Optimal control in the critical phase of movement: a functional approach to motor planning processes.

Grasping movements are often planned in a way that they end in a position where joints are in an anatomically medial position. This behaviour is termed the "end-state comfort" (ESC) effect (Rosenbaum et al., 1990). We suggest that the anatomically medial position is favoured to control the most difficult part of the movement. In most experiments investigating ESC, objects have to be placed onto a target location, and the highest precision demand occurs at the end of the movement. Thus, ESC is confounded with movement difficulty. In this study, we dissociate movement difficulty and ESC. In our experiments, participants had to execute a task where the critical part of the movement was either at the end or at the beginning of the movement. Participants' grasping behaviour confirmed the hypothesis that movement planning is constrained by a goal for optimal control during the part of the movement that demands the highest precision, rather than by a goal to end in a comfortable state (Rosenbaum, Chapman, Weigelt, Weiss, & van der Wel, 2012). We identified recall and movement plan generating processes of motor planning (Cohen & Rosenbaum, 2004), that ensure the optimal control in the critical part of movement. Our results indicate that recall processes depend on motor experience which is acquired in different time scales. We suggest that motor planning processes are triggered only if the costs for executing movements controlled by recall processes exceed the costs for generating a motor plan.