My understanding is that this advice is based on the idea that humans didn't develop to sit in chairs with backs. When you're sitting on the ground or on a bench, you are most stable sitting at 90 degrees since there's nothing to lean against. They also point to how young children often sit at 90 degrees even when sitting on chairs with backs as evidence that it is a more natural sitting position, though I would point out that those chairs are adult sized, so a small child can't really lean against the back of the chair comfortably.

The body must hold itself up against gravity which acts straight down along the vertical.  In principle there should be an ideal posture where the design of the body minimizes the amount of force the muscle must generate to counteract gravity, as you lean away from this ideal posture the muscles should have to generate more force.  This should be due to the structure of the skeleton.  You would expect that when near vertical the spine best supports the head abd shoulders.  Of course arm placement might affect all this.

As I know, the suggested degree is not only 90, but also it is suggested a range from 90 to 110. Finding he suitable degree depends on some environmental factors and personal characters as well. Online ROSA is one of the useful methods to check the ergonomic parameters among office workers.

To my knowledge, there is no evidence that supports the ideia that sitting at 90° angle is better for the spine. Actually, the classic study of Nachenson and Elfstron (1970) showed higher disc compression sitted than standing. This may be due to changes in mass center, that is shifted forward, requesting a higher effort of lumbar stabilizing muscles to counteract the increased resistant torque. To avoid this situation, Anderson et al. (1974) provided important data indicating that sitting at 120° may reduce significantly the disc compression.

Thus, in my opinion, the ideia concerning sitting at 90° as the best sitting strategy does not seems to be suportted by the scientific basis, being just a common advice.

As stated above, Nachemson et al. research regarding disk pressure and muscle activity led to the idea that sitting upright - with the lumbar lordosis preserved - reduces adverse stresses to the spine. Thinking about this needs to be fluid in that there is significant variability in sagittal plane spinal curves between individuals - especially as one ages. The degree of lumbar lordosis required to reduce adverse stress to the low back during sitting will vary from one individual to the next based on one's unique anatomy - including the presence of degenerative changes. In general, disk bulges and the like may be produce less pain when sitting with a greater degree of lumbar lordosis while those with facet joint arthropathy may be more comfortable with a lesser degree of lordosis (or even a more flexed lumbar spine). One can assume a sitting posture with or without lordosis while sitting in variable flexion angles (femur relative to the pelvis).

This 90 degree angle is an invention of wheelchair and chair makers but when someoone takes this attitude he sit on nhis tuber ischii. Dr, Staring has written an book with this position as base but he makes two holes for the tubers.. B.Engstrom do the opposite , he try to get the pelvis further than theis 90 degree angle to get an better position of the thorax and the head. But that is by patient with an problem in the lower trunk .

In my opinion is the 90 degree angle not the optimal position and must the pevis rotated further to the front and that ask for an support on sacral level befor an support on lumbar level. 

Thanks for the interesting answers. It really does look like the 90 degree/90 degree story so many (office)chair suppliers and even physiotherapist tell is based on absolutely nothing. Very interesting. 

Dear Ramon,

You concluded the answers very well.

Sitting is very important, especially for the preservation of the lumbar lordosis which is mentioned by some colleagues above. Tendency to kyphotic lumbar position is very harmful for the intervertebral discs. Slight anterior tilt of pelvis is very favorable. On the other hand, extreme hip flexion when seated is not recommendable because of disturbing blood supply to the femoral head. So I believe a balance between lumbar lordosis and hip flexion position should be taken into consideration when the sitting angle is discussed. In my opinion intermittent change of the sitting position for people who work seated is the best recommendation. 

Sitting in 90 degrees of the hips is mentioned as the deliverer of unhealthy forces on the spine, especially during the period of growth, from the very start of Orthopedics. The book Orthopedia out of 1741 by the French professor Andry is the start of many fields with interest in the human body, its growth and its functional capacities. That young spines cannot stand longstanding flexion was proofed in anniaml experiments more than 110 years ago by Wullstein.

They already understood that this flexion was a main cause for kyphosis and a progressing factor for scoliosis, and since then everybody is looking for seats that avoid the 90 degrees of hipflexion ( balls, saddles, tiltes seats as Stokke etc) .

An own design based on all this knowledge is the Zami, a double curved seat. It is brought by Zamilife.nl.   I found a fascimile copy of Andry's book and found by surprise he came on the same idea, only with the type chairs then available. German orthopedics succeeded to get all European schoolchildren in posture guiding schoolfurniture,

Even da Vinci draw the sitting human with at least one leg lowered. Variation as prof Tanyev says is the best! Keep moving in un upright position.

Now we end up with a youth with very bad postures, losts of lumbar degeneration and scoliosis, that will end up in adult spinal deformations. A hughe challenge for Orthopedics if you forgot prevention.

The position adopted by a person will affect the musculoskeletal system and especially the spine. The front spine is straight, but in profile it has a compound curvature: cervical lordosis, thoracic kyphosis and lumbar lordosis.

In the sitting position the normal or physiological curves tend to be modified passing from the normal profile (lordotic) when standing to a profile (kyphosis) and as a consequence of this can occur overloads.

An ergonomic design should promote lumbar lordosis without the need for muscular effort. This is achieved by a suitable lumbar support, a seat neither too low nor too deep, an obtuse angle between the seat and the backrest.

 Theoretically we can talk about ideal postures, but on the other hand it is not appropriate to maintain a fixed posture indefinitely. There are requirements that the chair must meet and changes in position should be possible.

I agree with you, Alicia. Low back pain is also known as the "sitting disease".

This publication may be of interest

Book BIOMECÁNICA

To perform the anthropometric measurements, the subject should be placed in a modified anthropometric position (sitting at a 90 degree angle), but for a correct design of the chair, the back should form an obtuse angle with the seat, approximately 100 degrees. The ergonomic design of the backrest is very important to ensure the natural curvatures of the spine (Lordosis and Kyphosis).

Thesis REQUERIMIENTOS COGNOSCITIVOS PARA EL DISEÑO DE UN LABORATORI...

With the huge neuromuscular thightness children and young people do have in their body nowadays ( test finger floor distance by bending and make a description of the curvature of the back you see), it is impossible to sit "correctly"  for a prolonged time on a chair, 90 or 100 degree. It is the variaton in placing the legs , one or two under your seat, that comes to the most functional , stressless, sitting posture.

Leonardo da Vinci and the European orthopedics were well aware of the anatomical-functional features involved in a natural sitting position. Free and active sitting positions should be trained by children as early as they are strong enough to sit by theirselves. Till the end of growth posture as a whole must be controlled and guided by all those involved in raising children.

Ramon, 

Quite a bit of work was done in the 70s and 80s to investigate if 90deg angle sitting posture was optimal. It seems that adjustable sitting, and semi-sitting might be optimal. 

I invite you to refer to chap 9 of Chaffin, Andersson, Martin, Occupational Biomechanics (2006). This text discuss many studies and their findings on 90deg angle sitting posture. 

Sitting in 90 degree posture exatly isn't wise ! Because your sitting on your tubers and that nobody hold for long. The question must therefore be;  must I sit for or behind my tubers

Thank you for all the replies. It's amazing to me that we have this knowledge. But everyday "workplace advisors" force people to sit in 90 degree angles. Tell them this is best for them.

What we do at the SitAcademy is we give a sitting course. In which we reset people to their natural sitting posture. Give them the tools to sit actively. We have people get angry after they discovered that 90 degree sitting was bad for them. That after years of pain and trying to sit in 90 degree angles. A 100-105 degree angle relieved them of this pain. Lots of work to do! For anyone who wants to reach out: [email protected]

Restoring knowledge on the huge power developing a healthy posture in childhood has for the durability of the body ( especially for prevention of early degeneration of discs and cartilage in our spine and joints) cannot be successful without warning parents and educators for the sitting position in 90 degrees. But for all people living "a western lifestyle" on chairs and sofa's starting in seats for babies and toddlers, avoiding these passive sitting positions and incorporate "active sitting"other preventive measurements this can reduce health costs and social security costs. If we will understand that a bad posture (malalignement) is the main cause of pain in the locomotor system.

All common sense measurements possible should be taken at least in childhood, to fulfill the promises of the original orthopedic knowledge from the Antique world on proper raising children that found its restart after Da Vinci's mathematic anatomical approach in the pedagogic work "Orthopaedia" of Andry , what ended in Europe in proper orthopedic schoolgymnastics, proper posture guiding furniture and corrective exercises and orthoses . In the TLI ( thoracolumbar lordotic intervention) bracing technique the value of this knowledge can be brought back in serious deformation of spines.There is one prerequisite for a healthy postore: the thoracolumbar spine should develop into lordosis/ extension and stay mobile in all directions.

Sedentary lifestyle in unhealthy postures is a main cause for the socio-economic burdens of musculoskeletal conditions. Lots of work to do, also i research.

I agree with Alicia and Panayot. Although many think that it should be 90 degrees, you should use an obtuse angle of around 100 degrees, this will not only help the comfort of the seated person, but it will also prevent musculoskeletal affectation.

Ramon Tilman The 90, 90, 90 sitting theory (90 degrees at the hips, knees, and ankles) has been perpetuated by the ergonomics community forever. No offense to the ergonomics community but they don't know anything, they can barely do statistics and are still showing pie charts to each other. The 90 degree sitting theory is actually from an old flight manual where they had to fit a grown man inside a jet cockpit without hurting him. The standard became 90 degrees as it took up the least room without the pilots rebelling (this insight was gleaned at an ergonomics conference).

That being said, it is incorrect. Modern adults can't sit in this position and maintain a proper posture. The psoas muscle connects the femur to the low back and as it comes up to the 90 degree angle, your hips only have 120 degrees of flex in them, so the rest comes from rounding your low back. Proper posture is when you have a nice lumbar lordosis (the nice s curve in your spine) and bringing the knees to 90 degrees destroys that. SO egonomisists shoved lumbar support into the low back to try and get that curve back. But of course we all end up slumping anyway. The only way to sit with proper posture is to have your knees lower than your hips. Check out www.qor360.com to see a chair that actually lets you sit with proper posture.

I see some people say sitting disease is low back pain Panayot Tanchev . This is also incorrect. Back pain is back pain. Sitting disease refers to the host of negative health effects that come from prolonged sedentary behavior, good cholesterol goes down, bad cholesterol goes up, your risk of heart disease, diabetes, and cancer all go up. It IS true that modern chairs cause back pain. In cultures without western chairs, the back pain rate is 3% compared to america's 80%. The way to offset sitting disease is movement. Not a standing desk or more lumbar support. Movement is what humans are meant to do and we should move often. However our modern society demands we sit for a living so it makes it difficult. You should be getting up every 30 minutes at a minimum and walking around. Or get one of the chairs I linked above.

Piet van Loon I see you have a fascination for weird chairs. Check out www.qor360.com and the founder of this company Dr. Turner Osler also started a project to provide active chairs for kids to promote better posture. Posture prophylaxis if you will:

www.buttonchairs.org

I only have a fascination for the true preventive possibilities to create healthy postures in children.

Interesting designs of dr. Osler . For the scientific backgrouns of the duoble curved active sitting solution on a stable stool ( as the Zami) we gatherd some important research.

Completelly agree with Eric! In the Netherlands the producer of the double curved active sitting solution, the Zami got the approval of the Dutch ergonomic Association Human Factors. It was the first time the orthopedic and biomechanic background ( helping the balancing mechanism, the need for the body) was the basic underground for their rapport.

It is a very good topic to investigate all aspects of sitting ( in different ways)on the development and maintainance of Health. Many researchinstitutions and many studies on Universities of Applied sciences can take care of that.