Nevertheless, other mechanical, physiological and musculoskeletal influences on radial pushrim forces are less obvious and are still not well understood. In particular, the role of the inertial forces has not been addressed in the literature to the best of our knowledge despite the large accelerations undergone by the upper extremities, even at moderate locomotion speeds. Quantifying these individual contributions requires a model of the wheelchair-user system, but the models proposed in the literature so far [10-14] have not been employed to address this particular question.