tendon, tough cord composed of closely packed white fibers of connective tissue that serves to attach muscles to internal structures such as bones or other muscles. Sometimes when the muscle involved is thin and wide, the tendon is not a cord but a thin sheet known as an aponeurosis. The purpose of the tendon in attaching muscle to bone is to enable the power of the muscle to transfer over a distance. For example, when one wants to move a finger, specific muscles in the forearm contract and pull on ligaments that in turn pull the finger bones to produce the desired action.

Achilles' tendon (tendo calcaneus), sinew prominent at the back of the ankle, connecting the tendons of the calf muscles to the heelbone. When the musculature contracts, the pull on the Achilles' tendon elevates the heel in the springy motion essential to running and jumping. Since the effect is to lift the entire body weight against a severely adverse leverage ratio, the Achilles' tendon by necessity is the toughest and strongest of human tendons. The name derives from the mythical Greek hero Achilles, who was vulnerable only in the heel.

Tissue attaching a muscle to other body parts, usually bones, to transmit the mechanical force of muscle contraction to the other part. Much like ligaments, tendons are composed of dense, fibrous connective tissue with a high collagen content, which makes them remarkably tough and strong, with great tensile strength to withstand the stresses generated by muscle contraction.

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