Path curvature in workspace and in joint space: evidence for coexisting coordinative rules in aiming

In this study we tried to establish whether point-to-point aiming movements are planned in workspace, joint space, or both. Eight right-handed subjects performed horizontal, vertical, and diagonal aiming movements on a transversal plane. Movements were performed at several speeds. Curvature variations of the hand and corresponding joint-space paths were investigated as a function of position, direction, and speed. Straightness of hand paths predominated for vertical movements but was systematically violated for horizontal and top-right to bottom-left movements. Furthermore, the hand-path curvature of the latter movements increased with speed. Joint-space paths showed more deviation from a straight line than hand paths except for top-left to bottom-right movements in which the paths were equally curved. A comparison of normalized path curvatures at the hand and joint level indicated that in aiming, the coordinative rule of straight-line production seems to apply to both workspace and joint-space planning. The present findings confirm Kawato's (1996) views that optimization processes operate concurrently at the two control levels of arm-trajectory formation under study.