What holds the shoulder in position is not joint compression, but muscle tone -- specifically the trapezius, levator scapula, serratus anterior, and rhomboid muscles, as well as ligaments. Two of these muscles, the levator scapula and the trapezius, also attach to the neck bones (cervical spine) and/or base of the skull. Because the shoulder joint is held in position by muscle tone rather than by joint compression, and some of these muscles attach to the neck and head, unnatural shoulder mechanics -- a functional problem (in which muscles or joints do not move optimally, creating stress to the tissues) -- can cause neck pain and headaches.
The upper portion of the trapezius muscle helps elevate the shoulder blade (scapula) both at rest and in overhead positions. Often the upper trapezius will become weak or long, allowing the shoulder blade to sink too far down on the trunk. This leaves the levator scapula vulnerable because it then must do the work of two muscles. The levator scapula attaches from the scapula to the upper four vertebrae of the neck. When repeatedly tugged on, this muscle contributes to neck extension, rotation, and/or compression -- especially when the deep neck- stabilizing muscles (located in the front of the neck) are weaker than the adverse forces acting on the neck. This can occur asymmetrically, creating compression, rotation, or extension on one side of the neck more than on the other. When the neck is unable to stabilize against these forces, neck pain or headaches result.
Now think about how heavy your arm is, including your shoulder blade. Think of all the muscles, bones, nerves, blood, tendons, and ligaments. How heavy do you think all that may be? Ten pounds? Fifteen? Twenty? If the muscles holding your arm in place -- your shoulder muscles -- aren't functioning properly, suddenly you've got a big pendulum hanging from your neck, dragging it around. This can contribute to compression, rotation, and extension of the cervical vertebrae, as well as disk problems.