Flexor Pollicis Longus
The Flexor Pollicis Longus is the long flexor of the distal phalanx of the thumb. Its tendon passes deep to the flexor retinaculum in its own synovial sheath lateral to the common flexor synovial sheath. This is the only muscle that flexes the interphalangeal joint of the thumb.
Fixing the proximal phalanx and flexing the distal phalanx against resistance
tests the flexor pollicis longus.