Manual wheelchair pushrim biomechanics and axle position

Objective: The biomechanics of wheelchair propulsion have been linked to upper extremity injury. Specifically, prior studies have correlated increased median nerve dysfunction with increasing propulsion frequency and a higher rate of rise of the resultant, or total, pushrim force. Despite this link, there is little research on the effect of wheelchair setup on propulsion biomechanics. The objective of this study was to determine the effect of rear axle position relative to the shoulder on pushrim biomechanics, Design: Case series. Setting: Biomechanics laboratory. Participants: Forty individuals with paraplegia who use manual wheelchairs for mobility. Intervention: Subjects propelled their own wheelchairs on a dynamometer at two different steady-state speeds and going from a dead stop to maximum speed, Bilateral biomechanical data were obtained using a force- and moment-sensing pushrim and a motion analysis system. Main Outcome Measures: Position of the axle relative to the shoulder at rest both horizontal (XPOS) and vertical (YPOS), and pushrim biomechanical variables including frequency of propulsion, peak and rate of rise of the resultant force, planar moment, and push angle. Partial correlation coefficients between relative axle position and propulsion biomechanics variables were calculated. Results: After controlling for subject characteristics, XPOS was significantly correlated with the frequency of propulsion (p <.01) and the rate of rise of the resultant force (p <.05). In addition, both XPOS and YPOS were significantly correlated with the push angle at multiple speeds (p <.05). Conclusion: Specific biomechanical parameters known to correlate with median nerve injuries were found to be related to axle position relative to the shoulder. Providing wheelchair users with adjustable axle position and then Fitting the user to the wheelchair can improve propulsion biomechanics and likely reduce the risk of injury.