Tendons and ligaments both consist of fibrous connective tissue containing collagen, a tough, fibrous protein. A tendon is a dense form of connective tissue that attaches muscles to bones. A tendon serves to move the bone by pulling on it as a muscle contracts. It also helps absorb the impact muscles experience as they move. The calcaneal tendon (also known as the Achilles tendon) connects the calf muscles to the heel (calcaneus bone) and is the largest tendon in the body. The calcaneal tendon is notoriously prone to injury.
A ligament is a tough, fibrous band of connective tissue that connects bones together at joints. Ligaments contain more elastic fibers than tendons to allow for normal movement yet are still tough in order to provide necessary strength and stability. This stability is critical for limiting the range of motion beyond normal movement. For example, the four main ligaments of the knee stabilize the sides, front, and back of the knee to keep the knee from moving too far in any one direction. A common knee injury is associated with one of these ligaments, the anterior cruciate ligament (ACL). An ACL tear is an injury that often occurs in athletes involved in sports, such as skiing or gymnastics, that involve sudden shifts in direction, stopping, or impact.
Both tendons and ligaments are responsible for many surface markings, or protrusions, on bones at the point where they attach. The force of the pull from these bands stimulates the bones to reinforce the area to prevent bone fractures, creating bony projections in the form of bumps and ridges on the surface. For example, when the deltoid (shoulder) muscle contracts, the arm lifts out to the side of the body. Where the deltoid muscle attaches to the humerus, the upper arm bone, there is a projection called the deltoid tuberosity.