Potential Clinical Applications for

Center of Pressure Measurement

Jean L. Minkel

Introduction

In his article, "Managing Maneuverability and Rear Stability of Adjustable Manual Wheelchairs: An Update" 1, Jamie Tomlinson outlines the factors which increase a person’s maneuverability and the effect these adjustments have on rear stability. He states, "a health care professionals challenge is to optimize wheelchair technology for each user." The article then presents several complex formulas to give the health care professional information about wheelchair characteristics and the effect adjustments will have on each characteristic.

I had just finished reading the article when I was asked if I could think of new applications for pressure mapping technology. I recognized the importance of the information in the article, but realized very few practicing health care professionals were going to take the time to calculate the effect of moving the rear wheel forward a couple of millimeters (or inches). Tomlinson uses a calculated Center of Mass in his formulas. I understood the pressure map would not be measuring the Center of Mass, but wondered if it could measure the center of pressure of a wheelchair system – chair and user- to get a visual presentation of the stability/maneuverability of a chair and a user?

Development Steps

A very rough prototype of a wheelchair pressure map, to prove the concept, was designed by Vista Medical2 using an FSA torso size pressure map. The torso mat was originally designed for reading pressure on a bed mattress surface. The overall size of the mat and the number of sensors is more suitable to capture the "footprint of a wheelchair", than the smaller wheelchair seat or back mats. By placing the torso mat on the floor and covering it with a thin, ¼ to ½ inch, high density foam, the relative pressure distribution on all four wheels of the chair can be measured, and the center of pressure of the system –the person and the chair can be calculated. The use of the foam distributes the pressure across several sensors on the mat and prevents a concentration of pressure under one or two sensors.

The prototype proved several interesting pieces of information:

Using a convenience sample of a wheelchair softball team, some preliminary testing was done. The members of the team were all men. Most of the men had a spinal cord injury, level of injury ranging from low cervical to low thoracic. Other team members had low extremity amputations, above knee, unilateral and bilateral. The team members were asked to roll onto the rubber mat and observe the read-out on the screen. The screen showed the weight distribution under the rear wheels and the caster wheels. A diagram to the side displayed the percentage of weight under each wheel.

During initial pilot testing, some interesting observations were made:

  1. The static stability of the system – chair and user – should be viewed looking at two pieces of information:
  1. There were two fairly consistent patterns of distribution of weight observed:
    1. 75% on rear wheels and 25% on front casters – lower level of injury
    2. 60% on rear wheels and 40% on the front casters – higher level of injury.
  1. Several rigid frame chairs were not "square". The weight distribution was not even in the right to left/front to back weight distribution.

Potential Clinical Applications

This information may be useful in varied ways. For new wheelchair users, learning to position in and to hold a chair in a "wheelie", watching the monitor and observing movement of the center of pressure marker on the screen may aid in their learning. For experienced users, who want to replicate the set-up of their existing chair in a new chair, a baseline read, in their current chair, can guide adjustments for a new chair, using objective, center of pressure measurements. For sports enthusiast, and all those perfectionists seeking the "perfect setting", this system provides an objective readout, which should be able to be reproduced on another chair or at another time, if desired.

Like any tool, this measurement system needs to be used with clinical judgement, which takes into account the context in which the information is being collected. The center of pressure marker is based on 2-dimensional location based on width and length) calculation and does not reflect the location of the 3-dimensional center of mass (width, length and height).

Conclusion

Chairs come with an increasing number of adjustments, but how may user and or health care providers know how to use these adjustments to optimize the performance of the chair. Perhaps if we could objectively record the effect each adjustment we made has on the user’s performance, we save time by using effective adjustments.

1Tomlinson JD. Managing Maneuverability and Rear Stability of Adjustable Manual Wheelchairs: An Up Date. Physical Therapy, 2000; 80: 904-911.

2 Vista Medical, Ltd. 120 Maryland St. Winnipeg, MB Canada R3G 1L1 www.pressuremapping.com

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