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

Abstract

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.

Choosing Actions

“…Ending in a comfortable state may be less important than occupying postures affording the most control. For further evidence, see Künzell et al (in press). …”

Habitual vs Non-Habitual Manual Actions: An ERP Study on Overt Movement Execution

“…It seemed that participants' initial grasp choices were influenced by the anticipated precision or control needed at the end of the task. Further findings indicating that movements are not planned towards end-state comfort but rather towards a comfortable posture at the moment, when control is needed, have been reported by Hughes et al [11] and Künzell et al [12]. Hughes et al [11] varied the precision demands at the beginning and end of a bar transport task and observed initial state comfort for 50% of their participants.…”

Perceptuo-motor planning during functional reaching after stroke

“…In addition, neither age nor cognitive impairment significantly influenced the grasp-height effect in the current study. Increased demand on precision has also been suggested to influence the end-state comfort effect such that a more comfortable hand or grasp position is selected in the phase in which the highest precision is needed (Kunzell et al 2013 ; Rosenbaum et al 2012 ). However, this was unlikely to have influenced our results since in our task, the requirements for precision were low, as the target platform was larger than the base of the plunger and the plunger itself had a steady base.…”