We saw in the last piece that, to support upright posture, muscles actively support the head and spine. At the same time, the forward balance of the head exerts stretch on muscles, which do not simply contract but are kept lengthened as part of their normal resting length. Posture is thus maintained by a combination of forces that includes the inertial force of head balance, which counteracts the pull of the neck muscles, as well as the action of the extensors of the back, which act on the head and spine even as they are acted upon.
But how do the muscles of the back act upon and support the spine and trunk, and how does head balance relate to the these structures? Two key groups of muscles maintain the erect posture. The first group is comprised of a series of small muscles running in between the vertebrae of the spine along its entire length, from the sacrum to the occiput of the head. Acting directly upon the spinous and transverse processes of the vertebrae, these deep small muscles maintain the spine's internal length and support. See the illustration to the left. Several muscles, including multifidus, rotatores, interspinalis, and others make up this deep internal layer.
The second group is the sacrospinalis or erector spinae muscles. These are the long muscles that run lengthwise up and down the back, from the sacrum right up to the base of the skull, in overlapping bundles that leapfrog from the bottom to the top, forming a continuous sheet of muscles supporting the entire back. When we are standing, and particularly if we incline forward, bend down, or lift weight, the trunk needs muscular support. It is the sacrospinalis muscles that maintain this support and thus play an absolutely essential role in our upright posture. See the illustration to the right. Taken as a whole, the overlapping bundles of muscles make up the sacrospinalis sheet.
But these muscles do not simply “pull” on the spine to maintain erect posture. We saw in the last piece that, when we slump, this musculature is disengaged, with the result that, when we need to sit upright, the only way to accomplish this is to arch the back. In this case, the muscles of the lower back quickly tire, as we all know when we carry a lot of heavy weight in our arms and the muscles in the lower back become fatigued and start to burn. In the normal upright posture, however, this is not how these muscles actually work. When the head is balanced forward and the neck muscles are lengthened, the back as a whole is lengthened and, instead of shortening, the sacrospinalis muscles are kept lengthened. In the illustration to the left, the forward momentum of the properly balanced skull is shown exerting an upward force on muscles. The spine is acting as a lengthening device, keeping muscles naturally lengthening and performing work.
As with the muscles of the neck, these muscles are lengthened within their bony attachments, and maintain postural support in the context of this length. They are designed to do work, but they are meant to be lengthened so that, when they do work, they do it in the context of length.