The flexor pollicis longus (FPL) muscle is located in the anterior compartment of the forearm, specifically in the deep layer alongside the flexor digitorum profundus and pronator quadratus. It's an extrinsic muscle of the hand because, despite being located in the forearm, it acts primarily on the thumb.
Detailed Location:
- Forearm: The bulk of the flexor pollicis longus resides within the anterior (flexor) compartment of the forearm. This means it's on the palm-side of your forearm.
- Deep Layer: Within the anterior compartment, it's situated in the deepest layer. This means it lies underneath other forearm muscles like the flexor carpi radialis and palmaris longus.
- Adjacent Muscles: The flexor pollicis longus shares space in this deep layer with the flexor digitorum profundus (which flexes the fingers) and the pronator quadratus (which pronates the forearm, turning the palm downwards).
- Extrinsic Hand Muscle: Even though the muscle belly is in the forearm, its tendon crosses the wrist and attaches to the distal phalanx (tip) of the thumb, enabling thumb flexion. Therefore, it's considered an extrinsic muscle of the hand.
In summary, the flexor pollicis longus muscle is found deep within the anterior forearm and is responsible for flexing the thumb.
