The flexor carpi ulnaris is the most medial superficial forearm flexor and the only forearm flexor innervated by the ulnar nerve, making it a reliable clinical indicator of ulnar nerve integrity. It runs from the medial epicondyle and posterior ulnar border to the pisiform, which it essentially moves as a sesamoid bone, transmitting force to the hamate and fifth metacarpal through the pisohamate and pisometacarpal ligaments.
| Origin | Medial epicondyle via the common flexor tendon; Medial margin of the olecranon and posterior border of the ulna |
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| Insertion | Pisiform bone, with extensions via the pisohamate and pisometacarpal ligaments to the hamate hook and fifth metacarpal base |
| Nerve Supply | Ulnar nerve (C7, C8) |
| Blood Supply | Ulnar artery |
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Its combined flexion and ulnar deviation action makes it the dominant wrist muscle during golf swings, hammering, and racket sport backhand strokes, and together with the extensor carpi ulnaris it produces pure ulnar deviation without any flexion or extension component.
FCU tendinopathy and calcific tendinitis at the pisiform insertion are common causes of ulnar-sided wrist pain in golfers, throwers, and baseball players. The ulnar nerve passes directly posterior to the FCU humeral origin in the cubital tunnel, and the two-headed origin of the FCU contributes to the compressive anatomy of this tunnel. Medial epicondyle tenderness at the FCU origin is a hallmark of medial epicondylalgia, commonly called golfer elbow.
The FCU tendon is the most medial wrist flexor tendon, palpable just ulnar to the ulnar artery at the volar wrist crease. Its insertion into the pisiform can be felt as the small bony prominence at the hypothenar base during resisted wrist flexion with ulnar deviation.
Degenerative overuse at the pisiform insertion producing ulnar-sided wrist pain reproduced by resisted wrist flexion with ulnar deviation, common in golfers, racket players, and manual labourers.
Degenerative tendinopathy at the FCU and flexor pronator common origin on the medial epicondyle, producing medial elbow pain reproduced by resisted wrist flexion and forearm pronation.
