Quantification of the User-wheelchair System Stability based on the CoP Trajectory within the Base of Support

Aim of the work was the introduction of a user-wheelchair Stability Index able to give objective information to find the most appropriate wheelchair settings both for daily and sports use. The Stability Index was introduced after asking two users to assume four pre-defined postures over a dynamometric force platform and measuring the centre of pressure (CoP) trajectory. After identifying the base of support centroid Ctd and the instantaneous CoP, the Stability Index expresses how far is the instantaneous CoP from the sides of the base of support outline: the index is calculated with respect to each direction (forward, rearward, right and left side) and referred to the centroid of the base of support. The Stability Index values increase proportionally with the stability: a zero value means that CoP is on the outline, a unit value corresponds to coincidence between CoP and Ctd. A preliminary evaluation of the Stability Index was performed by varying both back adjustment and user posture. The proposed method only requires a force plate or a stabilometric plate. The evaluation of user-wheelchair system stability can be performed in real time, merging this objective parameter with user subjective feeling in order to propose the most satisfactory solution to users, manufacturers and clinicians.