The occipital bone is a bone that covers the back of the head and is the only bone in the head that connects with the cervical spine. The occiput, ‘occ’ rests on the first cervical vertebra which is called the atlas or CI. The atlas rotates around what is called the odontoid process, dens or peg - a protuberance of the second cervical vertebra which is the axis or C2. This rotation allows the head to turn from side to side.
When you shake your head yes and no the atlas is at work, allowing the head to move forward and backward. The atlas axis, ‘atlanto-axial joint,’ is a synovial joint and has a greater range of motion then the other joints in the cervical column.
Placing the back of the head on the face pad of the neck machine and pushing backwards is called extension. When we completely contract in this position it is deemed “full-length extension.” Placing the front of the face on the face pad and pushing forward as far as possible without flexing the neck is called protrusion.
The skull can be extended on the atlas for about 15 degrees without participation of other cervical vertebra. With radiographic analysis of both protrusion and full-length extension you find that even though the upper cervical segments are positioned in the extension portion of their total range, only protrusion takes Occ-C1 and C1-C2 to the end-range of extension.
The implication is this in training the head and neck, that while we deem the back of a neck exercise on the neck 4-way or 5-way Pendulum machine as full-length extension, adding protrusion as part of a neck routine ensures that all the musculature involved in extension is being trained.
Retraction before Protrusion
Protrusion on the Pendulum Neck Machine