The 90 degree arm position has been accepted for god knows how many years but, it has critical flaws. The primary one is that the keyboard would have to be split with a gap of 6 inches or more or the user will deviate wrist angles or internally rotate the humerus to place their fingers on the keyboard. Internal rotation of the humerus would increase the amount of kyphosis in the user’s back. So, why do they still recommend 90 degrees?
As a technical illustrator and ergonomic evaulator, I am often frustrated that I can’t provide an image that expresses how dynamic an ergonomically correct work-style actually is. That is why it is so important to work directly with people, rather than throw statistics and diagrams at them.
I agree that the 90/90/90 posture image is misleading; but I thave used it successfully as a way to help people see the relationship between their eye, elbow and knee heights and corresponding screen, keyboard, and chair seat height. I tell them that the neutral posture is simply a starting point, and that frequent motion and regular breaks help mitigate fatigue and strain that result from holding a posture.
Manager, Engineering Publications & Ergonomic Advocate
When you can meet with someone and tell them "this isn’t the only way to work" the upright reference can be helpful. However, for the other 99% of all computer users whom we never get to speak with and educate are missing the story. Furthermore many of them don’t read the supporting text we might use to suggest other ways of working.
I like to use composite images to show movement. Allsteel has done a nice job in one of their free on-line brochures. Google "Allsteel ergonomics adjustment brochure" to see an example of a composite photo. When designing the American Express Ergonomics Guidebook we used similar imagery to illustrate acceptable ranges for keyboard placment, and also the benefits of a document holder/ drafting table.
These composite images can show how monitor placement can affect neck posture, how document placement can affect neck posture, and also suggested posture changes for the back, legs, etc. I’ve got a ton of images that can be combined into composites. Send me a note and we could meet to look at them. I’m based in Minneapolis and have met with some of your team on other occasions.
Gene Kay, MS CEA
Gene Kay, MS CEA
I have been away dealing with a long term illness with my mother who passed away earlier this year and and have been dealing with a couple of eye surgeries (detached lens and cataract). Thank you for your response. This is a subject I listed as one of my major complaints the last time i spoke at the Office Ergonomics Research Committee Meeting.
I think we tend to forget how many problems arise when we fail to look at the effects of bad hand and wrist position and how that affects the biomechanics. I have been working with Olympic athletes and how they can improve their mechanics by improving skeletal alignment and. In the past I have been a consultant for the Department of Health Services and Industrial Realtions (State of California), Hewlett Packard (Corporate Headquarters), Apple Computer (Cupertino), and Microsoft (Redmond, WA). I moved away from ergonomics because it seemed to focus on the environment and not the mechanics.
Certainly, ergonomics influences the user’s mechanics and is extremely helpful in resolving injuries but I think the user’s basic mechanics need to be addressed as well and I think is often overlooked. In athletics, technique and form are determining factors in performance and injury prevention, it has always been my belief that they are determining factors in workplace injuries along with workplace ergonomics as well.
Thank you for your insight and time.
I think the critical verbiage that has been lost over the years regarding the 90/90/90 postural guideline is "at least". I think (hope) there is a genral consensus that angles less than 90 degrees at these critical joints may be problematic in terms of increased stresses. Angles greater than 90 degrees have cetainly been recognized as potentilaly helpful (examples would include stading versus sitting, negative tilt in keyboard support surfaces, etc).
That said, there is an argument for 90 degrees at the elbow assuming this falls under the more general prescriptiion of a vertical upper arm and a horizointal lower arm. Leaving the keyboard aside for the moment, I have found an increased tendency when mousing to anchor the wrist and use the wrist for mousing movements (wrist deviation) as the arm deviates away from a vertical upper arm / horizontal lower arm. I feel the 90 degree elbow angle / horizontal lower arm has distinct benefits in terms of reducing awkward wrist postures when mousing.
Your keyboard observation is an important one so let me bring it back into the discussion. The variety of segmented keyboards available today have proven quite successful in addressing wrist deviation issues associated with conventional keyboards. I would argue in favor of a different keyboard to address awkward wrist postures versus a postural change that might adversely impact mousing.
Joe Selan, Ph.D., CPE
Advanced Ergonomics Inc
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