Posterior arm muscles. Computer artwork of an oblique posterior view looking up a human arm, showing its muscles and tendons (red and white), and also its arteries (red) and veins (blue). The uppermost muscle, wrapping around the shoulder, is the deltoid. Below it are the triceps. In the forearm are the extensor carpi radialis longus and brevis, extensor digitorum, and extensor carpi ulnaris muscles. All these muscles act to extend (straighten) the upper and lower arm, hand and fingers around the elbow, wrist and knuckle joints. Muscles on the other side of the arm cause the opposite action (flexion) to bend the joints.
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