By reading part 1 of The Desk Warriors Guide to Building a Healthy Back, we know that sitting for prolonged periods is something to be avoided. But what are we to do when sitting can't be avoided? After all, our jobs, our livelihoods are now spent hunched over computers. We can't all earn a living on our feet like pro-athletes, dancers or even bartenders for that matter. We have to sit at a desk and work.
I will first address the myth about "proper" posture. When we sit with "proper" posture, our feet flat on the floor, our knees and hips at 90', the spine long and upright; the head and chest up and our lumbar curve maintained we are actually damaging our lumbar spines. This may be aesthetic sitting posture or proper school boy/girl posture, but according to my man Stuart McGill it is only tolerable for 10 minutes max and should not be attempted further.
You would be correct. However, a back angle of anything less than 135* can't really be considered sitting since you are closer to laying down. Assuming we have to be at a desk/work station in a seated position, the ideal posture is one in which muscle activation is lowest via support (back of chair or lumbar support), maintenance of the normal lumbar curve and one in which we can change position easily.
The reason we need to change positions easily and regularly is to not overload any one tissue. Tissue loads need to be migrated from tissue to tissue in order to minimize the accumulated microtrauma of any one tissue. Think about it, you fidget for a reason. You are spreading the microtraua from tissue to tissue. McGill recommends changing postures very often. Atleast every ten minutes.
The chair is not helping the situation. Chairs are designed by architects, not biomechanists. Ideally, you would want a chair with a great deal of lumbar support. The Nottingham chair is one that supports the lumbar spine and aims to keep a posture more congruent with standing, thus sparing the spine. Standing desk stations are also a great way to preserve the spine. But yes, the Nottingham chair is expensive and unstylish and standing desk stations have their problems as well.
So when forced to sit for long periods of time. We want a chair that supports the entire back, rather than doing all the work ourselves via the core musculature and contributing to spinal compression in a flexed posture (contributing to discogenic pain). We want to be able to mimic normal standing posture as best as possible so a chair that supports the normal lumbar curvature is essential. If your work chair does not do this, simply buy a lumbar support. Arm rests also alleviate spinal compression, so use them when you get the chance. Most importantly, we want a chair with the ability to recline. The more the better. The optimal seated position would be in a lazy boy, but as most desk warriors aren't afforded this luxury, we must aim to achieve this recumbent position via lumbar support and reclining our chairs. Again, I reiterate that any position must be changed eventually to promote circulation and to spread the microtrauma to various tissues, thus our reclined lumbar supported position must to be changed regularly.
The least compressive the posture, the longer it can be tolerated. So Desk Warriors, arm yourselves with an arsenal of appropriate desk postures. Change the angle of your back via more or less lumbar support or varying degrees of reclination. Shift your weight to your right, then your left. Sit with your legs crossed, left and right. Cross your legs with your lean on. Essentially be mindful of maintaining your neutral lumbar spine while shifting into a variety of comfortable positions.
This stance on postural shifting can also be applied to sleep. No one sleeps in any one position and they shift subconsciously. They do this because it's our way of our body spreading the stresses to different tissues as we should do when we sit, stand, lie etc…
In conclusion, the ideal posture for prolonged sitting is a reclined ~135 back angle posture where our natural lumbar, thoracic and cervical curves are preserved. However, we learned that after a certain exposure, no position is safe and that we must find a variety of Desk Warrior poses to serve our productivity needs.
Callaghan, J.P., and McGill S.M. (2001) Low back joint loading and kinematics during standing and unsupported sitting. Ergonomics, 44 (4): 373-381
And of course the books cited in Step 1: Sit Less