An extensor muscle is a muscle that works to extend or straighten a body part, effectively enlarging the angle between body parts. Examples of extensor muscles include the muscles that straighten the elbow or the knee. The opposite equivalents of extensor muscles are flexor muscles, which work to close a joint, for example to bend the arm at the elbow or the leg at the knee.
In addition to the large extensor muscles that operate the limbs, there are a great number of smaller extensor muscles throughout the human body. Examples include the muscles that serve to extend the fingers and toes; the extensor muscle of the foot, called the extensor longus muscle, which flex the foot upwards at the ankle; and the important opponens pollicis, which serves to flex the basal joint that provides for thumb opposition. Other important extensor muscles are present along the spinal area of the back, and enable the spine to be flexed.
The forearm extensor muscles fall into two main groups, the superficial muscles, and the deep muscles. There are four main superficial extensor muscles in the forearm, called the extensor digiti minimi, the extensor carpi radialis brevis, the extensor carpi ulnaris, and the extensor digitorum muscle. All of these muscles are anchored to the bone of the upper arm, the humerus, by a common tendon, known as the common extensor tendon. These muscles serve primarily to extend the wrist and fingers.
The main muscle that works to straighten the arm by extending the elbow joint is the triceps brachii extensor muscle. This large muscle is situated on the back of the upper arm. Its name means "three headed muscle" in Latin, and it is so called because it has three points of origin, two on the humerus and one on the shoulder blade, or scapula. Three muscle bundles arise from these points, and then join together at the elbow. In addition to extending the elbow, the triceps brachii extensor muscle can also hold the arm at a fixed angle, for example for writing.
All of the extensor muscles of the human body are muscles of the striated type. They are formed from a mass of parallel fibers called myofibers. These myofibers are built up of many long, cylindrical cells, each of which has multiple nuclei. In the structure of an extensor muscle, the myofibers are held together by means of connective tissue.