Flexibility

Your joints allow you to move your body and manipulate objects. Movements can be small and controlled, such as when writing your name, or large and powerful, such as when throwing a ball. 

Joint structure

A joint forms where two bones come into close contact. Some joints are fixed, with the bones locked together, such as the suture joints in an adult skull. Some joints have a limited range of movement, such as the elbow or knee, while others can move more freely, such as the shoulder. 

Inside a joint

The bone ends within a mobile joint are coated with a slippery cartilage and oiled with synovial fluid to reduce friction. These synovial joints are held together by bands of connective tissue, called ligaments. Some joints, such as the knee, also have internal stabalising ligaments to stop the bones sliding apart while bending. 

Types of joints

Although your body as a whole moves in complex ways, each individual joint has only a limited range of movement. A few joints have a very limited amount of movement so that they can absorb shock, such as where the two long bones in your lower leg (tibia and fibula) meet or some of the joints in the feet. The temporomandibular joints between your jawbone and each side of the skull are unusual in that they each contain a disc of cartilage that allows the jaw to glide from side to side and protrude forwards and backwards during chewing and grinding your food. 

• Ellipsoidal 

These complex joints involve a bone with a rounded, convex end fitting into a bone with a hollow or concave shape. This allows a variety of movements, including sideways tilting, but not rotation. 

• Ball and Socket

Found in the shoulders and hips, this type of joint allows the widest range of movement, including rotation. The shoulder joint is the most mobile joint in the body. 

• Gliding

These allow one bone to slide over another in any direction within one plane. Gliding joints allow the vertebrae to slide over each other when you flex your back. They are also found in your feet and hands. 

• Saddle

This is only found at the base of the thumb and allows a similar but wider range of movement to ellipsoidal joints, including a circular motion, but without rotation. 

• Pivot

This allows one bone to rotate around another, for example when you move your forearm to twist your palm to face up or face down, (or pronation and supination). A pivot joint in your neck allows your head to turn from side to side. 

• Hinge

This type of joint mainly allows movement in one plane, rather like a door opening and closing. Good examples are found in the elbow and knee. 

"The smallest joints are found between the three tiny bones of the middle ear that help to transmit sound waves to the inner ear."

---DOUBLE-JOINTED PEOPLE---

People who are said to be double jointed have the same number of joints as everyone else, but their joints have a wider than normal range of movement. This trait is usually due to inheriting unusually elastic ligaments or a gene that codes for the production of a weaker type of collagen (a protein found in ligaments and other connective tissues).