EFFECTIVENESS OF REAR SUSPENSION IN REDUCING SHOCK EXPOSURE TO MANUAL WHEELCHAIR USERS DURING CURB DESCENTS

Andrew M. Kwarciak, BSE; Rory A. Cooper, PhD; Erik J. Wolf, BSE.

slide 1: Title and Authors


Background

slide 2: Background

Slide text:

  • W/C users are get whole-body vibrations on a daily basis from:
    • bumps, curb descents, and uneven terrain.
  • Extended exposure to such vibrations causes discomfort and physiologic trauma:
    • e.g. cardiovascular excitation, herniated discs, degradation of vertebrae, & chronic low back pain.
  • Compromises the ability of the spine and back muscles to distribute suddenly applied loads, thereby increasing rider vulnerability towards further injury.
  • Response from wheelchair manufacturers: incorporate suspension elements into frame.

Research Question

slide 3: Research Question

Slide text:

  • Though manufacturers have developed suspension wheelchairs in an effort to reduce whole-body vibrations and improve rider comfort, no data are available to assess the effectiveness of their designs.
  • The purpose of this study was to determine whether selected manual suspension wheelchairs reduce shock vibrations transmitted to users during curb descents as compared to folding x-brace wheelchairs.

Methods

slide 4: Methods

Slide text:

  • 3 rear suspension wheelchairs:
    • Colours Boing, Invacare A6S, and Quickie XTR
  • 3 folding x-brace wheelchairs:
    • E&J Epic, Invacare Action Xtra, and Quickie 2
  • 3 different curb heights:
    • 2”, 4” and 6”
  • Three trials were performed per chair, each consisting of three randomized curb descents of varying height.

Data Collection

slide 5: Data Collection

Slide text:

  • Data were collected from a tri-axial accelerometer fitted to an aluminum seat plate
    • At 200 Hz and filtered with a 2nd order, 100 Hz Butterworth low-pass filter
  • A MATLAB program was implemented to detect the raw and frequency weighted (ISO standard) peak-to-peak accelerations
    • Accelerations within the 4-8Hz, the resonant frequencies of the human body in the seated position, receive the highest weighting
  • Repeated measures ANOVA was performed to determine statistical difference between chairs

Results

slide 6: Results

Slide text:

  • Both plots exhibit similar trends amongst chairs
  • Repeated measures ANOVA between chairs: raw (p = .243), frequency weighted (p = .160)

Graphic description:

The plots show the mean peak-to-peak seat accelerations and the mean frequency weighted peak-to-peak seat accelerations for every wheelchair tested. The accelerations recorded from each chair increase along with each increase in curb height. The Quickie XTR experienced the lowest accelerations for each curb height. Overall, there does not appear to be any significant difference between the performance of the standard and suspension manual wheelchairs.


Discussion

slide 7: Discussion

Slide text:

Behavior of the accelerations indicate that the shock reducing elements do not adequately reduce seat vibrations transmitted to the user during curb descents.
  1. Wheelie descents effectively reduce the benefits of rear suspension.
    • Fixed alignment of the suspension units cannot adapt to, and therefore may not be able to reduce, vibrations introduced at angles not perpendicular to seat pan
  2. Oscillations within suspension, caused by impact, increase the overall amount of vibrations.

Type Comparison

slide 8: Type Comparison

Slide text:

  • Repeated measures ANOVA between XTR and x-brace chairs:
    • raw (p = .057), frequency weighted (p = .077)

Graphic description:

This plot shows the average seat acceleration at each of the three curb heights for the folding x-brace wheelchairs, the suspension wheelchairs, for just the Colours Boing and Invacare A6S wheelchairs, and for just the Quickie XTR wheelchairs. These four curves demonstrate a near linear relationship between curb height and measured seat acceleration and the difference in performance between the XTR and the other two suspension models.

The folding x-brace wheelchairs had the highest seat accelerations at both the 4” and 6” curb height, whereas the Boing and A6S group had the highest seat acceleration at the 2” curb height. Conversely, the XTR had the lowest seat acceleration at every curb height. As a result the XTR skewed the combined suspension wheelchair data, lowering the overall averages for the group.

There was no significant difference between the XTR and the x-brace wheelchairs; however, the differences between the two were much greater than those calculated between both wheelchair types.


Performance

slide 9: Performance

Slide text:

  • The Quickie XTR was the only wheelchair that demonstrated a significant amount of shock reduction.
    • Colours Boing also demonstrated reduced seat accelerations over most trials
  • It is suspected that spring-based suspension systems provide a better method of vibration dampening than elastomer-based systems during curb descents.
    • Might compensate for the misalignment of the fixed element.

Conclusions

slide10: Conclusions

Slide text:

  • Type and orientation of integrated frame suspension is critical in determining its effectiveness of reducing shock vibrations.
  • The Quickie XTR demonstrated the highest level of vibration reduction and greatly improved the performance of the suspension wheelchair group.
  • Overall suspension wheelchairs may reduce vibrations during level orientation, but they did not prove effective in reducing shocks imposed during curb descents.

References

slide11: References

Slide text:

1. Pope MH, Wilder DG, Magnusson ML. A review of studies on seated whole body vibration and low back pain. Proc Instn Mech Engrs 1999; 213(H): 435-446.

2. Griffin MJ, Handbook of Human Vibrations. San Diego: Academic Press Inc., 1990. p 173-186.

3. Dupuis H, Hartung E, Haverkamp M, Acute Effects of Transient Vertical Whole-Body Vibrations. Int Arch Occ Env Health 1991; 63: 261-265.

4. Evaluation of Human Exposure to Whole-Body Vibration – Part 1: General Requirements, (1985). ISO 2631-1, Washington, DC: ANSI Press.


Acknowledgements

slide12: Acknowledgements

Slide text:

This study was partially supported by the National Institute on Disability and Rehabilitation Research, Rehabilitation Engineering Research Center (H133E990001), and the U.S. Department of Veterans Affairs, Rehabilitation Research and Development Service (F2181C).

The End

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Updated: May 6, 2003

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