Tendons fix muscle to bone and ligaments fix bone to bone, enabling bodily motion. Tendons are thin, tough and flexible strands of fibrous tissue that connect skeletal muscles to bone. Ligaments are present at the joints to stabilize the point of contact between two bones.
Tendons and ligaments are both mainly comprised of collagen, a structural protein that accounts for 25 to 35 percent of the body's protein. Tendons consist of dense bundles of this protein encased in protective protein sheaths. Healthy tendons consist of an array of collagen fibers that run parallel to one another. Other proteins also present in tendon composition include elastin and proteoglycans. The former facilitates the restoration of the tendon to its original shape after straining, while the latter serves to stabilize the first two proteins of the tendon.
The connection between the tendon and bone enables the passive modulation of the forces transmitted form the former to the latter. The tendons themselves do not actively supply any additional force. The elasticity of tendons is essential to the force-generation role of muscles, where tendon stretching enables muscles to generate force with little or no accompanying strain. Ligaments serve in almost the same capacity as tendons, except they connect bones together at the joints. They are also composed of collagen and elastin.