Fig. 10.

a, Stiffness ellipses during force regulation tasks without cocontraction at the proximal center hand position (subject A). Each ellipse represents the spatial characteristics of the elastic property of the arm at the corresponding hand position. All ellipses are aligned in a polar manner according to the force directions and magnitudes requested in each task. The ellipses placed in the innermost circle represent the hand stiffness during force regulation tasks at 5 N in eight directions. Similarly, 16 ellipses placed in the second, third, and fourth circle positions represent those during force regulation tasks at 10, 15, and 20 N in each direction, respectively. The arrow on each ellipse denotes the force magnitude and direction. The stick picture in the center of the polar graphs shows the arm configuration of each subject.b, The characteristics of the stiffness ellipses (sizeA, shape s, and orientation ϕ_e–ϕ_h) of all subjects during force regulation tasks without cocontraction at the proximal center hand position. Thetop graphs represent the normalized torque at the shoulder and elbow. In the second, third, and fourth graphs, solid,dashed, dotted, anddash-dot lines denote each index (see ordinate label) during force regulation tasks at 5, 10, 15, 20 N, respectively. The force regulation tasks at 20 N were not applied to subjects C and D. The numbers on the abscissa denote the force directions applied to the handle.