The shared job of tendons and muscles is to function together to produce a pulling force. Muscles that are voluntary, such as skeletal muscles, contract to exert the force, and tendons in turn modulate this force. The tendon is a strong, flexible band of fibrous connective tissue that joins muscle to bone, typically across a movable joint, and enables muscle to function and exert greater force. The tendon's role is to withstand tension and distribute the force, essentially allowing the muscle to do its job properly. Without tendons, muscles would be rather useless; they would not be properly connected to bones, and therefore be unable to generate the necessary force to trigger body movement.
Until as late as the 1990s, it was thought that the only purpose of the tendon was to link muscle and bone. It was assumed that tendons merely transmitted forces between muscles and bones in a passive manner, without really affecting them. Recent research has revealed, however, that tendons, which are made of strong, elastic protein collagen, also have the ability to function as springs when muscles contract.
For example, when a person is walking, the muscles in his legs are generating forces while tendons are adapting these forces, applying them wherever they are most needed. This action allows the person to remain stable and keep his balance. The tendon performs this job without producing any extra work.
In addition, tendons and muscles can act together to efficiently store and recover energy. When tendons are stretched, they store the energy produced by muscles, then release the elastic energy when they are returned to their natural state. This means that muscles can operate with little to no difference to their resting length, which enables them to create greater force.
While parts of the leg and foot are most commonly referred to as examples of where tendons and muscles function together, the two perform as a team throughout the human body, anywhere there is skeletal muscle. Tendons have a different appearance in different areas of the body. For example, in the leg, tendons are thin and short structures that resemble rods, while in the abdomen and thorax, tendons take on the form of large sheets to match the large sheets of that area's muscles. While their appearances may change, tendons and muscles work together to serve the same function throughout the body: to animate the skeleton, with muscles contracting to generate force and tendons distributing this force properly.