The APB is the most superficial thenar muscle and is exclusively innervated by the recurrent motor branch of the median nerve — the most clinically important motor branch in the hand. Its atrophy visible as flattening of the thenar eminence is the diagnostic sign of long-standing carpal tunnel syndrome, and its weakness detected by resisted thumb abduction is the most sensitive clinical test for carpal tunnel syndrome motor deficit.
| Origin | Flexor retinaculum, scaphoid tubercle, and trapezium |
|---|---|
| Insertion | Radial side of the base of the thumb proximal phalanx and extensor expansion |
| Nerve Supply | Recurrent motor branch of the median nerve (C8, T1) |
| Blood Supply | Superficial palmar arch |
| Actions | Abduction of the thumb — the only muscle that moves the thumb palmarly (anteriorly away from the plane of the palm); Assists in thumb opposition |
|---|
True palmar abduction of the thumb (moving it anteriorly perpendicular to the palm plane) is unique to humans and essential for precision pinch and opposition. The APB initiates this movement, positioning the thumb for the opposition that follows.
APB bulk and strength assessment is the primary motor examination for carpal tunnel syndrome. Thenar eminence wasting visible on inspection indicates established nerve compression requiring urgent decompression. The MMT of APB is tested by resisting upward thumb abduction (perpendicular to the palm) against the examiner's resistance. Recovery of APB after carpal tunnel release confirms median nerve reinnervation.
The APB forms the lateral bulk of the thenar eminence between the first metacarpal and the flexor retinaculum, palpable as the largest muscle belly during thumb abduction.
Thenar wasting from median nerve compression in the carpal tunnel requiring urgent decompression to prevent permanent motor deficit.
