Tendons consist of strong fibrous tissue that connect muscle to bones, muscles, or other structures. These tough cord-like structures transfer the forces exerted by muscles to the skeletal system, enabling body movement. Sometimes the attachment points are near, but in other cases the tendon can be rather long (several inches/many centimeters), such as the tendons that attach the muscles in the forearm to the bones at the tip of the fingers.
Tendons not only transfer force, but the elastic properties of some tendons enable them to store energy. The Achilles tendon that attaches to the back of the heel stores energy during running and releases it a short time later, improving the efficiency of gait.
Tendons contain collagen fibers that provide strength. Generally, these are aligned with the long direction of the tendon. Electron microscope images show tendons to have a wavy microstructure prior to loading. This causes tendons to have an initially low elastic modulus that can be characterized as the exponential portion of the stress-strain curve. As the fibers straighten, the stiffness stabilizes and the material exhibits linear elastic behavior, yielding a straight section of the stress-strain curve.
