The horizontal curvature of point-to-point movements does not depend on simply the planning space

Point-to-point movements constrained to the horizontal plane are generally straight, although they exhibit slight deviations from straightness. Unconstrained horizontal movements (i.e., movements where the hand is lifted) are more curved in their projection onto the horizontal plane than constrained movements. It has been argued that this difference in horizontal curvature is due to differences in the space in which the movements were planned (joint space versus work space, respectively). The current study challenged this explanation. We found that horizontal curvature of constrained movements increased when moving over a round surface compared to moving over a flat surface and when friction was low compared to when friction was high. Because all these movements were constrained movements this suggests that the earlier reported differences in horizontal curvature between constrained and unconstrained movements may not originate from a difference in the movements' planning space. The discussion addresses how factors related to planning and factors related to biomechanics may contribute to the magnitude of horizontal curvature in unconstrained and constrained movements.