At QuadshoX, its not just rear suspension or front suspension, it is suspension. It has been medically proven that adding some type of suspension or shock absorber to a wheelchair, bicycle, stroller, or car can improve the overall functionality of the device as well as the quality of ride for the user.
Raoul F. Reiser II, PhD, FACSM, CSCS Associate Professor
Director – Clinical Biomechanics Laboratory Health & Exercise Science
Colorado State University Fort Collins, CO 80523
Objective: The goal of this project was to determine the shock reducing effectiveness of an aftermarket rear-wheel suspension system manufactured by QuadshoX, LLC (Fort Collins, CO) for tilt-in-space manual wheelchairs.
Methods: Peak resultant accelerations were analyzed from a tri-axial accelerometer (Model 339A31, PCB Piezotronics, Depew, NY, sampled at 2k Hz) mounted either to the rear of the seatpan of Quickie tilt-in-space wheelchairs (Phoenix, AZ) (Figure 1) or to a bicycle helmet worn by the subjects. Five QuadshoX users volunteered to participate. Subjects were pushed by a trained caregiver over four preselected common outdoor surface scenarios in both their QuadshoX equipped wheelchair and one without. Surfaces included a 1) building exit door with elevated threshold followed by new concrete sidewalk with regularly spaced expansion seems (Figure 2), 2) stretch of fractured asphalt (Figure 3), 3) road crossing with bubbled transition ramp from sidewalk (Figure 4), and 4) pea gravel (Figure 5). All surfaces were horizontally level. Subjects performed 3 acceptable trials over each surface first in the rigid wheelchair followed by trials in their suspension equipped chair with the accelerometer mounted on the seatpan. Rigid chair trials all had to be within 0.5 s as determined by hand timing. Suspension chair trials also had to be within 0.5 s of each other with the average within 0.5 s of those in the rigid chair. Similar requirements existed for the trials conducted with the subjects wearing the helmet. All shocks were inspected and adjusted prior to data collection. The suspension system of one chair was determined to not be functioning properly, and could not be fixed on site. As a result, only four subjects are included in the analysis (2 men, 2 women; 3 with cerebral palsy, 1 with quadriplegia; age = 27.7±7.0 yrs; height = 162±24 cm; mass = 50.2±17 kg (mean±SD)). Three of the suspension equipped wheelchairs had rear wheels with 15” diameter, one with 12” diameter. The rigid chair had 12” diameter rear wheels. Posterior tilt of all chairs was 15°. Suspension system use ranged from 1-24 months.
Figure 1: Triaxial accelerometer mounted to the rear of the wheelchair seatpan.
Results from the seatpan mounted trials conducted on all four surface scenarios are included in this report. From each acceptable threshold and new sidewalk trial, peak resultant accelerations from 10 points were extracted (Figure 6). In addition to analyzing points A through I, the averages of A + B, C + D, E + G + I, and F + H + J were also analyzed. For the other three surfaces the root mean square (RMS) over a region of interest (ROI) of the resultant acceleration was calculated. For the fractured asphalt, the ROI was 10s in the middle of the section were fractures were the greatest (Figure 7). For the road crossing, the ROI was the average RMS on both sides of the street, inclusive of the sidewalk seams and bubbled metal (Figure 8). For the gravel, the ROI was 10s after getting the wheelchair to a steady speed (Figure 9).
A representative value for each point/ROI was created for each subject by averaging values from each of their three successful trials. To assess differences between rigid and suspension equipped chairs, paired, double-sided T-Tests were conducted on pooled individual data in Excel 2013 (Microsoft Corp., Redmand, WA). Statistical significance was set at p≤0.05. Trends were identified at p≤0.10.
Results: There were no significant differences in the time to complete the threshold and new sidewalk section in the seatpan mounted condition in the two chairs (rigid seatpan = 10.11±0.71 s; suspension seatpan = 10.08±0.50 s; p=0.783). Peak accelerations at the rear wheel were significantly reduced by the shocks at all points (C, D, F, H, & I; p<=0.011) and the combined points (CD & FHJ, P<=0.002), with differences ranging from 47 to 79% (Figure 4). Peak accelerations were also reduced at the front wheel by the suspension system, but not to the same extent. Significant reductions only existed at point E (p=0.008) with a trending difference at combined point EGI (p=0.091) associated with the seams in the new sidewalk (Figure 4). No significant reductions were observed at the front wheels associated with traversing the threshold (p>=0.147).
There were no significant differences in the time to complete the fractured asphalt section in the seatpan mounted condition in the two chairs (rigid seatpan = 7.98±0.52 s; suspension seatpan = 7.86±0.76 s; p=0.441). The 10s RMS of the resultant acceleration was reduced by 50% with the shocks (p=0.013) (Figure 11).
There were no significant differences in the time to complete the road crossing section in the seatpan mounted condition in the two chairs (rigid seatpan = 12.94±0.96 s; suspension seatpan = 12.77±1.09 s; p=0.354). The average RMS of the resultant acceleration over the two ROI was reduced by 50% with the shocks (p=0.050) (Figure 11).
While the trials conformed to the time constraints set in place for this study, those in their suspension chair were completed slightly faster than those in the rigid chair when traversing the gravel section in the seatpan mounted conditions (rigid seatpan = 8.93±0.74 s; suspension seatpan = 8.78±0.82 s; p=0.037). The 10s RMS of the resultant acceleration was reduced by 56% with the shocks (p=0.002) (Figure 11).
Conclusions: QuadshoX significantly reduce resultant accelerations (aka, shock) at the seatpan of tilt-in- space manual wheelchairs. From the threshold and new sidewalk surface results it is clear that reductions in shock are greatest when impacts are at the rear wheel of the chair. However, reductions in shock are also reduced when impacts are at the front wheels, though not to the same extent. These rear-axel mounted suspension systems appear to be able to absorb smaller impacts at the front wheels, but not larger impacts. Across all surfaces, reductions in shock are on the order of 50-80%. These 2 reductions are 1+ g (1 g = acceleration of earth’s gravity). Differences of this magnitude in peak accelerations between the rigid and QuadshoX suspended chair are greater than those reported by Kwarciak et al (2008) when comparing rigid wheelchairs to old design suspended wheelchairs traversing 5, 10, and 15 cm curb descents. Differences of this magnitude in RMS between the rigid and QuadshoX suspended chair are similar to the differences reported by Wolf et al. (2007) when comparing a manual wheelchair to a powered wheelchair traversing concrete with an interlocking textured brick pattern and 6 mm bevel. Overall, it is anticipated that the absorption of shock by QuadshoX dramatically improve the comfort of the user when traversing rough terrain. Acceleration magnitudes are a component of whole body vibration (Lis et al., 2007), which has been linked to low-back pain, neck pain, muscle ache and fatigue, and other harmful effects in seated individuals (Griffin, 1990). Further, it has been shown that manual wheelchairs experience levels of vibration that could cause fatigue and injury (Van Sickle et al., 2001). Therefore, with reductions of shock on the order of 50% and greater, it is expected that QuadshoX not only improve comfort, but also reduce risks for secondary injuries of the neck, spine, and trunk.
Figure 2: Exit door threshold (left) and sidewalk seam (right) traversed in the first over ground scenario.
Figure 3: Section of fractured asphalt (left) with overhead close-up of roughest section (right) traversed in the second over ground scenario. Subjects stayed to the right of the white line.
Figure 4: Road crossing with bubbled transition ramp from sidewalk (left) and close-up of bubbled section (right) traversed in the third over ground scenario.
Figure 5: Pea gravel section (left) and overhead close-up of typical matrix (right) traversed in the fourth over ground scenario.
Figure 3: Exemplar resultant acceleration profile with 10 peak values marked. A = front wheel first contact with threshold, B = front wheel leaving the threshold, C = rear wheel first contact with threshold, D = rear wheel leaving the threshold, E = front wheel contacting first sidewalk seem, F = rear wheel contacting first sidewalk seem, G = front wheel contacting second sidewalk seem, H = rear wheel contacting second sidewalk seem, I = front wheel contacting third sidewalk seem, J = rear wheel contacting third sidewalk seem.
Figure 7: Exemplar resultant acceleration profile of fractured asphalt section. The RMS of the resultant acceleration was calculated over the middle 10 seconds, bounded by dark vertical lines in graph. ROI = Region of Interest.
Figure 8: Exemplar resultant acceleration profile of the road crossing section. The RMS of the resultant acceleration was calculated over each sidewalk to road transition, inclusive of both the seams in the concrete and the metal bubbles, bounded by dark vertical lines in graph. ROI = Region of Interest.
Figure 9: Exemplar resultant acceleration profile of gravel section. The RMS of the resultant acceleration was calculated over 10 seconds, bounded by dark vertical lines in graph. ROI = Region of Interest.
Figure 10: Group mean and standard deviation resultant accelerations of rigid seatpan (RS) and suspension seatpan (SS) trials over the threshold and new sidewalk section. A = front wheel first contact with threshold, B = front wheel leaving the threshold, C = rear wheel first contact with threshold, D = rear wheel leaving the threshold, E = front wheel contacting first sidewalk seem, F = rear wheel contacting first sidewalk seem, G = front wheel contacting second sidewalk seem, H = rear wheel contacting second sidewalk seem, I = front wheel contacting third sidewalk seem, J = rear wheel contacting third sidewalk seem, AB = average of A + B, CD = average of C + D, EGI = average of E + G + I, FHJ = average of F + H + J . * p≤0.10, ** p≤0.05. Percent reduction indicated for differences with p≤0.10.
Figure 11: Group mean and standard deviation RMS of the resultant accelerations of rigid seatpan (RS) and suspension seatpan (SS) trials over the fractured asphalt, road crossing, and gravel sections. ** p≤0.05. Percent reduction indicated for each.
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A RESNA Study (012700) June 1995, conducted by Gerald Weisman and Dryver R. Huston of the Vermont Rehabilitation Engineering Research Center at the University of Vermont Burlington, Vermont studied whole body vibration as “ride quality” and concluded that:
“Whole body vibration has been identified as a contributing factor to the “ride quality” of wheelchair mobility. Ride quality in this context generally refers to the comfort of the wheelchair user.”
“Low back pain is a common problem among people with disabilities, particularly those who use wheelchairs. “Ride quality” of manual and powered wheelchairs has been a topic of discussion and research for some time. The recommendations of the Wheelchair III Workshop held in 1982 included a statement that, “the ride of quality of the equipment is considered to be a research item.” However, at the same time it was acknowledged that ride quality was ill defined and “research and development effort is needed to provide good safety and comfort to the user.” While whole body vibration has been identified as contributing to “ride quality” for wheelchair users, no one has suggested that this vibration may be a contributing factor to low back pain experienced by wheelchair users. Numerous studies have determined that there may be some correlation between whole body vibration exposure and low back pain. The most commonly reported effects noted in the literature are low back pain, early degeneration of the lumbar spinal system and herniated lumbar disc.”
If suspension is necessary for a bike, why not a wheelchair…
A study conducted by Dr. Frobose from the German Sports University of Cologne confirmed my observations. The goal of the study, conducted by Dr. Frobose and RockShox, a Colorado Springs- based manufacturer of bicycle suspensions, was to learn more about the affect of absorption of hits and vibrations on the human body while riding various mountain bike configurations.
“The average reduction in impacts and vibrations throughout the whole body was more than 20 percent for the suspension bikes vs. the rigid bikes. The biggest benefits were found on lower backs, with a 33 percent reduction on full suspension bikes. The suspension-seat post was surprisingly efficient, with a 25 percent reduction.”
Dr. Frobose sums up all these experiences by saying, “While riding a bike, shocks and vibrations mainly stress the spine and the vertebra discs. Therefore, back problems occurring during or after a ride are not simply a result of tired muscles but more the outcome of consistent stress fatigue caused by shocks and vibrations, which hammer straight into the weak spot of the human body, the back. This proves that suspension on mountain bikes becomes more important in relation to health. If someone wants to put suspension on their bike, they should start at the rear; that is where the spine needs it most.”
“Spine problems and back pain are among the top health problems people have to cope with today.”
“Suspended bicycles will reduce physiological fatigue and increase comfort. Riding a suspended bicycle will increase your satisfaction, improve bicycle control, improve your efficiency and decrease muscle trauma and fatigue.”