Charles SK, Hogan N. Stiffness, not inertial coupling, determines path curvature of wrist motions. J Neurophysiol 107: 1230 -1240, 2012. First published November 30, 2011 doi:10.1152/jn.00428.2011.-When humans rotate their wrist in flexion-extension, radial-ulnar deviation, and combinations, the resulting paths (like the path of a laser pointer on a screen) exhibit a distinctive pattern of curvature. In this report we show that the passive stiffness of the wrist is sufficient to account for this pattern. Simulating the dynamics of wrist rotations using a demonstrably realistic model under a variety of conditions, we show that wrist stiffness can explain all characteristics of the observed pattern of curvature. We also provide evidence against other possible causes. We further demonstrate that the phenomenon is robust against variations in human wrist parameters (inertia, damping, and stiffness) and choice of model inputs. Our findings explain two previously observed phenomena: why faster wrist rotations exhibit more curvature and why path curvature rotates with pronation-supination of the forearm. Our results imply that, as in reaching, path straightness is a goal in the planning and control of wrist rotations. This requires humans to predict and compensate for wrist dynamics, but, unlike reaching, nonlinear inertial coupling (e.g., Coriolis acceleration) is insignificant. The dominant term to be compensated is wrist stiffness.kinematics; dynamics WRIST ROTATIONS ARE ESSENTIAL for proper upper limb function but have received little attention, despite the fact that common disorders such as stroke often result in significant wrist impairment. To improve the assistance and rehabilitation of wrist function requires a more thorough understanding of wrist rotations, in terms of both biomechanics and neural control. Wrist rotation paths exhibit a distinctive pattern of curvature (Charles 2008;Charles and Hogan 2010;Hoffman and Strick 1999), but the origin of that curvature is unclear. This report reveals the probable cause of this curvature pattern and what it implies about how the nervous system controls wrist rotations.During wrist rotations, the hand draws paths on a roughly spherical surface surrounding the wrist joint. These spherical paths could a priori be straight, like great arcs on a globe, or curved. We recently characterized the spatial characteristics of wrist paths between a central target (in neutral wrist position) and peripheral targets requiring flexion-extension (FE), radialulnar deviation (RUD), or a combination (Charles and Hogan 2010). We found that while individual wrist rotations show substantial variability, on average wrist rotation paths show an intriguing pattern of curvature: In general, 1) outbound and inbound paths curve to opposite sides of a straight line (Fig. 1); 2) movements in the same direction but to opposite targets curve to the same side (for example, moves from an extended wrist position to neutral position and moves from neutral position to a flexed position, both of which in...