Effect of handrim velocity on mechanical efficiency in wheelchair propulsion

To study the effect of tangential speed of the handrims independent of external power output on gross mechanical efficiency (ME), nine able-bodied subjects performed wheelchair exercise tests on a stationary ergometer. The ergometer allowed for measurement of torque and three-dimensional forces on the rims and tangential velocity of the rear wheels. The experiment comprised two series of submaximal tests against constant external power outputs (0.25 and 0.50 W.kg-1) and four wheelchair speeds (0.83, 1.11, 1.39, and 1.67 m.s-1), which simulated a wheelchair speed of 1.67 m.s-1 and mechanical advantages of 0.43-0.87. ME stayed below 10.5% and changed inversely with speed of movement of the handrims. Peak torques on the right handrim stayed even with speed, leading to a significant increase in peak power output. Energy losses owing to braking torques at the beginning and end of the push phase increased with handrim speed but hardly exceeded 5 W. The effective force component applied to the handrims was below 71% of the magnitude of the total force vector and dropped up to 13% with increasing handrim speed. It is suggested that an ineffective direction of forces on the rims might (partly) be responsible for the low ME and for a decrease in ME in relation to tangential handrim velocity. This suggestion is discussed from a number of theoretical perspectives. It is concluded that the use of handrims with a lower mechanical advantage will increase wheelchair propulsion efficiency.