Consequences of a cross slope on wheelchair handrim biomechanics.

OBJECTIVE To test the hypothesis that pushing on a cross slope leads to increased handrim loading compared with that found on a level surface. DESIGN Case series. SETTING Biomechanics laboratory. PARTICIPANTS Twenty-six manual wheelchair users. INTERVENTION Subjects pushed their own wheelchairs on a research treadmill set to level, 3 degrees , and 6 degrees cross slopes. Propulsion speed was self-selected for each cross-slope condition. Handrim biomechanics were measured for the downhill wheel, using an instrumented wheelchair wheel and a motion capture system. MAIN OUTCOME MEASURES Speed, peak kinetics (force, rate of loading, torque), push angle, cadence, push distance, and power output were averaged over a 20-push set for each subject and each cross-slope condition. Outcomes were compared across cross slopes using a repeated-measures analysis of variance. RESULTS Push angle and cadence were unaffected by cross slope. A trend of decreasing self-selected speeds with increasing cross slope was not significant. There were considerable increases in the peak kinetic measures, with the axial moment increasing by a factor of 1.8 on the 6 degrees cross slope (P=.000). More pushes were required to cover the same distance when on a cross slope (P<.034). The power required for propulsion increased by a factor of 2.3 on the 6 degrees cross slope (P=.000). CONCLUSIONS Users must push harder when on a cross slope. This increased loading is borne by the users' arms, which are at risk for overuse injuries. Exposure to biomechanic loading can be reduced by avoiding cross slopes when possible.