Normal internal force (ordinate) as a function of the sine of the handle orientation angle (abscissa). The force is normalized by object weight (corresponding to the masses of 830, 1080 or 1330 g) and hence is dimensionless. Group average data are shown. Note that the curves to the right (supination) and to the left (pronation) on the zero inclination line are slightly concave and do not converge to the zero force levels. (Adapted by permission from T.C. Pataky, M.L. Latash, and V.M. Zatsiorsky. Prehension synergies during nonvertical grasping, I: experimental observations. Biological Cybernetics, 2004, 91: 148–158.)