Your thumb is capable of a wide range of motions, in part because of the work of three tendons. These tendons assist your joints by abducting and extending the parts of your thumb. Each tendon in the thumb begins as a larger muscle in the forearm before passing through the wrist and into your thumb.
Extensor Pollicis Longus
Your thumb has two tendons located at the top of the digit. One is called the extensor pollicis longus. This tendon begins as a long muscle in the forearm before extending through the wrist and into the thumb. The tendon is responsible for extending the distal phalanx of your thumb at the interphalangeal and carpometacarpal joints. In other words, the tendon allows you to flex your thumb at the joint nearest the tip of your thumb.
Extensor Pollicis Brevis
The second tendon located at the top of the thumb is the extensor pollicis brevis. This tendon also begins as a muscle in your forearm before passing through a groove in the lower radius and being inserted into your thumb. The extensor pollicis brevis works in conjunction with the extensor pollicis longus to flex and abduct the thumb at the metacarpophalangeal and carpometacarpal joints, allowing you to move your thumb at the base and middle thumb joints.
Abductor Pollicis Longus
The abductor pollicis longus begins as a muscle attached to the ulna at the exterior of the arm before crossing the forearm to the radius and ending in a tendon at the interior base of the wrist and thumb. The tendon is responsible for extending and abducting the thumb at the base, or carpometacarpal, joint.