Projection of the point of force application onto a palmar plane of the hand during wheelchair propulsion

The objective of this study was to develop and test a method for projecting the pushrim point of force application (PFA) onto a palmar plane model of the hand. Repetitive wheelchair use often leads to hand and wrist pain or injury. The manner by which the hands grasp the pushrim and how the forces and moments applied to the pushrim are directed may contribute to the high incidence of pain and injury. The projections of the PFA onto the palmar surface model of the hand reside primarily within zone II. These results are in agreement with previous studies which have assumed the PFA to be coincident with one of the metacarpophalangeal (MP) joints. However, the results from three subjects show different PFA patterns within the palmar surface of the hand which can be related to each subject's propulsion pattern, and the PFA is not focused at a single MP joint. Projection of the world coordinates of the four hand marker system onto the palmar plane show the resolution to be within 3 mm, or one half the diameter of the passive reflective markers. The errors in the planar model assumption were greatest for the second and fifth MP markers. This was expected because as the hand grasp changes these markers do not remain coplanar. The results of this study indicate that new knowledge about how forces are applied by the hand onto the pushrim can be obtained using this method. This technical note provides insight into understanding the details within the kinetics of wheelchair propulsion and describes a technique for estimation of the PFA on the palmar surface of the hand. This technical note provides initial results from three different wheelchair users.