Posture and Motion

 The word posture brings to mind images of ideal body positions, where everything is held in perfect alignment.  In real life, the maintenance of upright human posture is a complex, dynamic activity, because the body is designed by our evolutionary history to be constantly in motion.  If we wish to arrive at a viable theory for a healthy posture while sitting, we first have to understand how the body maintains upright posture during the normal activities of standing and moving.

Spinal stability

The human spine is a highly flexible stack of 24 interlocking vertebrae (plus the sacrum and coccyx), requiring elaborate bracing from the core musculature in order to support the weight and movement of the upper body. The erector spinae muscles pull the head and trunk upright against the force of gravity; the deep multifidus, quadratus lumborum, psoas and transverse abdominus muscles stabilize and compress the spine like guy wires on an unstable tower, allowing it to assume sufficient stiffness to bear the upper body’s weight.  Without the bracing of these spinal support muscles, the lumbar/thoracic spinal column can only support a weight of 5 lbs. before buckling (1), or a weight of 20 lbs. for the lumbar spine alone (2).

The crucial point to understand regarding back health is that in order to keep our spine upright, core muscles must provide support at all times when we are standing (and when we are sitting).  However, human muscle fibers cannot contract endlessly; they have to alternate exertion with recovery to maintain blood flow and avoid fatigue. (3)  So how do postural muscles support the spine all day long without becoming exhausted?

Posture’s natural motion

The simplest way to let postural muscles vary their exertions is to vary the weight-bearing stress on the core.  Humans have two points of contact with the ground, so gravity's force upon the core structures can only be varied between these two points, by shifting weight from one leg to the other.  Since people rarely stand still during daily activities, this weight transfer is constant.(5) 

 Research shows that the body uses this motion to avoid static postures, regularly shifting weight distribution 2-3 times per minute even when standing still.(6) This automatic postural variation, called postural sway, is remarkably consistent, but most people are unaware of their body’s subliminal motion.  These movements are made discreetly, because there is a strong social prejudice against “fidgeting”.  However, once you know what you’re looking for, it’s easy to observe these constant small postural shifts in any group of people who are standing still.  We all do it constantly, every 20 seconds or so!  When you see someone swaying back and forth with a child in their arms, they're speeding up the sway motion to relieve the increased load on their lower back. 

An experiment: verify postural sway for yourself

To better understand why the body utilizes this motion, try this simple experiment:Stand with your back just barely touching a wall, straight and tall but relaxed, with knees not locked, and arms hanging comfortably at your side.  Try to hold perfectly still, using your light contact with the wall as a gauge.  In less than a minute you’ll feel a strong urge to move to one side or the other, or to sway forward and back, but try to keep motionless.  You'll notice that even with your arms and legs staying fairly relaxed, a tension will start to build in your lower back, and around the sides of your trunk.  See if you can continue to hold absolutely still for three or four minutes; it’s not easy!  When the tension gets uncomfortable, shift your weight slightly over on to one leg, then slowly shift it over to the other leg.  You’ll notice immediate relief as the transfer of force to one side of the pelvis allows your lower back muscles to adjust to the redistributed load.  This is why our body maintains this motion at all times while we are standing.

It’s not surprising that our core muscles are conditioned to this side-to-side weight alternation, since walking is the most common human motion.  Walking is often called the best exercise, because it utilizes over 200 muscles in the legs, trunk and arms.  As you have just experienced, simply transferring one’s weight from side to side has a major affect on the spinal support muscles.

Active sitting: continuing postural sway while sitting

How does this relate to sitting?  In our experiment, if you continued standing motionless, your postural muscles would tire and eventually give out.  You would find yourself slouched over even while standing, putting damaging stresses on your spinal discs and ligaments.  Forcing someone to maintain a static posture is, in fact, a common method of torture.

When we sit down in a normal chair and can no longer shift postural stress from side to side, our core muscles eventually become exhausted, and we collapse into the slouched sitting position.  A healthy sitting posture, with the spine supported in a neutral position, is only possible if you can continue the constant adjustment of posture that the body is accustomed to.  This is what we mean by Active Sitting, and only the Core-flex chair brings you the patented technology to benefit from this revolutionary understanding about posture’s necessary motion.

1  Lucas, D., & Bresler, B. (1961). Stability of the ligamentous spine. In: Tech report No. 40, Biomechanics Laboratory, University of California, San Francisco.

2  Crisco, J.J., & Panjabi, M.M. (1992). Euler stability of the human ligamentous lumbar spine, part I theory, 7: 19-26 and part II, experiment, Clinical Biomechanics 7: 27-32

3  McGill, S. M., Hughson, R. L., & Parks, K. (2000). Lumbar erector spinae oxygenation during prolonged contractions: implications for prolonged work. Ergonomics, 43(4), 486-493.

4  McGill, S.M., Grenier, S., Kavcic, N., & Cholewicki, J. (2003). Coordination of muscle activity to assure stability of the lumbar spine. Journal of Electromyography and Kinesiology,13, 353–359

5  Bridger, R. S. (2003). Introduction to ergonomics. Crc Press.

6  Duarte, M., Freitas, S. M. S. F., & Zatsiorsky, V. (2011). Control of equilibrium in humans—Sway over sway. Motor control. Oxford University Press, Oxford, 219-242; Duarte, M., Harvey, W., & Zatsiorsky, V.M. (2000). Stabilographic analysis of unconstrained standing. Ergonomics 43(11), 1824-1839