Extensors and Head Balance
In the last piece, we looked at the center of gravity of the skull in relation to head balance. Because the center of gravity is forward of the point of balance at the atlantooccipital joint, the human head is off-balance on the atlas and falls forward at this joint, which acts as a hinge. In four-footed animals, the head sits out in front of the spine with no support from below; in this cantilevered position, the center of gravity is well forward of the atlantooccipital joint and acts quite powerfully to hinge the head at the head-neck joint. In the upright human posture, the head sits atop the spine, with the center of gravity located much closer to the point of balance than in the quadrupeds. For this reason, the head in humans does not exert a force as great as that of the quadruped’s skull or even that of an ape, and the neck muscles, which serve to both move and support the head, do not have to exert a great deal of force to move the head or to maintain its balance on the atlas.
There are several key structures that move and support the head in humans. The first is the nuchal ligament, which extends from the 7th cervical vertebra to the occipital protuberance of the skull, forming a septum between the right and left halves of the neck. The second key structure is the network of suboccipital muscles, which arise from the atlas and attach to the occiput. The third are the semispinalis capitis muscles, which originate at the upper thoracic and cervical vertebra and, passing upward at an oblique angle, attach to the occiput. The splenius muscle arises from the 7th cervical vertebra and nuchal ligament and passes upward and outward to attach to the mastoid process of the temporal bone and the occiput. Finally the trapezius muscle attaches at the most superficial point to the occiput. The trapezius muscle is mainly a mover and stabilizer of the scapula, but it can affect head balance because, at its uppermost edges it attaches to the occiput at a point that can pull forcibly on the skull.
At the most obvious level, the purpose of the muscles and ligaments of the neck is to support and move the head. The suboccipitals, splenius, and semispinalis muscles extend and rotate the head. The trapezius functions largely as a scapula muscle but, when forcibly and chronically contracted, will pull the head back. All these muscles, in addition to actively moving the head, maintain its balance on the atlantooccipital joint, preventing the head from falling forward and thus maintaining the posture of the head. As part of the extensor system on the back of the body, these muscles thus play a central postural function—a function that becomes even more pronounced during the performance of strenuous activities that require the forceful action of the extensors to support and stabilize the neck and head.
But muscles do not just act upon the head; the balance of the head, in turn, affects muscles. Consider, for instance, how the balance of the head influences the action of the semispinalis muscles. These muscles, as we saw, extend and maintain the balance of the head—an obvious function that clearly plays a role in posture and balance. In the absence of a counterposing force, these muscles would pull the head back, which would in turn disrupt not only the balance of the head but posture in general. But the head is acted upon by gravity and, because of this, creates forces of its own. Because the head falls forward at the AO joint, the semispinalis muscles don’t simply pull he head back but are kept stretched by the action of the head which, by falling forward, keeps the muscles from shortening. In this way, the balance of the head influences the neck muscles, just as the neck muscles influence the balance of the head. In the illustration, the AO joint is circled in green; the red dot marks the approximate location of the center of gravity of the head.
The balance of the head, and its relationship to the neck muscles, plays an even more global role in upright posture. To support upright posture, the extensor muscles of the spine maintain the support of the trunk—a role that becomes quite obvious when we go into a postural slump. In this posture, the extensor muscles of the back are inactive and, because the spine lacks the support needed to maintain upright posture, we collapse in the chair—a situation that is reversed only when we re-engage these extensor muscles and sit “up” in the chair. This “corrected” posture, however, is not the natural state. To sit normally, we do not actively need to arch the lower back but utilize the entire musculature of the back—a condition that can be readily observed in young children, whose normal upright posture is maintained by the active lengthened tone of the extensor muscles of the back.
This erect posture, in turn, is directly related to the balance of the head. In the slumped posture -- as in the photo to the right -- the head is no longer sitting atop the spine but, instead, is actively pulled upon by the muscles of the neck which, in this shortened state, clearly pull the head back and down.
With the head pulled back in this way, upright posture is virtually impossible because, instead of exerting an upward force on the neck muscles, the head is pressing down upon the spine and the spine is unable to lengthen. To restore upright posture, the neck muscles must lengthen to allow the head to go up in space and, at the same time, to release forward at the AO joint.
This allows the spine to lengthen and the extensor muscles of the neck and back to regain their supporting role. We can see this in young children, whose natural, upright posture includes a skull balanced forward on the spine and lengthened neck muscles. In this natural state, the head acts as an inertial device to counteract the pull of the extensor muscles of the neck and to help lengthen the spine.
It is useful at this point to contrast this upright human arrangement to that of a four-footed animal. In a cat or horse, the head is cantilevered and, in this position, exerts stretch on the neck muscles, which in turn maintain the posture of the head.
Although radically altered in humans, something of this four-footed arrangement is preserved in the upright human posture, where the head, sitting on top of the spine, sits off balance and thus continues to exert stretch on the neck muscles. In contrast to the horizontal pull of the cantilevered skull in the four-footed animal, however, the forward balance of the skull on the vertically-positioned human spine acts upward against the downward pull of the muscles.
This is a very different arrangement than in the four-footed animal. There, the cantilevered head produces a very powerful horizontal pull on the neck muscles which, in turn, maintain tone in the neck to support the weight of the head.
In the upright human arrangement, the muscles of the neck and spine pull directly downward, and the forward tilt of the head acts upward to counteract the downward pulls. This arrangement is more easily disturbed than that of the quadruped because, unlike the animal’s head, the human head can easily be pulled backward, thus disrupting the upright support system
—a fact that at least partially explains why humans are so prone to slumping. To counter these downward forces, the head must retain its off-balance position on the spine. The forward balance of the head in relation to the spine is thus an essential feature of the upright human postural system.
To support upright posture, then, the muscles of the back must actively support the head and spine. At the same time, the head must actively balance forward so that the muscles of the neck are stretched and toned in the context of length. In this sense, both muscles and head balance are involved in upright posture. Posture is maintained by the extensors of the back and the small spinal muscles. And head balance, acting upon the muscles of neck and spine, exerts stretch on the muscles. To support upright posture, both muscles and skeletal forces are required. Without muscles, there would be no forces to act upon the skeleton. Without the forward balance of the head, there would no inertial force to counteract the downward pull of the neck muscles acting upon the head. In the illustration to the right, the red mark is at the approximate location of the center of gravity of the skull. Notice that it is now directly above the AO joint.
It is important to mention that, although we often speak of “good” and “bad” posture, posture is dependent on the natural, balanced working of bones and muscles required to produce efficient forces. We are also not speaking here about a method for making adjustments but identifying the elements involved in posture as a normative function. We are not saying that, when the system works one way it is working better and the other way, worse; it is designed to work with the balance of the head counteracting muscles; length is a function of its design.
Thus the forward balance of the skull on the spine plays a central role, not only in the maintenance of the posture of the head but of upright posture in general. We are speaking here of an anatomical relationship, and have not addressed how this relates to the various neural mechanisms (proprioception, stretch reflexes, and so on).