We began this series by noting that the weight of the skull is balanced toward the face, in front of the atlantooccipital ("AO") joint. In order to add some precision to this discussion, we located the center of gravity of a skull at the rear interior portion of the sphenoid bone, using the simple experimental method of suspension. In this piece, we discuss some implications of the location of the skull’s center of gravity.
With its weight mostly ahead of the AO joint, there is a tendency for the skull to roll forward off the joint, as in the illustration to the left. To keep this from happening, muscles and ligaments attach to the base of the skull behind the AO joint, where they exert counteracting forces. In the illustration to the right, we show the counteracting force of the muscles as a weight hanging from the base of the skull.
There are many muscles attaching to the skull’s base, and it is important to our present discussion to note that they attach at various distances from the joint. Some are quite close to it. The sternocleidomastoid, for instance, attaches at nearly the same plane as the joint, though still behind it. Others attach at longer distances from the joint, like the trapezius at the occipital ridge.
These different distances have a dramatic effect on the forces necessary to counteract the forward momentum of the skull. This principle is known as mechanical advantage. The farther away from a hinge that a force acts, the smaller that force needs to be to move the hinged object. A given amount of force acting far from a hinge may move the object; the longer distance gives it higher mechanical advantage. But apply that same amount of force much closer to the hinge, and it may not be enough to move the object. A muscle with higher mechanical advantage, acting farther from the joint, moves bone more efficiently than one with less. We can demonstrate this by hanging weights from the base of the skull behind the AO joint. From directly behind the joint, a given weight works to stabilize the skull; from farther away, perhaps half that weight suffices. The drawings below show weights hung from two distances from the AO joint (the weight figures are intended to illustrate the concept, and are not exact).
We might expect nature to exploit this basic principle by placing smaller, weaker muscles in positions of higher mechanical advantage, since a smaller force from there will suffice to move the bone. However, we actually find larger, more powerful, more superficial muscles at these positions. At the skull, we find long sheet muscles like the trapezius attaching at the outer edge of the occipital bone, while we find shorter muscles like the suboccipitals attaching closer to the AO joint.
There is another way of looking at this situation, one that allows us to restate these same facts but emphasize a separate implication of the principle of mechanical advantage. The skull’s tendency to roll forward means not only that muscles must act on it to keep it balanced, but that the skull itself acts back on the muscles with the force of its forward momentum. The implication is that the skull is an active agent. It is not simply an inert structure being moved about by muscles. It is a relationship of bone and muscle.
As we note above, the force with which muscles must act on the skull is inversely proportional to their distance from the AO joint; in the same way, the force that the skull’s forward momentum exerts on muscles is inversely proportional to their distance from the AO joint. Muscles attaching farther back on the skull will experience the least force, while muscles attaching closer to the joint will experience greater force. In fact, the amount of force has mostly to do with this distance. Double the distance, and the force quadruples; triple the distance, and the force is nine times greater. A muscle farther from the joint, even just a little farther away than the next, has a very different relationship to the skull than the one closer in.
In our next piece, we discuss the relationship of head balance to the musculature of the back.