The APB is the most superficial thenar muscle and the most reliable test of median nerve motor function — it is the first muscle to be assessed in carpal tunnel syndrome evaluation (resisted thumb palmar abduction). Its atrophy produces the visible thenar wasting of longstanding median nerve compression. The APB forms the medial border of the anatomical snuffbox radially.
| Origin | Flexor retinaculum, scaphoid tubercle, and trapezium crest |
|---|---|
| Insertion | Radial side of the thumb proximal phalanx base and the extensor expansion radially |
| Nerve Supply | Recurrent (thenar) branch of the median nerve (C8, T1) |
| Blood Supply | Superficial palmar arch |
| Actions | Palmar abduction of the thumb (lifting the thumb away from the palm in the plane perpendicular to the palm); Assists thumb opposition and MCP flexion |
|---|
APB weakness is assessed by resisted thumb palmar abduction (lifting the thumb perpendicular to the palm) against resistance. Thenar wasting from APB atrophy is the sign of severe or longstanding CTS requiring urgent surgical decompression. The APB provides the clearest EMG signal for median nerve evaluation because of its superficial position.
The APB forms the most radial thenar muscle, palpable just radial to the opponens pollicis during thumb palmar abduction.
Abductor pollicis brevis atrophy from severe median nerve compression indicating longstanding CTS requiring urgent surgical carpal tunnel release.
