Age, stress, and disease can make joints stiffen in much the same way as wear and a lack of lubrication can cause stiffness and squeaking in mechanical hinges. The most common joint problem is arthritis, which afflicts about 43 million Americans, or nearly 20% of the population.
In patients with arthritis, the joint's cartilage lining wears away, allowing the bones to rub against each other, resulting in friction, swelling, stiffness, instability, and sometimes deformity. Severe arthritis can cause crippling pain when sufferers bend, walk, climb stairs, or even sit.
Joint pain can also be caused by deformity or by direct injury to the joint, as in the case of trauma or a sports injury. Sometimes, joint pain is made worse by avoiding use of a painful joint, which weakens the muscles and makes the joint even more difficult to move.