Joints and Movement

Structure and mobility are related, not identical

A joint is where bones meet. Fibrous joints use dense connective tissue and allow little or no movement, as in skull sutures. Cartilaginous joints join bones with cartilage and allow limited movement, as between vertebrae. Synovial joints have a fluid-filled cavity and usually allow the most movement, as at the knee. Structure predicts movement, but the categories are not perfect synonyms for fixed, slightly movable, and freely movable.

Do not judge a joint only by its location. Ask what connects the bones and whether a cavity is present. Dense fibers point to a fibrous joint, cartilage between bones points to a cartilaginous joint, and a capsule around a fluid space points to a synovial joint.

Synovial joints also differ by shape. A hinge joint mainly bends and straightens, as at the elbow. A pivot joint allows rotation, as in the upper neck. A ball-and-socket joint moves in many planes and rotates, as at the shoulder and hip.

The synovial membrane lining the joint capsule produces synovial fluid, which lubricates the moving surfaces.

A low-friction, stabilized cavity

In a synovial joint, smooth articular cartilage covers bone ends. Synovial fluid reduces friction. A capsule surrounds the space. Ligaments connect bone to bone and add stability. Tendons connect muscle to bone and carry the muscle’s pull. A torn ligament affects joint stability; it is not the same as a torn tendon.

Cartilage has little direct blood supply, so it repairs slowly. Bursae are small fluid-filled sacs that reduce rubbing near some joints. Menisci are cartilage pads that improve fit and spread force in joints such as the knee. These structures support smooth motion but do not create the muscle force.

Keep connective structures distinct. A ligament joins bone to bone and helps stabilize a joint; a tendon joins muscle to bone and transmits muscle force. Articular cartilage reduces friction and distributes load at bone ends, while synovial fluid lubricates the joint cavity. A movement problem caused by a torn tendon is therefore not the same as joint instability from a torn ligament.

Structures of a synovial joint

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Name the change in angle or direction

Flexion decreases a joint angle, such as bending the elbow. Extension increases the angle. Abduction moves a part away from the midline; adduction brings it back. Rotation turns a bone around its long axis. Circumduction makes a cone-shaped path by combining movements.

Use anatomical position as the reference. Moving the arm sideways away from the trunk is abduction; bringing it back is adduction. Turning the palm upward is supination, and turning it downward is pronation. Plantar flexion points the toes down; dorsiflexion brings them toward the shin.

Joint shape constrains movement

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Force, fulcrum, load, and range

A bone acts as a lever. A joint is the fulcrum, muscle force is the effort, and the body part or object is the load. Most body levers favor speed and range of motion rather than force. Moving a weight farther from a joint increases the turning demand, so the muscle must create more force.

Worked example: Hold a book close to your elbow, then hold it at arm’s length. The book weighs the same, but the longer distance from the elbow creates a larger turning effect. The elbow flexors must pull harder. This shows why load position changes effort.

Your quick summary

• Fibrous joints move least; synovial joints usually move most.
• Cartilage, fluid, ligaments, and tendons have different jobs.
• Flexion closes an angle; extension opens it.