The ‘S’ shaped spine is central to ergonomic sitting and seating. Does anybody out there know where the data of this ‘S’ shape has come from. In anthropometry hundreds of people are measured, has the equivelent been done for the spine (internal anthropometry)?
Also how does the ‘S’ shape spine differ from the surface shape of the back?
Any responses would be greatly appreciated.
>> The ‘S’ shaped spine is central to ergonomic sitting and seating. Does anybody out there know where the data of this ‘S’ shape has come from. In anthropometry hundreds of people are measured, has the equivelent been done for the spine (internal anthropometry)? Also how does the ‘S’ shape spine differ from the surface shape of the back?
You are actually asking a number of complicated questions. The spine is not just the vertebra – it’s a composite of spinal processes. The contours of the back are influenced by more than the spinal contours – women deposit fat in ways that tend to promote low back contours more than men, for example. There are also differences between races as an evolutionary response to dissipation of heat. Women’s center of gravity is forward of their hip socket, whereas in men it’s close to the hip socket. Women have lower center of gravities also, affecting their spine.
The best place to read the research on the measurement of lumbar contours is in the journal Spine. There is more than one way to measure contours of the spine – and different schools on the topic (e.g., long lever arms vs. short lever arms)
A lot of common assumptions by ergonomists about how the spine responds to postures don’t hold up in the literature. For example, the research does not really support common assumptions that lumbar lordosis increases as we recline. It is also not consistent about the lumbar flexion as we sit.
Rani Lueder, CPE
Thanks for the reply Rani.
I have come accross a book edited by yourself entitled ‘hard facts about soft machines’. I am conducting research into what data or methods have been used to develop some of the backrest contours on the market currently.
Have the designers developed contours based on the shape of the spine of ‘the average person’ or is a bit of guesswork?
I haven’t got a copy of the above book but would appreciate if you could let me know whether it discusses the methods that have been used to develop backrest contours.
Yes, that book does have some research about the measurement of contours of the spine but I don’t know if it would answer your question.
I don’t really know how to define an average spinal contour, or what that would be. Paul Branton once published an article in Applied Ergonomics that showed contours of the spine based on percent torso length – but that study has been ignored, I think because it’s hard to interpret. There was an in-depth study of spinal contours in the 1950’s by Clara Ridder at the University of Arkansas that has really interesting data even though it was taken long ago and in a very homogeneous white population. I think it’s best to not expect to find an answer to your question in one study, though. Good luck!
Rani Lueder, CPE
There’s a history to the Ridder study, by the way. In the late 1970’s a head of design of a leading manufacturer came out and claimed that Ridder’s research proved that there was a one-inch variability in the whole population, obviously to justify their lack of backrest height adjustability in their new seating line. But my review of Ridder’s research clearly demonstrated that for the one backrest angle, there was a four inch variability (5th to 95th pcs) at the middle of the lumbar and a 6 inch variability at the top of the lumbar. And those numbers were conservative since it was taken from a homogeneous white adult population who were not obese.
I called that designer several times to try to get him to explain how he came to his interpretation, but he never gave me any kind of a response. His claim though had a major impact on the seating industry. For the next two decades the whole furniture industry was in lock step, claiming that there was only a one inch variability in the population, and therefore the backrest height adustability was unimportant.
Being a pushy broad, in the late 1990’s I made it part of my agenda when I spoke and wrote to make sure that everyone understood that the one inch lumbar range was total fabrication and that this myth had harmed our user populations. Finally, BIFMA came out and told its furniture members to stop making this claim – and they did.
But this history is the reason that to this day most chairs in the industry still provide insufficient backrest height adjustability.
Rani Lueder, CPE
The background to Ridder’s study is very useful. My search so far has revealed that Ridder was probably the first researcher to define a seat contour similar to the one we use today. This was further devleoped by Grandjean and others with time to take into account the different tasks we undertake and the different requirements of modern day work.
Obviously there are many schools of thought on this issue i.e. Mandal and Gorman but generally a supported back seems to be reccommended by research.
It seems to me that the backrest profiles do not account for individual differences as we know that there is a difference between males and females, ethnic groups, people of different shapes and sizes etc. The modern backrest does not seem to accomodate for these differences which are further exacerbated by the relationship between the spinal curves and the surface curves of the modern worker, many of which are obese!
This lack of accomodation seems to lead to voids between the backrest and back particularly in the reclined position.
The modern chair can still be improved!
Some chairs on the market flex to accommodate individual differences in contours of the spine.
One question is whether their new shape accurately reflects the best shape.
The biggest difference is of course not at the lumbar spine but at the thoracic and (even more) cervical levels.
Grandjean has great information but take him with a grain of salt, as I mentioned research on the spine contradicts common ergonomics assumptions and guidelines.
I’m glad my comments were of use. Good luck on your project!
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