What Butts Tell Us About Biomechanical Risk

Photo of Sharon Sonenblum Editor’s Note: Ride Designs has graciously donated their blog post space to Sharon Sonenblum, a rehabilitation engineer and researcher at Georgia Tech, to share her thoughts and research findings on some very important topics: sitting mechanics, pressure ulcers, wheelchair cushions, and butts. Sharon is Senior Research Scientist at Georgia Tech, and she has been studying wheelchair use, pressure ulcer prevention, and the response of buttocks to loading for over a decade. This is the first blog post of two, so stay tuned. 
On a related note, see the interview with Tom Hetzel, CEO of Ride Designs, with a manufacturer's perspective on the need for evidence-based seating systems. 
Sitting-acquired pressure ulcers are a critical problem for people who use wheelchairs for mobility. More than 50% of people with SCI develop a pressure ulcer during their lifetimes, with over 20% requiring surgery. Pressure ulcers reduce activity and participation, affect quality of life, and increase the risk of premature death. And once you get one, you are at increased risk for future pressure ulcers. 
These ulcers most often occur at the ischial tuberosities (the sitting bones that bear most weight), and sometimes at the greater trochanters. Our research has shown that individuals using power wheelchairs spend about 12 hours per day in their chairs, while manual users spend about 10.5 hours. That’s a long time. 
You might ask, “Surely they are doing weight shifts every 30 minutes, just like they were taught?” Not exactly. In our studies, we have found that people do move around a lot, performing small tilts 3 times per hour in power wheelchairs and weight shifts (or partial unloading of the buttocks) 2.4 times per hour in manual wheelchairs. But in terms of bigger movements that unload more of the buttocks, people using power tilt-in-space only did large tilts past 30⁰ every few hours. People in manual chairs only fully unloaded their buttocks (with a depression lift or full front lean) once every few hours. Most individuals we studied in manual wheelchairs had at least one 2-hour stretch during the day without any weight shifts at all. 
Now here is where things get interesting. Nearly half of individuals do NOT experience tissue break down, even though they have stretches of time without weight shifts, and even though they don’t do full pressure reliefs very often. Some individuals seem to have much more tolerance for loading than others, who seem to get away with anything without developing a pressure ulcer. I like to call this difference a person’s “Biomechanical Risk.” 
What it means is that we all have different buttocks, and the same amount of load applied causes a different amount of deformation on different buttocks. For those who have palpated many rears, you might have noticed a variety of differences that I will describe here in completely non-technical terms based on my non-clinical “expertise:”
  • No butt: very skinny, often a young man with chronic paraplegia, and there is almost no tissue covering the pelvis.
  • Saggy butt: This individual has more tissue covering the buttocks, but it is very loose and on palpation the ischial tuberosity can easily be palpated.
  • Well-padded butt: This rear end may be larger, but that isn't what makes it stand out. More important is the fact that the tissue holds up well to palpatation. It doesn’t collapse, making the ischial tuberosities more difficult to locate.
  • Surprising butt: This rear end might appear well padded to the uninitiated, based on size and adipose content, but upon palpation, it is noted that the adipose does not have any ability to support load and the ischial tuberosity is easily identified and not well supported.
Genetics and demographics certainly play a role. Tissue is different in men versus women, lighter skin behaves differently than darker skin and skin compliance changes with age. Behavioral risk factors play a role, too. Smoking, dietary choices and hydration levels, for example, all can change tissue compliance. In addition, our recent MRI research at Georgia Tech showed that while some people sit on muscle, many do not – more people sit directly on connective tissue and fat. 
Our research shows that if we could consistently identify someone’s Biomechanical Risk, we could prevent more pressure ulcers. MRI is a great research tool that we are using for visualizing the buttocks' anatomy and deformation during sitting, but it’s not going to help in the clinic. 
So here’s what I want to know from you: what different buttocks do you see in your practice, and which ones seem to be at the greatest risk of breakdown and which one seem to have greater tolerance? Is there a clinical tool out there (besides some well-trained hands) than can be used to identify someone’s Biomechanical Risk?


Submitted by Marlene Adams (not verified) on

Hi Sharon, I am an OT working full time in a seating clinic at a spinal cord rehab centre in Toronto, Canada. I have seen the types of butts you described (good descriptions by the way!) and I have seen wounds on them all. I do find that sometimes people who have spasms/clonus tend to maintain muscle bulk that may help prevent skin ulcers. I would also say that I see less ulcers on the well padded butts. That being said I think you're missing a key component - it's not so much the "butt" as the person in the butt. I use the PMAT, Pressure Management Assessment Tool, to help determine what's causing a skin ulcer. Many team members are quick to point to the cushion when someone has an IT or coccyx ulcer but there are many other potential offending surfaces that people sit on,,, car seats, bath benches, couches to name a few. People's activities also play a big part (and the PMAT captures this). For example my patients that do "crash landing" transfers often end up with skin problems. Another variable I would wager on is social support. There is literature that says people who return to work have better social support, and I wonder if there's a correlation with skin. Good luck with your work. It sounds very interesting and I look forward to seeing what you dig up!

Submitted by Sharon (not verified) on


Thanks for your reply! I agree with you that pressure ulcers are highly multi-factorial, and that plenty of comfortable couches, bath benches and road trips have contributed to skin breakdown. In my follow-up post (stay tuned!), I'm planning to talk about my little dream to create a tool to help clinicians more easily match an individual's butt with an appropriate cohort of cushions, not because the cushion is the only important factor, but so that clinicians can have an evidence-driven approach to narrow their selections. Ideally, that would leave more time and attention to deal with all of the other person and lifestyle factors that impact cushion selection and/or contribute to skin problems on their own.

Your comment about the role of social support is very interesting and consistent with some of the research that has been done. Studies out of Rancho Los Amigos and the Medical University of South Carolina through the SCI Model Systems have looked at the roles of lifestyle, socioeconomic status and health care access in pressure ulcer prevention and their findings suggest the importance of the entire support network (social, financial, and medical). How would you approach pressure ulcer prevention differently in someone you know to have a poor social support network?

Submitted by Sharon (not verified) on

One more thought. As a researcher, I've never had the chance to use the PMAT, but I'm glad you take the time to really investigate the likely cause of the pressure ulcers you see. How much time do most clinicians have to spend on that 'investigation' during a typical appointment? I know everyone is pressed for time... I did want to point out that from my studies, answers to questions such as "How frequently do you move for the purpose of pressure redistribution?" and "How long do you hold each pressure redistribution?" tend to be highly unreliable, so be skeptical of the responses. As an example, I asked 28 people asked how often they did their pressure reliefs (options ranged from every 15 min to less than once an hour), and approximately half of the participants overestimated their weight shift frequency and the other half underestimated or were accurate. So on the whole, there was no way to predict what they actually did in real life based on what they told me. Thankfully, we had sensors hooked up to measure their behavior!

Submitted by Barbara Sweet M... (not verified) on

Hi Sharon, I've been working many years with this issue here at the Center for Rehabilitation Technology at Helen Hayes Hospital and elsewhere. I'd love to see some additional research to support cushion choice and design - not sure how to collect the information that would be helpful to you. I'm not aware of any specific tool that looks at bio-mechanical risks. The PMAT is a good tool. As you know a lot of what we do re cushion choice is based on extensive pressure mapping, interviews, experience/instinct and let's not forget money!! . I only look at the patients buttocks when there is a wound, not before! Perhaps this is a good practice for all new injuries.
So I was wondering : does buttocks tissue change over time? ie new injuries vs older injuries. or is it more related to the type of individual body make-up. Are you looking to collect a data bank of buttock types to look at outcomes a year or two later?
Finally, in my experience pelvic asymmetry is one of the biggest contributor to pressure sores. Have you looked at this as one of your bio-mechanical risk factors?
Let me know if we can be of any help. Looking forward to your research..

Submitted by Sharon (not verified) on

Hi Barbara, and Happy New Year! Thanks for your great questions and your offer to help.

Buttocks tissue, like other tissues in our body, certainly change over time, as well as depending on the individual. In fact, one reason I want to create a tool to assess the buttocks itself is to help people aging with an SCI. (Where is Susan Johnson-Taylor when I need her - if you're reading, please add your voice!) Tell me - how often in your practice have you seen someone using the same cushion for years without incident, and all of the sudden they break down? I would like to develop a tool that says "hey look, your butt has changed enough that even though this cushion has provided enough protection until now, it might be time to consider something different."

A database of buttocks types and outcomes would be incredible, and I would love to do that study. But since practically, that is really tough and expensive to do, we would consider starting with a retrospective approach. That is, we would look at people who have a history of breaking down and those who don't, and see if their buttocks differ. That would give us more justification for the longitudinal study.

Lastly, let's talk asymmetry. I have not spent much time looking at this, but it fits right in to this idea of biomechanical risk. Asymmetry will put more load on one side, which will lead to greater deformation, and therefore greater risk.

Thanks again for the feedback and great questions.

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