The dynamic interplay between bones and muscles also plays a central role in how the spine functions in upright posture. The spine is made up of twenty-four movable vertebrae, as well as the sacrum and coccyx. As the central bony structure upon which muscles act, the spine forms a compression-resistant column for bearing the weight of the skull and the trunk as a whole. This column is made up of a series of vertebral bodies that increase in size as they descend toward the sacrum. Between the vertebral bodies are intervertebral discs that act as shock-absorbers. In the illustration to the right, the intervertebral discs are shaded in blue.
Posterior to the vertebral body is the vertebral arch, which protects the spinal cord and also provides attachments for ligaments and muscles. These ligaments bind the vertebrae together and limit the range of motion
of the vertebrae and intervertebral joints. In the illustration to the left, the ligaments are the threaded webbing between the bones.
With only ligaments supporting it, however, the spine is a rather inert structure that would collapse under the weight of the trunk. It is the intervertebral muscles, acting upon the vertebral attachments, that support the curves of the spine and thus maintain its internal length. Thus the spine acts as a supporting column but is itself supported by a series of small postural muscles that maintain its internal support, aided by muscles on the anterior spine in the lumbar and cervical regions. In the illustration to the right, we see some of the deep postural muscles of the spine, which run from vertebra to vertebra up the entire length of the structure.
But these intervertebral muscles cannot perform their function simply by “pulling” on the vertebra. When the spine is collapsed, as in the familiar postural slump, the postural muscles are disengaged and the spine has no internal support. For the postural muscles to function in their supporting role, the neck muscles must be lengthened and the head must balance forward so that the spine can regain its natural length. This allows the deeper postural muscles to come into play and to act upon the vertebral processes. The spine then acts as a lengthening device in which muscles act upon bones, but bones oppose each other so that the muscles perform their function within this larger context. In the illustration to the left, we see the forward balanced head exerting upward pull on neck muscles; the lengthened spine exerts pull on deep postural muscles.
When the spine lengthens in this way, we can see that, instead of being compressed by the head and trunk, the spine as a whole lengthens and the intervertebral discs, which regain their natural buoyancy, exert a hydraulic force against downward pressure. In this sense, the spine is a lengthening device with hydraulic as well as compression-resistant properties. In this dynamic state, the spine does not function as a fixed column to support the head and trunk; it is acted upon by muscles and at the same time acts upon muscles to form a complex tensegrity structure that is able to dynamically lengthen in response to gravitational forces.
Thus we see that head balance in relation to the lengthening spine acts as a central mechanism which acts upon muscles and within which muscles act, to produce active lengthening against gravity.