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Lumbar Spine in Sitting


The Lumbar Spine in Sitting
Prof. E Nigel Corlett

©E Nigel Corlett. Reprinted with permission

Why should we spend our time talking about seating? After all there have been seats around for hundreds of years so surely we must know all about it by now. If we do, why are so many chairs uncomfortable, and why do so many people get backache after using some chairs? Perhaps, if we do know all about seating, the people who design chairs haven’t yet got the knowledge.

Chairs are most often designed by designers or architects. What is lacking from both these camps is any detailed knowledge of the human body and what happens when we sit.

In what follows we are talking about working seats, for offices, schools, factories and other business places, e.g., checkout stations. Easy chairs are a different matter, although much of the same knowledge is needed.

First, though, a bit of anatomy.

The curve in the small of our backs allows us to stand upright with a minimum of back muscle activity, because the vertical line through the centre of gravity, (CG) of the upper part of our body is over the peak of that curve. So there is no line of force trying to bend us forward or backward. As a result the back muscles are at a very low level of activity, just enough to keep our trunk upright.

Point 1: Maintaining the lumbar curve allows the minimum back muscle activity.

When we sit on a flat seat, which is horizontal, our thighs are also horizontal. It was shown many years ago that rotation of the thigh from in-line with the body to being at right angles at the hip was achieved, for the first 70° or so by hip rotation and, for approximately the last 20°, by a backward rotation of the pelvis.

The effect of this backward rotation is to flatten the lumbar curve. In consequence, when we sit upright, the line of thrust through the CG of our upper body lies forward of this (now flattened) curve, so the back muscles have to become active to hold us upright.

The lumbar spine in sitting. Corlett & Eklund How does a backrest work
From Corlett & Eklund (1984)
How does a backrest work?
Graphic with permission

This not only produces discomfort and fatigue, but it loads the spine, and its discs. Add to this that, when the lumbar curve is flattened the discs are slightly pinched at their front edges. This is another load on them, adding to that from the counteracting moment of the back muscles.

Point 2: Sitting upright with the thighs horizontal loads the spine even before any work is done.

Almost all of the load arising from work with the arms, as well as forces exerted by the trunk, are transmitted to the ground via the seat, and thus through the spine and pelvis. These are the loads created by working.

We expect such loads. But if we pre-load our bodies as in point 2, then we reduce the available capacity of the spine for transmitting these work loads. The load we introduce is a continuous pressure, unlike the usually varying loads arising from work activities, so recovery is not so readily available.

It has been shown that a continuous muscular load, (static load), maintained for a long period requires a long time for the muscles to recover. Also, continuous loadings due to posture eventually cause the body to adapt to the posture, distorting our bodies if poor postures are held for long periods of time.

Point 3: A poor sitting posture reduces our physical capacity for effort and can lead to bodily distortions in the long run.

The human body is an organism which thrives on change, not on stasis. It is beneficial to change our posture frequently, to allow loaded muscles to recover and this can be helped if we can stand up easily. If a chair is low, standing up is difficult, because our leg muscles are not in an advantageous arrangement for rotating our hip and knee joints.

If the seat is higher, not only are we part way to standing but the mechanical advantage of our leg muscles allows them to exert more force, making it easier to stand. Note, also, that if we sit higher, our thighs will be sloping downward, which will allow us to retain our lumbar curve, reducing the load on our spines, (point 1).

Point 4: Having a sloping thigh reduces back loading and makes it easier to stand up.

But, how can we sit with a sloping thigh without sliding off the seat? We have to arrange it so that, although it is possible for our legs to slope downward, our weight is always borne on a horizontal part of the seat. This apparent contradiction can be achieved if the seat is curved from front to back, with the weight being taken on the top of the curve, which is horizontal.

As we raise the seat, to prevent our feet lifting from the floor we rotate the seat forward. This gives clearance for the thighs so that the feet reach the floor whilst there is still a horizontal bit of the seat for bearing the weight. Such a seat makes it possible for people of different heights to work comfortably at the same bench, without a footrest, and to stand or sit as the work requires. This is the purpose and the basis of the design of the Nottingham sit/stand seat.    More about the Nottingham seat

Point 5: A seat to the Nottingham design is the most effective work seat available for those who must be seated at their work.

The idea of a sloping-forward seat is not new. An illustration from the middle of the 19th century shows such a seat. For at least thirty years a Danish surgeon, Chresten Mandal, has pressed for the adoption of a sloping forward seat and several designs have appeared. A well known one, intended to cover the problem of people sliding off the forward slope, is the "kneeling chair", where a resisting force is provided from the shins in contact with pads. This solves one problem at the cost of creating another, discomfort in the shins!

The unique feature of the Nottingham seat is the curved surface, which allows well supported sitting, clearance for the thighs and retention of the lumbar curve. It has been widely tested and has demonstrated that it is effective in improving access to the work whilst reducing the loads on the spine and the discomforts arising from the more conventional seating usually provided.

A few of Professor Corlett’s many other inventions with his colleagues include the RULA / Rapid Upper Limb Assessment, the Body Part Discomfort Scale, and the Stadiometer for assessing shrinkage of the spine.     He also paints.

Sit/stand seat from Univ. Nottingham IOE

More about office ergonomics

Rethinking sitting   htm    pdf    zip

Seat height revisited  by Rani Lueder

Ergonomics for children
A practitioner’s manual

Ergonomics of sitting & seating  |  Slides
National Ergonomics Conference & Expo

The case for & against movement
Courtesy of Allsteel  |  Abstracts

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Review of the ergonomics research


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