Effect of Chair Designs on Sitting Tissue Pressure and Perfusion

Researchers Mohsen Makhsous, Fang Lin, David Hanawalt, Shannon Lynn Kruger, and Angie LaMantia conducted a study that compared five different office chair designs. Specifically, they looked at pressure distributions between the body and chair, and tissue perfusion in the buttock-thigh area.

In the following review, text between quotation marks, (example: "text") is quoted directly from the Makhsous et al article, referenced below, and text that appears in italics preceded by my initials (example: PB: text) are questions or thoughts that came to my mind as I read the article.

Background

The researchers begin their article by summarizing low back pain (LBP) statistics, referencing numerous other researcher articles:

• roughly 100 million lost workdays each year are attributed to LBP;
• although there can be many factors, "it is accepted that prolonged sitting … may contribute significantly to the symptoms";
• different sitting postures produce different "stresses within the spine";
• prolonged sitting has been associated with pathologies in the L4-L5 disc;

They postulate that chair design and its influence on sitting posture may play "an active role in causing and relieving LBP associated with sitting." This theory is intuitively appealing, and reflects the anecdotal experience of many ergonomists, but how strong is the scientific evidence to support it (PB: Recall Marras' recent review of our understanding of LBP and low back disorders; The Complex Spine: Understanding and Preventing Low Back Pain and Disorders)? Makhsous et al point to previous studies that touched on the question:

• using a back support has been shown to significantly change buttock-thigh pressure distributions;
• "Long periods of sitting during office work has been found to be one of the significant determinants for predicting LBP occurrence in office workers" (PB: Is this the same as saying that 'sitting for long periods is predictive of LBP in office workers?')

Makhsous et al recognize that the weight of the head, arms and trunk are carried primarily by the ischia (PB: the so called 'sitting bones') and surrounding tissues during sitting (PB: especially in the absence of arm rests and back rests/supports). They go on to suggest that:

• pressure differences on the ischium "may change the curvature of the lumbar spine";
• not having a back rest/support "may promote lordosis in the lumbar area and increases the stress on the spinal structure"; and that
• "Therefore, sitting interface pressure distribution between the buttocks and the seat may be used to evaluate new chair designs;" and
• a previous study of interface pressures found that both chair design and posture did affect pressure distributions, but "chair design produced the greatest effect."

PB: This is an ongoing challenge in chair research; what quantitative biomechanical or physiological measures do we have that correlate well with comfort, let alone pain, discomfort or injury, specifically in the lower back region? 

As a physiological measure, Makhsous et al use tissue perfusion (PB: capillary level blood flow), which they state "has a significant effect on the occupant’s sitting comfort." Specifically, they measured transcutaneous partial pressures of oxygen (tcPO₂) and carbon dioxide (tcPCO₂) from the posterior buttock-thigh area with sensors attached directly to the skin, citing research by another researcher that "reported that transcutaneous oximetry is a useful tool in assessing tissue comfort in a study on mobile and immobile participants."

Methods

Study Participants:

• 15 healthy Caucasian women;
• mean age = 23.5 yrs (range = 23 to 25);
• 58.2 kg (range = 49.9 to 67.1); or 128.3 lb. (range = 110 to 147.9);
• mean height = 167.3 cm (range = 158.5 to 180.4); or 65.87 in. (range =  62.4 to 71); 
• mean body mass index (BMI) = 20.8 kg/m² (range = 19.1 to 23.4);
• all in good physical condition with no current of previous LBP, pelvic surgery, or cardiac or vascular dysfunction.

Office Chairs:

• Suspension A Chair: "a Pellicle suspension design with its seat and backrest made of glass-reinforced polyester material;"
• Suspension B Chair: "an Elastometric mesh design with glass-filled nylon frame material for both seat and backrest;"
• Foam A Chair: "a polyurethane foam chair; the seat was made of a black plastic frame, and the backrest was made of double mesh;"
• Foam B Chair: "a urethane foam chair with a seat made of polypropylene shell and glass-reinforced nylon material and a backrest made of chlorofluorocarbon-free foam;"
• Bicompliant Chair: "a contoured suspended pixels suspension design with seat made of Amitel and Sarlink and backrest made of polypropylene Krayton mat."
• seat height, seat tilt, and recline were adjustable for all chairs; and 
• seat depth was adjustable only for the Foam A, Foam B, and Bicompliant chairs;
• Chair models and manufacturers are not identified, and the authors state that the research was not supported by a chair manufacturer, though they do note that Herman Miller donated "different chairs for this study."

Posture Instructions; participants were:

• "asked to maintain an upright posture with minimal movement of the limbs;"
• assisted in adjusting the chairs such that they "sat with 90° angles at the knee and hip to standardize the posture in data collection;" and
• "were instructed to sit in the testing chair using the backrest with buttocks to the rear of the chair, hands on the thighs, and feet flat on the floor."

Pressure distributions and tissue perfusion variables were measured using equipment detailed in the article, as are detailed testing protocols and data analysis methods, and interested readers are encouraged to read more in the original source. A few key portions of the protocol are worth mentioning here: 

• participants sat, and seated measurements were taken, for only 10 minutes per chair;
• for data analysis, the buttocks-thigh area was divided into three zones; posterior (closest to the backrest), which included the ischium, middle and anterior.

Results …

The remainder of this articles are comments from the reviewer, Peter Budnick:

I found it challenging to sift through the results section and piece together a coherent description of their findings. Makhsous et al present their data in summary bar graphs and paragraph form, but do not provide a summary in table form, which would have made it much easier to understand, compare chairs, and appreciate their findings. The bar graphs do provide a visual understanding and comparison for the pressure distributions, but the tissue perfusion data was not summarized in a useful comparison format. So, I turned to their discussion to see if they might pull things together there, and rather than bringing clarity, it left me with even more questions.

Regular readers of The Ergonomics Report know we do a lot of research reviews, which means we often pour through pages and pages of sometimes boring and sometimes confusingly written text to pull applicable meaning from the articles. If you read a lot of research articles, you begin to understand that what is not said, or the peculiar way in which something is said, is where the "meat" of the findings and conclusions sometimes lives. This article kept making me ask more questions than it was answering, and one of those questions had to do with "bicompliant," the descriptor for one of the chairs tested. So, I turned to Google, which quickly turned up some interesting additional information that left me asking even more questions.

Here's what I found:

A search for "bicomplaint", which is not recognized by spell checkers, turned up links to the very article I was reviewing, followed by a series of links to a specific chair from a specific manufacturer that used the term "bi-compliant" to describe a portion of their chair design. One of those links included a summary of research, dated 2008, by the same first two authors for the article being summarized herein. That research, being described by a representative of the manufacturer, contained a great deal of interpretation, something that seemed to be missing in the paper I was reviewing. I could not be certain whether the data and study she referred to in the 2008 source was the same as that being described in the 2012 article I was reviewing, or whether this was a new study, presenting new data and findings. Further, what conclusions can really be drawn from this research?

Rather than speculate, I am going to reach out to the researchers and the manufacturer to see if they would be willing to comment and help us understand. I'll write a follow-up that I hope will shed additional light on this research and its applications.

Reference

Mohsen Makhsous, Fang Lin, David Hanawalt, Shannon Lynn Kruger and Angie LaMantia, (2012), The Effect of Chair Designs on Sitting Pressure Distribution and Tissue Perfusion, Human Factors, published "online first," 27 August, 2012, DOI: 10.1177/0018720812457681. At the time of this writing, it was available to Human Factors subscribers at: http://hfs.sagepub.com/content/early/2012/08/24/0018720812457681

This article originally appeared in The Ergonomics Report™ on 2012-09-18.