A UNIFIED METHOD FOR CALCULATING THE CENTER OF PRESSURE DURING WHEELCHAIR PROPULSION

The measurement of the center of pressure (COP) has been and continues to be a successful tool for gait analysis. The definition of a simila r COP for wheelchair propulsion, however, is not straightforward. Prev iously, a COP definition similar to that used in force plate analysis had been proposed. Unfortunately, this solution has the disadvantage o f requiring a separate COP definition for each plane of analysis. A de finition of the generalized center of pressure (GCOP) which is consist ent in all planes of analysis is derived here. This definition is base d on the placement of a force-moment system, equivalent to the force-m oment system at the hub, on a line in space where the moment vector (w rench moment) is parallel to the force vector. The parallel force-mome nt system is then intersected with three planes defined by anatomical landmarks on the hand. Data were collected using eight subjects at pro pulsion speeds of 1.34 m/s and 2.24 mis (1.34 m/s only for subject 1, 0.894 m/s and 1.79 m/s for subject 8). Each subject propelled a wheelc hair instrumented with a SMART(Wheel). A PEAK 5 video system was used to determine the position of anatomical markers attached to each subje ct's upper extremity. The GCOP in the transverse plane of the wrist fo rmed clusters for all subject's except subject 2 at 1.34 m/s. The clus tering of the GCOP indicates that the line of action for the force app lied by the hand is approximately perpendicular to the transverse plan e through the wrist. When comparing the magnitude of the moment vector part of the wrench with the moment of the force vector of the wrench about the hub, the wrench moment is approximately an order of magnitud e smaller. This indicates that the role of the wrist for wheelchair pr opulsion is primarily to stabilize the force applied by the arm and sh oulder. (C) 1998 Biomedical Engineering Society.