How do joints work?

The surfaces of the joint are separated by a thin space. The joint capsule seals the joint airtight from the outside. The space that results is called the joint cavity (or synovial cavity). The joint capsule has an inner and an outer layer. In the inner layer there are blood vessels and nerves. The nerves pass on information about what position the joint is in, and send pain signals to the brain.

The outer layer of the joint capsule is made up of firm fibrous tissue. If a joint is immobilized (kept still) for a long period of time, these fibers may become shorter: The joint capsule shrinks and the joint is no longer as moveable. This is why it’s important to do mobilizing exercises to keep moving joints after injury or when confined to bed for a longer time.

Some joints have other structures inside of the joint cavity, such as the cruciate ligaments in the knee joint and cartilage discs like the meniscus in the knee joint. Other joints have a special shape. In the hip joint, for instance, the hip socket almost completely surrounds the top of the thigh bone (the femoral head). Three bones meet in the elbow joint: the elna and radius of the forearm and the humerus of the upper arm.

Cells of the inner joint capsule layer produce synovial fluid. This thick fluid nourishes the cartilage, makes sure that the surfaces of the joint glide smoothly over one another, and acts as a shock absorber by spreading out the pressure put on the joint.

The direction that a joint can move in is determined by the shape of the joint surfaces. The joints are moved by muscles. A joint's range of motion also depends on the soft tissue, ligaments or bones that are part of it.

The flexibility of some joints can be improved by doing stretching exercises. Your overall flexibility is also determined by your build, age, level of fitness and other individual factors.

As well as movable joints, there are also joints that hardly move or don’t move at all, called synarthrosis joints. Examples include the ribs and the breastbone, or the bone at the base of the spine (the sacrum) and the pelvic bones. In these immovable joints, the ends of the bones are directly connected to each other with cartilage or connective tissue. So they don’t have a joint cavity, synovial fluid or joint capsule. Some synarthrosis joints fuse and become rigid in children as they grow older. This happens to the joints between skull bones (called sutures), for instance.