The presumption that pain originates at the site of that pain is often an incorrect one. Many times a painful area of the body is not the reason for that pain. While it's true that sometimes the cause of a pain is the result of some other area at fault—knee pain due to poor mechanics in the foot for example—the knee pain itself comes from the knee. But at times knee pain itself does not come from the knee, it may be hip pain that's sent to the knee, where there is actually nothing wrong with the knee. This is called referred pain. Sound confusing? It's just another example of the complexities of the human body. But when understood, they become more simple.

Referred pain is common. It's defined as pain from a malfunctioning or diseased area of the body, perceived in another area, often far from the origin. A common example is found in a person having a heart attack. They often have pain down the inside of the left arm and forearm. Other areas may include:

… the gall bladder referring pain on the top of the right shoulder.

… a diaphragm problem may be felt in the shoulder and neck.