Methods for determining three-dimensional wheelchair pushrim forces and moments: a technical note

This technical note illustrates that some of the differences that have been reported regarding wheelchair propulsion may be due to the methods used to calculate key variables. Wheelchair ambulation is a very important form of locomotion that lacks a standard pushrim force and moment analysis system. We have developed tools for analyzing upper limb biomechanics during manual wheelchair propulsion. Among the tools is a system that allows the direct measurement of global coordinate forces F(x), F(y), F(z) and corresponding moments. The analytical techniques presented here allow calculation of radial (F(r)) and tangential (F(t)) forces, the determination of point of force application (PFA), and the moment applied by the hand (M(hz)). Our results show that the PFA can be calculated from kinetic data. Comparison of the PFA to the second metacarpophalangeal (MP) joint, calculated from kinematic data and used in previous studies, resulted in a 0.2 radian difference on average, with the PFA showing greater variation near the beginning and ending of the propulsion phase. Analysis of methods for calculating the applied tangential force showed that using the PFA provides a more accurate measurement of this force than the previous method of assuming negligible hand-moment contribution. The hand moment was compared using the calculated PFA and assuming the PFA was coincident with the second MP joint. Both methods provided similar results with a mean difference of 0.6 N x m. The methods presented in this paper provide a framework for analyzing wheelchair propulsion forces and moments.