Motion in the cervical spine is critical for almost any function. As we look at our surroundings, focus on objects to pick up, and track things that are moving, we use the motion in our cervical spine. Our eyes are powerful drivers of the head, and unless the trunk moves simultaneously the exact same amount as the head, cervical motion will occur. At the Gray Institute, we call this top-down driven motion.
Whenever the head moves on a stationary trunk or moves faster than the trunk, whatever the head does in three-dimensional space the cervical spine will experience the same motions at the joints. With a top-down driver, the real bone motion of the head and the relative joint motion in the cervical spine are the same.
However, understanding the motion occurring in the cervical spine joints becomes less clear when the arms are the primary driver of motion (rather than the eyes). When the trunk moves in response to the arms, the cervical spine will almost always experience motion. This motion is driven bottom-up. The table below lists the arm and trunk motions with the resulting cervical motion if the head does not move or moves less than the trunk.
A quick glance, confirms that in the spine when the motion is driven bottom up, the real bone motion (trunk) and the relative joint motion (cervical) are opposite. During function we often see the head and trunk move together. The REAL and RELATIVE are the same when the eyes drive the head more than the trunk. The REAL and RELATIVE are opposite when the arms drive the trunk faster than the head.
To learn more about real bone motion and relative joint motion that is part of Chain Reaction Biomechanics please visit the Gray Institute website.