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In human anatomy, the mobile wad describes the portion of the forearm where three distinct muscles join and work together. The wad is responsible for such actions as flexing, twisting, and most wrist movements. These are normally thought of as more complex muscular commands that often require coordination of several groups in order to execute efficiently. In the case of the mobile wad, these muscles are the extensor carpi radialis brevis, the extensor carpi radialis longus, and the brachioradialis. All three of these forearm muscles act collectively as flexors to bend the elbow joint, lessening the angle of this joint and allowing the arm to curl upward. In some places and references this group of muscles is also referred to as the wad of Henry, the lateral compartment, or the radial group.
Muscles are usually thought of as bundles of dense fibers that assist the body in executing movement. They wrap around the skeleton adding flexibility and stability, and while each is distinct few work alone: in most cases they’re wrapped around and intersect with each other in important ways. This is certainly true where the lower arm and wrist are concerned, and one of the reasons the muscles here are referred to as a “wad” is because of how closely they’re stacked and almost woven together.
Muscles of the legs and arms are normally grouped together in compartments. These compartments are bound by a strong protective covering of connective tissue and are innervated with exclusive supplies of nerves and blood. The three primary compartments of the forearm are the mobile wad, the volar, and the dorsal.
Forearm muscles are responsible for a variety of finely controlled motions, such as bending the wrist inward and rotating the wrist around. These complex movements are possible because the forearm is composed of approximately 20 muscles layered over and anchored to the forearm bones, the radius, and the ulna. Three of these 20 muscles — the extensor carpi radialis brevis, the extensor carpi radialis longus and the brachioradialis — make up the mobile wad and work jointly to bend the arm up and inward.
The top part of the brachioradialis muscle originates at the lateral supracondylar ridge of the humerus, or the far end of the humerus where the bone begins to flare out into the nub called the epicondyle. The bottom part of the brachioradialis attaches to the styloid process of the radius, or the bump on the radius bone near the wrist. The brachioradialis muscle functions together with the other radial group muscles to flex the forearm. This muscle also works to pronate the forearm, which basically means to twist the arm toward the center of the body, and to supinate the forearm, or turn it outward.
Also originating on the top at the lateral supracondylar ridge of the humerus is the extensor carpi radialis longus muscle. This muscle is normally much longer than the brachioradialis and is anchored at its far end on the top side of the index finger’s first bone. Along with acting in concert with the other muscles in the wad as an elbow joint flexor, this muscle also controls a variety of wrist movements including extending and bending the wrist outwards.
Near the elbow, the extensor carpi radialis brevis originates at the lateral epicondyle of the humerus, which is the protrusion at the far end of the bone. This muscle extends down the forearm and is bound onto the base of the middle finger. The extensor carpi radialis brevis flexes the forearm and, much like the extensor carpi radialis longus muscle, also controls extension and outward movements of the wrist.
"That runs along the inside of the forearm" is potentially misleading because the forearm pronates. In the anatomical position, the thumbs are pointed outward placing the mobile wad in a lateral position. Otherwise it would be the medial compartment and not the lateral compartment.