Fig. 11.
The theoretical (surface) and experimental (open circles) variations of orientation (ϕe–ϕh; top graphs) and shape (s; bottom graphs) of the stiffness ellipse at a distal ([x, y] = [0.0, 0.5]m; left graphs) and proximal ([0.0, 0.35]m; right graphs) postures according to the change in stiffness ratios (Ree/Rse,Rcj/Rss). In thetop graphs, the surface representing theoretical variation of orientation split away at 0 rad, indicating that the major axis of ellipse is in the hand–shoulder direction. Experimental data points (open circles) on the surfaces were derived from Equations 5 and 6 with the stiffness ratios of subject A, realized in all tasks. The ellipses for two sets of stiffness ratios (indicated on the bottom axes plane of top figures) are depicted for distal and proximal postures. Each ellipse size is normalized by the major axis of the ellipse. Their orientations and shapes are indicated by filled diamonds on the corresponding surfaces.