End posture selection in manual positioning: evidence for feedforward modeling based on a movement choice method

Although it is well known that the postures adopted at the ends of movements depend on where one starts and how one moves, it is not yet clear whether those differing end postures are selected before movements begin. Two experiments were designed to test the hypothesis that end postures for positioning movements are chosen before movements commence. The experiments were further designed to check whether movements are internally simulated before overt movements occur and end-postures are still being selected. To address these questions we used a movement choice method. Participants were presented with two possible end postures and were asked to choose between them by moving to the one that seemed easier to adopt. End-posture choices were affected by starting positions and also by the movements that would have to be made, as affected by having obstacles in the way. The results suggest that participants relied on feedforward modeling of prospective movements as they selected end postures prior to overt movement production. The fact that the movement choice method could confirm this suggests that the method holds considerable promise as a tool for investigating motor planning.