RESNA 27th International Annual Confence
Vibrations During Manual Wheelchair Propulsion Over Selected Sidewalk Surfaces Are Sensitive To Weather-Related Surface Wear
Little information is known about the vibrational exposure experienced during wheelchair propulsion even though there is convincing evidence suggesting vibration doses can have detrimental health effects. We performed this study to determine if the vibrational dose value (VDV) experienced during manual wheelchair propulsion over six common sidewalk surfaces was sensitive to weather-related wear. We recorded the three-dimensional wheelchair seat accelerations while subjects (n=10) propelled over six sidewalk surfaces, and repeated the experiment after 16 months. Analysis of variance (ANOVA) results revealed that VDV was sensitive to both surface type and time-point. Our results demonstrate that particular sidewalk surfaces can minimize the risk of detrimental health effects from vibrations, and that weather-related wear preferentially reduces the surface roughness of the roughest surfaces.
“vibrations; wheelchair; sidewalk; injury”
There is convincing evidence that long-term exposure to whole body vibrations is linked to low back pain  and causes detrimental effects to the musculoskeletal and peripheral nervous systems [1,2, 3]. Published studies focus primarily on occupational sources of whole body vibration, which are the major sources of potentially harmful vibration for the majority of the population. The situation is different for wheelchair users since they experience vibrations constantly as they propel their wheelchair over different surfaces and terrains during mobility. The few studies that have investigated vibrations experienced during wheelchair propulsion reported that whole body vibration levels can contribute to fatigue among manual wheelchair users , and are sensitive to the type of surface traversed [5, 6] and rate of travel across the surface . These important initial studies indicate that vibration exposure during wheelchair propulsion may be a source of secondary injury to the user, and a more complete understanding of the life-activities of the user and their environment ( e.g., surfaces) are necessary to characterize the impact vibrations may have on the user.
We performed this study to investigate the environmental aspects of the vibration dose, and specifically tested whether VDVs experienced by users propelling manual wheelchairs over common sidewalk surfaces are sensitive to weather-related wear. This study is an extension of a previously reported study  and looks specifically at the sensitivity of VDVs to weather-related wear of common sidewalk surfaces. We hypothesized that weather-related wear (after 16 months) would not significantly affect the VDV of manual wheelchair propulsion over the selected sidewalk surfaces.
Consenting unimpaired subjects (n=10) propelled an instrumented manual wheelchair along each of six sidewalk surfaces three times each (3x6=18 trials per subject) in a random order. Subject demographics mean (SD): age 29.4 (9.35) years, weight 69.7 (17.1) kg, height 170.18 (10.89) cm. The five matched subjects in the second time-point did not have significantly different height (p=0.97) or weight (p=0.93) distributions then their matched counterparts when tested with an independent sample t-test.
A standard sidewalk surface (poured brush-finished concrete (Surface 1)) was used as the standard comparison. Surfaces 2, 3, and 4 were interlocking pavement arranged in a 90-degree herringbone pattern and had square edges, 3.2mm beveled edges, or 6.4mm beveled edges, respectively. Surfaces 5 and 6 were fired clay brick arranged in a 45-degree herringbone pattern and had square edges and 3.2mm beveled edges, respectively.
Instrumentation and data collection: For all trials we used a rigid-frame manual wheelchair (Quickie GP) with a standard foam cushion instrumented with a three-axis accelerometer on the seat. Subjects were required to propel the wheelchair down the surface at 1m/s (+/- 0.05 m/s) while acceleration data were collected through a tether attached to an analog-to-digital card in a personal computer running customized data-collection software. One experimenter triggered the computer to begin and stop collecting data when the subjects crossed marked lines at the beginning and end of the (4.8m) sidewalk surfaces. Another experimenter walking behind the subject held the tether clear of the wheelchair and timed the subject to assure the 1m/s (+/-0.05 m/s) rate was achieved. In cases where the subject propelled too fast or slow, the trial was redone. Each subject was given approximately three initial trial runs to adjust the required rate.
Like in our previous studies of this type [4, 5, 6], we used the International Organization of Standards (ISO) 2631-1  to guide the calculation of the VDV and interpret the results. The ISO standards describe the mathematical weighting of accelerations and the exposure limits that are associated with comfort, decreased performance, and an exposure limit. We calculated the VDV as specified by the standards by summing over the weighted accelerations (over the entire trial) taken to the forth power, and then taking the forth root of the sum (Eq. 1):
We repeated the experiment after 16 months to determine if vibrational dose value is sensitive to weather-related wear. Five of ten subjects were not able to repeat the experiment the second year and height- and weight-matched subjects were used in their place.
Statistics: We performed a mixed-model repeated measures ANOVA on the data in SAS (, using seat VDV as the dependant variable, and surface (6 repeated levels), and year (2 levels) as the independent factors. If the surface factor was significant, we used a Tukey post-hoc test to determine significant differences among factor levels. Significant differences were assumed using an alpha level of 0.05.
The repeated measures ANOVA revealed that both main factors (time and surface) were significant effects. Significantly lower VDVs were found in the second year, and a post-hoc on the interaction term (surface*year) indicated that this was due to a decrease in VDVs on surfaces 1 and 4 in the second year. A post-hoc analysis on the surface factor reveal identical results to those reported in our previous study : surface 4 had significantly higher VDVs than all other surfaces, surface 1 (reference) had significantly higher VDVs than all other surfaces except 4, and surface 2 had significantly lower VDVs than all other surfaces (Fig. 1).
Our results are consistent with our previous results  suggesting surfaces 1 and 4 induce the highest levels of vibration to the wheelchair user, while surface 2 induces the lowest vibrations. These results are intuitive when considering the geometry of surfaces 1,4, and 2. The standard sidewalk (brushed-concrete) surface (1) has joints between the concrete sections, which cause acceleration peaks when the wheels (especially the front caster wheels) cross the joints. Similarly, surface 4 has the largest bevels (6.4mm), which effectively make it rougher than the rest of the surfaces. Surface 2 has square edges, with the interconnecting blocks forming a smooth surface.
When investigating the source of the VDV decrease in the second year, our post-hoc revealed this effect was due to decreases in VDV for surfaces 1 and 4, which correspond to the surfaces with the highest VDVs. (Fig. 1). This intuitive result suggests that weather-related wear reduces surface roughness, and the roughest surfaces are most affected. Thus, weather-related wear has an ameliorating effect on the VDV induced from sidewalk surfaces. We assume that these effects slow over longer periods of time, and are accelerated due to other types of wear (such as pedestrian wear) that does not cause large-scale damage to the sidewalk. Confirmation of this is the topic of further studies.
While this study presents a well-designed controlled environment to evaluate VDV for selected surfaces, it does have limitations. Due to subject availability and variability, we tested unimpaired individuals instead of expert wheelchair users. Subject attrition also made it necessary to use height- and weight-matched controls for 5 of the 10 subjects used. These two limitations may reduce the internal and external validity of this study. Future studies should focus on wheelchair users and investigate vibrations induced by a larger selection of surfaces.
This study was partially funded by a consortium of the Interlocking Concrete Pavement Institute (ICPI), Brick Industry Association (BIA) and the National Concrete Masonry Association (NCMA). In addition, funding was provided by the VA rehabilitation Research Affairs (F2181C), the U.S. Department of Education, National Institute on Disability and Rehabilitation Research (NIDRR) Rehabilitation Engineering Research Center on Wheeled Mobility (H133E990001), and a National Science Graduate Fellowship.
Jonathan Pearlman, M.Sc.
Human Engineering Research Laboratories,
VA Pittsburgh Healthcare System
7180 Highland Drive, Building 4,
2nd Floor East, Pittsburgh, PA, 15206
Phone: (412) 365-4850,
Fax: (412) 365-4858,