A quick guide to bones and joints

Your body has hundreds of joints, bones and muscles. Here's a quick read on how they work together to keep you moving.

Bones are connected to each other at joints. The body's joints can be grouped into three classes based on the amount of movement they allow: fixed joints, slightly movable joints and freely movable joints.

• Fixed joints: These joints allow no movement whatsoever. Examples of fixed joints include those separating the bones of the pelvis, which bend only during delivery to ease the baby's movement down the birth canal and the joints (known as sutures) between the bones of the skull.
• Slightly movable joints: These joints allow a limited amount of motion. For example, the bones of the spinal column are held together by tough pads of fibrocartilage. These so-called intervertebral joints secure the bones tightly while still providing the flexibility to let you bend and stretch.
• Freely movable (or synovial) joints: These are the joints that usually leap to mind when we think of arthritis. Examples are the hips, knees, elbows, fingers and — most mobile of all — the shoulder joint, whose ball-and-socket structure enables you to move your arm in a complete circle.

• The human skeleton contains more than 200 bones, which are connected by almost 150 joints.

• The bones of the joint are covered by a tough, fibrous covering called the joint capsule. The capsule's outer layer is made of interwoven bands of collagen fibres, which provide the joint capsule with strength and flexibility.

• This is a delicate layer of tissue that lines the joint capsule's inner surface, producing and releasing synovial fluid.
• This clear, sticky fluid is 95 per cent water, has the consistency of egg white (synovia means "like egg white") and helps nourish and lubricate the cartilage and bones within the joint capsule.
• Aided by the synovial fluid, the cartilage-tipped bones in a healthy joint glide over each other smoothly, creating even less friction than ice sliding on ice.

• These strong, flexible bands of connective tissue are made of collagen fibres, and help to stabilize a joint by binding together the bones within it.
• Most ligaments lack elasticity, but some stretch a little to allow the bones that they connect to separate slightly. Others prevent the joint from moving a certain way.
• If you dislocate a joint, it is best to get it corrected immediately, as ligaments stretch when under that kind of force, which inevitably leads to a weakened joint that will likely dislocate again and again.

• Also known as sinews, tendons help the ligaments to stabilize and support the joints, these strong white cords of fibrous tissue serve to attach muscles to the bones of the joint. They work almost like springs, storing energy and adding support to a joint through the cycle of its normal movements (for example, the achilles tendon).

• Muscles provide the forces that move the bones within a joint. Even the simplest movement requires at least two muscles acting in equal and opposite ways: One contracts and pulls on its attached tendon, which in turn pulls on a bone to move it. At the same time, the opposing muscle relaxes to allow the movement to occur.
• By age 65, more than half of all people have X-ray evidence of osteoarthritis in at least one joint.
• Exercises that protect and strengthen the muscles, tendons and ligaments that surround a joint can minimize pain and stiffness within the joint itself.