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“The Way You Sit Will Never Be the Same!”
On Tuesday the 28th November 2006 a group of Scottish and Canadian researchers presented the findings of a study to the Radiological Society of North America. They told the Radiological Society of North America that the best position in which to sit at your desk is leaning slightly back, at about 135 degrees. They used a new form of magnetic resonance imaging (MRI) to show it places an unnecessary strain on your back to sit bolt upright or to slump forward by increased pressure in the discs.

For abstract Click Here
Full title: The Way You Sit Will Never Be the Same! Alterations of Lumbosacral Curvature and Intervertebral Disc Morphology in Normal Subjects in Variable Sitting Positions Using Whole-body Positional MRI by Bashir,W.  et al

Sitting upright with 90 degrees at hip/lumbar spine is not put forward as a good posture by most professionals in the field. There are a number of reasons for this:

It’s difficult to have an s-shape spine during sitting unless you are a bit reclined. Sitting bolt upright with 90 degrees at the hips takes a lot of muscular effort to overcome the tension of the hamstrings. Commonly, this muscular work is released by the pelvis rotating backwards. When the posterior rotation occurs at the pelvis the muscles relax but the ligaments are tensioned as the pelvis flattens and this transfers the loading onto the ligaments from the muscles. Also, if you recline and allow the pelvis to rotate backwards into a flat or forward bent (flexed) spine, this effectively creates the slouched position that increases disc pressure. So, the ideal is to sit reclined but with the lumbar arch maintained to form an s-shaped spine.

It takes a fair amount of muscle work to maintain a reclined position, with an s-shaped spine, for prolonged periods and so fatigue is likely. Therefore the seated spine should supported by a well designed backrest that is adjusted to a reclined angle and offers good lumbar support (or a bulge in the backrest) to support the s-shape.

As the report says this theory was put forward in 1974 - Anderson et al found that during relaxed sitting the intra discal pressure is up to 40% higher and pressures when sitting upright about halfway between standing and slouching. He concluded that the best posture, based on his findings using needles into the discs to sense pressure, was a reclined posture with the lumbar arch supported.

Considered purely from a mechanical point, it is evident that a reclined posture allows some of the weight bearing to be transferred onto the backrest which must be a good thing as this unloads the body structures a little.

The important point that people seem to be missing on this story is that they are not saying that slouching is good - they are saying that sitting reclined is good -with the s-shape still maintained. This has been the general advice and premise behind office seating design for some time so it actually supports what professionals in the field have been saying rather than discounting it.

Grandjean in a study in 1973 found that both people with and without back pain, chose a reclined posture with lumbar support as their desired position in a chair.

A few comment about the angle put forward of 135 degrees: There are two points here – the first is that as you increase a reclined posture, more horizontal forces come into play – resulting in slipping forward on the seatpan and subsequent loss of the full benefit of contoured support, especially in the lumbar region. The spine then reverts to the undesirable slumped posture. This can be overcome by some degree, by high friction seat covering.

The second point is that when working at a desk, using a PC, the arms have to move forwards to the work area. Therefore if you are very reclined, the mid - spine is pulled forward and support from the backrest is lost in this region. Often a c-shaped posture occurs. Further, the resultant neck and upper limb postures are also not good (another discussion). A very reclined posture is therefore impractical if used at a computer workstation. The more generally accepted norm is between 100 and 110 degrees reclined as this allows the benefits outlined above as well as allowing acceptable neck and upper limb postures during work.

A lot of this discussion was put forward by Stephen Pheasant in 1991, a well known Ergonomist.

Jan Vickery
Chartered Physiotherapist and Registered Ergonomist

Ergonomic Seats and Desking