I already introduced the monkey posture and explained why it has to be assumed while carrying out any dangerous activity, e.g. computing, driving, playing music. In this page, I will show how anoffice chair should be set so that this healthy posture can be kept without excessive effort.
Remember however that the purpose is not to keep the monkey posture for 8 continuous hours per day! Rather, one should remain aware of his posture as well as of his activity, and match both: either you are carrying out a dangerous activity and you must stand or sit like a monkey, or you are in another posture and you should not be doing anything dangerous (i.e. you should be reading, discussing, etc) and you should not do it for too long (beware the slouched postures in front of TV!).
The importance of a lumbar support
In sitting, the human body is subjected to gravity only, which creates vertical descending forces. Therefore, the weight of the upper body is entirely born by the lower back. Under such strain, the lower spine tends to "collapse", which reverse the naturally inward lumbar curvature. Lumbar and thoracic kyphosis are compensated by an unhealthy cervical hyper-extension, as illustrated below.
Roughly, postures with a round low back overload the intervertebral discs, increasing the risk of hernia, whilst postures with a hollow back overuse the vertebral facet joints.
A good chair bears the weight of the upper body by stabilizing the pedestal of the statue, i.e. the hips: Your weight should neither be borne by the discs nor by the facet joints but by the lumbar support.
In other words, a chair will carry the weight of the upper body (i.e. reduce the load on the spine) only if the low back (belt level) rests against the lumbar support. The adjustements of the chair should therefore be made in such a way that the usage of the lumbar support is maximized. This has consequences on the way you should sit down, on the chair adjustements (sitting height, angle and depth of the seat pan, vertical position of the lumbar support), and the use of the backrest at shoulders level.
Adjusting the chair
A number of rules are generally valid:
- Set the height of the seat pan so that the knees form an angle of 95°-100° when the lower legs are vertical. This keeps the hips angle open and thereby promotes blood flow in the legs while ensuring that the body weight is carried both by the chair and the feet.
Placing the feet under the chair creates a hollow low back. Leaving them far to the front tends to induce a round low back. Hence the feet have to remain flat and firmly on the ground, with the lower legs vertical.
- Keep the seat pan close to horizontal so that equal pressure is felt along the thighs. On some chairs, this may suppose to lock the free-float mechanism.
Crtain studies have shown that a seat pan angle of 7° forward reduces the electromyographic activity of certain back muscles. In my opinion, such setting tends to let the upper body slide away from the backrest, like when sitting too high, thereby reducing the effect of the lumbar support. Further, it creates a hollow back. I therefore recommend to consider such results with care.
On the other hand, inclining the seat pan backward is similar to sitting too low: it closes the angle in the hips, pushes the shoulders against the backrest, the low back away from the lumbar support and induces a round low back (see below).
- Adjust the depth of the seat pan so that sufficient space (approximately one fist) remains between its front edge and the back of the knee, so as to avoid compression in this region.
When the seat pan is too deep, one tends to sit on the front edge, away from the lumbar support, to avoid compressions behind the knee.
- The lumbar support should be adjusted at belt level (at least for men). The trends of female fashion renders such rule often difficult to apply, although the absolute position of the lumbar region is the same for both genders (between L4 and L5).
If the lumbar support is too low, you will feel that you tend to rotate your hips backward and therefore slouch when sitting down. If it is too high, it tends to overstretch the lower spine, inducing a hollow back. Both are uncomfortable and can on the long run cause low back pain.
- The armrests should be adjusted in width so that the arms are hanging loose along the torso and in height so as to support the elbows without raising the shoulders. Further considerations about the use of the armrests are detailled on the next page. Adjust them rather too low than too high.
The final working posture is illustrated below. Note on the right picture how the low back is fully supported.
Lumbar vs. shoulders-support
When sitting in the monkey posture, the thoracic spine is kept slightly in front of the hips. On an office chair, this implies that the shoulders blades are kept away from the backrest when the lumbar support is fully used.
This statement is consistent with the following considerations:
- On a blocked chair, resting the shoulders blades against the backrest pushes the low back away from the latter, i.e. provoques slouching and sliding away from the lumbar support.
- On an unblocked chair, resting the shoulders blades provoques backward rotation of the backrest. This results in a hollow back and an increase of the effort needed to bring the arms forward (e.g., towards the keyboard).
- Resting the shoulders blades while still trying to rest on the lumbar support provoques spinal extension (i.e. induces a hollow low back), as illustrated below (the postures on the left and on the right pictures are exactly the same, on the chair and out of the chair).
