tendon(redirected from annular tendon)
Also found in: Dictionary, Thesaurus, Medical.
A cord connecting a muscle to another structure, often a bone. A tendon is a passive material, lengthening when the tension increases and shortening when it decreases. This characteristic contrasts with the active behavior of muscle. Away from its muscle, a tendon is a compact cord. At the muscle, it spreads into thin sheets called aponeuroses, which lie over and sometimes within the muscle belly. The large surface area of the aponeuroses allows the attachment of muscle fibers with a total cross-sectional area that is typically 50 times that of the tendon. See Muscle
Tendons are living tissues that contain cells. In adult tendons, the cells occupy only a very small proportion of the volume and have a negligible effect on the mechanical properties. Like other connective tissues, tendon depends on the protein collagen for its strength and rigidity. The arrangement of the long, thin collagenous fibers is essentially longitudinal, but incorporates a characteristic waviness known as crimp. The fibers lie within a matrix of aqueous gel. Thus, tendon is a fiber-reinforced composite (like fiberglass), but its collagen is much less stiff than the glass and its matrix is very much less stiff than the resin. See Collagen
The function of tendons is to transmit force. They allow the force from the muscle to be applied in a restricted region. For example, the main muscles of the fingers are in the forearm, with tendons to the fingertips. If the hand had to accommodate these muscles, it would be too plump to be functional. Tendon extension can also be significant in the movement of a joint. For example, the tendon which flexes a human thumb joint is about 7 in. (170 mm) long. The maximum force from its muscle stretches this tendon about 0.1 in. (2.9 mm), which corresponds to rotation of the joint through an angle of about 21°. See Joint (anatomy)
Some tendons save energy by acting as springs. In humans, the Achilles tendon reduces the energy needed for running by about 35%. This tendon is stretched during the first half of each step, storing energy which is then returned during takeoff. This elastic energy transfer involves little energy loss, whereas the equivalent work done by muscles would require metabolic energy in both stages. See Connective tissue, Muscular system
a cord consisting of connective tissue; a tendon attaches a muscle to a bone and causes a contracting muscle to move. Tendons are composed of thick, strong, inelastic collagen fibers. The fibers are continuous with the muscle fibers at one end and are interwoven into the periosteum at the other end. Tendons vary in shape; those attached to long muscles are cylindrical, and those attached to transverse muscles are flattened and are termed aponeuroses. The centrum tendineum and galea aponeurotica are distinctive in shape. Some tendons, for example, those of the long flexor muscles of the fingers and toes, are surrounded by a synovial membrane that releases a fluid enabling the tendons to slide easily during motion.
Tendon function may be impaired by inflammation or injury. Diseases of the tendons and synovial bursae are treated conservatively. Surgery is indicated when tendons are ruptured as a result of injury.