Different methods of calculating body sway area

Posturography is used to assess the steadiness of the human body by measuring the movement of the centre of pressure of a standing subject on a force platform (stabilometry). This paper presents three different methods of calculating the centre of pressure trajectory. The first method ('Convex hull') is characterized by the area enclosed by the path of movement (body sway area), approximated by the area of a convex hull. PROC G3GRID is applied for the triangulation of the data points necessary for calculating the convex hull. This approach is compared with the second and most common procedure ('principal component analysis, PCA) which calculates an ellipse enclosing the sample points. PROC PRINCOMP is used to calculate the eigenvectors that represent the derived ellipse of the PCA. A third approach used in clinical studies ('Mean of Circle Areas') calculates the body sway area by summarizing the mean of the circle areas defined by the sample points and their distance to the point of origin. Simulation data are used to compare the different analysis methods. The SAS® annotate facility displays the results graphically.