Static rear stability of conventional and lightweight variable-axle-position wheelchairs.

Wheelchair users with high or posteriorly placed centers of mass often complain of rear tipping on inclines or when accelerating. In this study we compared the rear stability of occupied conventional and lightweight wheelchairs and determined the effect of various rear axle positions. Ten normal subjects were studied in both a conventional (22.6 kg) and a lightweight (12.3 kg) wheelchair. Twenty-one experienced wheelchair users were also studied in their own wheelchairs. Stability points were determined with brakes unlocked on a tilt platform. Inter- and intraobserver test-retest reliabilities were 0.97 and 0.974, respectively. For the normal subjects the mean (+/- 1SD) stability point in the conventional chair was 28.6 (+/- 2.7) degrees. In the lightweight chair their mean stability points ranged from 21.7 (+/- 2.6) degrees to 9.4 (+/- 2.9) degrees, depending upon axle position. The mean difference between the conventional chair and the lightweight chair in its most stable position was 6.9 (+/- 1.7) degrees (t = 13.04, p less than 0.0001) and in its least stable position 19.3 (+/- 2.4) degrees (t = 24.97, p less than 0.0001). Experienced wheelchair users in their own lightweight wheelchairs were less stable than users of conventional chairs (t = 2.16, p less than 0.05) or chairs with posteriorly offset axles (t = 3.64, p less than 0.01).