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Flexor Pollicis Longus (FPL)

Key Points:

• Flexes the thumb at the interphalangeal joint.
• Extrinsic thumb muscle.

Muscle Function:
Primary: Flexes the interphalangeal joint of the thumb.
Secondary: Assists in metacarpal-phalangeal and carpal-metacarpal joint flexion. May assist in flexion of the wrist.
Palpation: Palpate the flexor pollicis longus tendon as it crosses the proximal phalanx of the thumb on the palmar side.
Strength Testing: Position – the hand in neutral. Stabilize – the hand. Resist - the distal phalanx from interphalangeal joint flexion of the thumb. The direction of resistance is towards extension of the interphalangeal joint. Or put the patient’s interphalangeal joint of the thumb into a flexed position.  Tell the patient not to move from that position as the examiner tries to extend the interphalangeal joint.
Possible Substitutions: Includes the extensor pollicis longus by hyperextending the interphalangeal joint and allowing it to relax. This can sometimes appear to be flexor pollicis longus function when actually it is the relaxation of the extensor pollicis longus.
Relevant Anatomy:
• Roots: C7, C8, T1.
• Nerve: Median nerve.
• Innervation Route: C7, C8, T1 → median nerve → flexor pollicis longus branch.
Origins: Anterior surface of body of radius below tuberosity, interosseous membrane, medial border of coronoid process of ulna, and/or medial epicondyle of humerus.
Insertion: Base of distal phalanx of thumb, palmar surface.