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The extensor carpi ulnaris is a muscle located in the posterior compartment of the forearm, or on the same side as the back of the hand. Of the many muscles found here, which are all innervated by the radial nerve, it is one of the most superficial, or closest to the skin. Located alongside the extensor carpi radialis longus and brevis, this muscle is primarily responsible for extension of the wrist, or decreasing the angle between the back of the hand and the forearm as in bending the wrist backward. It is also involved in angling the hand backward and inward toward the pinky side.
A narrow, band-shaped muscle, the extensor carpi ulnaris is found on the side of the ulna bone in the posterior forearm, or on the back of the lower arm aligned with the pinky finger. It originates on the lateral epicondyle of the humerus, which is one of the two rounded bony prominences at the lowest point on the large bone in the upper arm. The lateral epicondyle, which is the prominence farthest from the trunk of the body, can easily be felt about an inch in front of the elbow on the outside of the arm. It also arises just below this point from the posterior border of the ulna on the back of the upper forearm.
From here the muscle runs down the posterior side of the forearm alongside the extensor digitorum muscle, which extends the fingers, to insert at the bottom of the fifth metacarpal bone. The fifth metacarpal lies beneath the pinky finger and is one of five long bones contained within the palm of the hand. Connecting the extensor carpi ulnaris to this bone is a tendon, which begins above the wrist. It is the tendon of this muscle that radiates into the hand, crossing the carpal bones and thus acting on the wrist joint.
As its name suggests, the extensor carpi ulnaris is an extensor of the wrist or radiocarpal joint, contracting in the forearm to bend the wrist backward. Since with continued action this muscle is involved with extension or straightening of the elbow joint, it can be exercised indirectly by performing cable triceps extensions, particularly when utilizing a rope attachment. This is because the extensor carpi ulnaris also inclines the hand toward the pinky side, and when pushing downward on the rope, which is shaped like an inverted V, the hands perform this exact angling motion at the bottom of the extension when the elbow and wrist are fully extended. Thus all the actions of this muscle are incorporated into a single exercise.