flexion and ulnar deviation. When the muscle contracts, it shortens the distance between its origin points on the elbow and its insertion points on the hand. Wrist flexion is the movement of bending the wrist forward, bringing the palm closer to the forearm.
The FCU is a highly effective flexor, working with other muscles like the Flexor Carpi Radialis to execute this bending motion. Ulnar deviation is the movement of tilting the hand toward the little finger side. The FCU is the most powerful muscle responsible for this side-to-side movement, working in conjunction with the Extensor Carpi Ulnaris on the back of the forearm to achieve pure ulnar deviation.
The FCU is unique in its nerve supply, as it is the only muscle in the anterior compartment of the forearm to be completely innervated by the ulnar nerve (from nerve roots C7, C8, and T1). This innervation pattern means the FCU’s function can be an indicator of ulnar nerve health. While its main actions are at the wrist, the FCU can also provide a weak assist in elbow flexion due to its connection to the medial epicondyle.
Preventing Strain and Common Injuries
Given its involvement in gripping and wrist movements, the FCU is susceptible to overuse injuries, particularly FCU tendinitis. This involves inflammation or irritation of the tendon, typically where it crosses the wrist near the pisiform bone. Tendinitis can arise from repetitive, forceful wrist motions seen in activities like weightlifting, racquet sports, or occupations requiring constant gripping or tool use.
Symptoms include pain and tenderness on the pinky side of the wrist, which worsens when flexing the wrist or attempting to grip firmly. Preventing this strain involves addressing the underlying cause of overuse through adjustments in activity and technique. Incorporating proper warm-up and stretching routines before and after activities that stress the wrist is a useful preventative measure.
Ergonomic adjustments, such as ensuring correct wrist positioning while typing or using tools, can significantly reduce the load placed on the FCU tendon. Strengthening the wrist flexors and extensors, along with maintaining flexibility, helps create a more resilient and stable wrist joint. Recognizing early signs of discomfort and applying rest and ice therapy can prevent a mild strain from progressing into a more chronic issue.