Moving Forward with Center of Gravity

Moving Forward with Center of Gravity

As someone who spends a great deal of time consulting others on manual wheel chair prescription and teaching about seating and configuring manual wheelchairs, I cannot help but observe and analyze every wheelchair I see at a mall, airport, playground, or wherever else I may roam.

One of my biggest sources of frustration is to see the rear wheels of an ultra lightweight chair in the most rearward setting along the rear axle plate when I cannot see any physical reason (i.e. bilateral amputee) why the end user needed their chair set in that configuration. This has bothered me so much, at times, that I once bent down on my hands and knees in the checkout line to reach under a chair and show a stranger where and how to make the center of gravity (COG) adjustments on his son's chair – much to the embarrassment of my wife (I will also add that I located the label of the company who sold the chair to this client and recommended the father call them so one of their ATPs could do a quick evaluation and reset the COG as appropriate.).

We prescribe manual mobility bases with adjustable axle plates for a reason – to increase the ease of propulsion for our clients. Over the years, I have boldly approached end users and asked them if they were aware that their rear wheel placement was adjustable. Just about all of the responses I received were complemented with a look that was similar to the one I had when my 5-year-old daughter told me she kissed a little boy on the bus and she now had her first crush – say what?

Seeing as there are so many different manual chairs on the market these days, there are also many different rear axle plates and ways to configure each chair. However all custom complex rehab manual wheelchairs (K0005) allow for COG adjustment. This is completed by moving the axle of the rear wheel forward or rearward along an axle plate on most folding frame chairs or sliding the axle plates (together in unison) along the side frames of most rigid frame mobility bases. Some chairs allow for a combination of both.

Center of Gravity

No matter how the adjustment is performed I believe we owe it to our end users to inform them of this option and allow them to try propelling their chair with a minimum of .5” of COG adjustment forward. (This measurement is typically taken from the front of the back posts to where your axle lines up along the side frame.) Once comfortable, most end users will successfully propel their chair with 1” of COG forward. This allows for better alignment of hand over the hub of the rear wheel when sitting in the chair, which leads to proper hand placement on the hand rim during propulsion ideally completing a 10-2 push (if you imagine the rear wheel represents a clock). Multiple studies have shown that even with minimal forward movement of the rear wheel, end users experience less work to propel their chair, find it easier to overcome thresholds and obstacles, reduce stress on their upper extremity muscles, and sit with better position in their chair. Ideally an end user wants 70-80% of their body mass over the rear wheels and the remaining percent of weight over the front caster for better propulsion performance.

Next time you prescribe and deliver a manual chair with an adjustable axle plate, or just happen to see a chair in your community with the axle in the most rearward position, take two minutes to ask the end user about their chair and if they know about adjusting the axle. You might just make a new friend that day!

Disclaimer: The contents of this blog are intended to be utilized as a general resource for clinicians and suppliers to then use clinical reasoning skills to determine optimal seating and mobility solutions for individual clients.  Steve and Angie are unable to answer questions from members of the general public including caregivers and end users. Members of the general public should direct specific questions to their own clinicians, medical, suppliers, or other health care professionals.


Posted by: Date: 4/18/2014 12:00:00 AM
Filed under: AdjustableAxlePlates, Configuration, Guest, ManualWheelchairs, Positioning
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