Effect of rear wheel suspension on tilt-in-space wheelchair shock and vibration attenuation

Abstract

Suspension systems are designed to reduce shock and vibration exposure. Prior to the QuadshoX LLC suspension kit (Fort Collins, CO), manual tilt-in-space wheelchairs did not have rear wheel suspension available for use. Furthermore, it was anticipated that rear wheel diameter would have an independent effect on shock and vibration transmitted to the wheelchair. The aim of this study was to investigate the shock and vibration reducing capabilities of the newly available aftermarket rear wheel suspension system and wheel diameter for manual tilt-in-space wheelchairs. Ten healthy non-wheelchair users volunteered for the study (7 men, 3 women: age 22.1±3.36 yrs, height 1.75±0.067 m, weight 73.9±8.87 kg (mean±SD)). Subjects were pushed by the same trained investigator over four different obstacles while using a Quickie IRIS® Tilt-in-Space manual wheelchair (Sunrise Medical, Phoenix, AZ) with two different diameter solid wheels, (0.381 m and 0.508 m), Primo Cheng Shin Tires (Cheng Shin Rubber, Yuanlin, Taiwan). Surfaces included a/an 1) exterior door threshold, 2) truncated domes, 3) 2 cm descent, and 4) 2 cm ascent. The subjects traversed the obstacles with the wheelchair as manufactured, and followed ~ 2 weeks later with the QuadshoX suspension kit installed. A tri-axial accelerometer, (Model339A31, PCB Piezotronics, Depew, NY), was mounted to the rear of the wheelchair seat pan with signals sampled at 2000 Hz. Peak resultant accelerations were analyzed from surface 1, 3-4, root mean square (RMS) resultant accelerations were analyzed from surface 2, and vibration dose value (VDV) and total power were analyzed from all surfaces 1-4. Unweighted and ISO 2631-1 frequency weighted (FW) accelerations were analyzed. The use of suspension decreased the un-weighted peak acceleration at the rear wheel when it impacted the door threshold, and when the rear wheel traversed the 2 cm descent and ascent (p=0.043, p=0.001, p=0.001, respectively) and FW peak accelerations at the rear wheel when it impacted and left the door threshold, and when the rear wheel descended 2 cm (p=0.049, p= 0.001, p= 0.005, respectively). With suspension, RMS and total VDV significantly decreased 14% and 10- 22% respectively (p=0.011, p=0.004). There were no significant differences between the rigid and suspended chair in total vibration power in frequency octaves most harmful in human exposure (4 – 12 Hz). The results of wheel diameter were not evaluated because there were significant differences in time spent over the obstacles between the two diameters (door threshold p= 0.018, truncated domes p= 0.028, 2 cm descent p= 0.029, 2 cm ascent p = 0.024). However, there were not differences in time spent over the obstacles between rigid and suspended conditions (p ≥ 0.064). The results indicate the aftermarket rear wheel suspension reduces some aspects of shock and vibration exposure, specifically at the rear wheel. While low back pain, neck pain, discomfort, and muscle fatigue correlate with shock and vibration exposure there is no set threshold of reduction in shock and vibration exposure to decrease the health risks with exposure. Considering how much time tilt-in-space users spend in their wheelchairs, we expect the observed reductions in shock and vibration with the use of the aftermarket rear wheel suspension may decrease the health risks, such as pain and muscle fatigue.