Planning for manual positioning: the end-state comfort effect for manual abduction-adduction.

How people take hold of objects depends on what they plan to do with them. Such anticipatory effects reflect motor planning. One class of such anticipatory effects is the end-state comfort effect, a tendency to take hold of an object in an awkward way to permit a more comfortable, or more easily controlled, final position. Here we asked whether the end-state comfort effect extends to manual abduction-adduction, a kinematic dimension that has not previously been studied in connection with the end-state comfort effect. Our participants brought their right hand from a start position (hand resting on a table) to an intermediate position (hand on top of a covered bowl that had an arrow extending from it) and then slid the bowl to a final position such that the arrow had to occupy a target range. We found that participants placed their hands on the bowl at an angle that was inversely related to the final angle of the hand. This relation was uninfluenced by whether participants chose between two possible final positions rather than being told which final position to adopt. In addition, there was no effect of the rejected final position on the intermediate or final hand angles adopted. The results provide evidence for the generality of the end-state comfort effect and for the flexibility of motor planning in general.