Axle Position of Wheelchair
Most lightweight and ultralight weight wheelchairs offer adjustable axle position. This allows the center of gravity to be adjusted appropriately for each individual, improving biomechanical efficiency and effectiveness of propulsion.
Discussion
There were four studies addressing the effect of rear axle position on wheelchair propulsion with individuals with a spinal cord injury.
Boninger et al. (2000) completed a study that showed axle position relative to the shoulder was associated with significant differences in pushrim biomechanics. They found that with the axle further back relative to the shoulder there is more rapid loading of the pushrim, and increased stroke frequency was required. Additionally, individuals attained a slower speed when starting from a dead stop and there was a decrease in the push angle. An increase in the vertical distance between the axle and the shoulder resulted in a decrease in push angle. With a decrease in push angle, force was applied to the pushrim for a shorter period and thus the frequency of propulsion had to increase to maintain speed. They suggested that providing users with a wheelchair with adjustable axle position and setting up the chair to meet the user’s needs could improve propulsion biomechanics and reduce the risk of secondary injuries because of wheelchair propulsion.
Mulroy et al. (2005) studied the effect of changing the fore-aft seat position on shoulder joint forces, moments and powers during three levels of effort of wheelchair propulsion. They found that the seat posterior position resulted in a statistically significant reduction in peak superior shoulder joint forces during free, fast and graded propulsion. They concluded that the posterior seat position may reduce the risk of rotator cuff tendinopathy.
Samuelsson et al. (2004) also studied the effect of rear wheel position on wheelchair propulsion and seating aspects. A more forward position of the rear wheel had a significant effect on stroke frequency and push angle. They also reported an increase in the weight distribution with the more forward position of the wheel. However, in their study they did not find any difference between the two-wheel positions with respect to mechanical efficiency, estimated exertion, and breathlessness, seating comfort, estimated propulsion qualities, pelvic position or activity performance.
Freixes et al. (2010) also assessed the changes in speed, acceleration, stroke frequency and shoulder ROM in relation to four different axle positions. The study showed that the up and forward axle position resulted in an increase in speed and acceleration with a higher stroke frequency and a decreased shoulder ROM. The axle position of down and backward axle position resulted in a lower speed and acceleration with a lower stroke frequency and an increased shoulder ROM. The authors indicated that these were clinically important findings for wheelchair propulsion in their homes.
Conclusion
There is level 4 evidence (from four post-test studies: Mulroy et al. 2005; Samuelsson et al. 2004; Boninger et al. 2000; Freixes et al. 2010) that the more forward the rear wheel is positioned, the greater the improvement in pushrim biomechanics, shoulder joint forces, push frequency, speed, acceleration and stroke angle.
