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The extensor carpi radialis brevis is a muscle of the posterior compartment of the forearm, found on the same side as the back of the hand. Its name is a nod to its function as an extensor of the wrist or radiocarpal joint, meaning that it bends the wrist backwards. The second part of this muscle’s name refers also to its location, as it is situated on the thumb side of the arm above the radius bone, and to its form, as it is slightly shorter than its sister muscle, the extensor carpi radialis longus. In addition to its action as an extensor of the wrist joint, the extensor carpi radialis brevis abducts the wrist, or bends the hand sideways toward the thumb side.
This muscle finds it origins on the inside of the elbow joint, where it begins on a small bony bump on the base of the humerus bone known as the lateral epicondyle. The extensor carpi radialis brevis shares this point of origin with several other muscles in the forearm, as all arise from a tendon attached to the lateral epicondyle known as the common extensor tendon. It also originates on a ligament of the elbow called the radial collateral ligament as well as on the sheath of fibrous tissue called the aponeurosis that envelops the muscle.
From this point on the inside of the elbow, the extensor carpi radialis brevis runs down the radial side of the posterior forearm, where its fibers converge and form a tendon that crosses the wrist joint and enters the hand. Specifically, this tendon runs to the inside of the fibers of the extensor pollicis brevis and abductor pollicis longus, small muscles that extend and abduct the thumb, and enters the wrist alongside the tendon of the extensor carpi radialis longus. It then passes under the dorsal carpal ligament, a broad flat ligament that crosses the base of the hand laterally and holds all of the ligaments together in a bundle. Finally, it attaches to the base of the third metacarpal bone, the long bone just beneath the middle finger, on its dorsal surface toward the lateral or thumb side.
Contractions of the extensor carpi radialis brevis, via its insertion in the hand, act to pull the back of the hand toward the forearm and inward slightly toward the thumb side. As such, this muscle is commonly utilized during such movements as typing on a computer keyboard. Should it become tight from overuse, it can be stretched by bending the wrist so that palm side of the hand, especially the thumb side, approaches the anterior forearm.