Development of motor planning in children: Disentangling elements of the planning process.

Second-order motor planning of grasping movements is usually measured using tasks that focus on the relative (dis)comfort of end posture of the arm and hand regardless of the objective outcome of performance. This may underestimate the ability for forward planning in young children. In the current study, we aimed to examine the developmental mechanisms of motor planning in children using a task that necessitates second-order motor planning for its successful completion. We tested 311 children (aged 5-12 years) who were instructed to grasp and rotate a hexagonal knob over 60°, 120°, 180°, or 240°. The 180° and 240° rotation conditions necessitated adjustment of the preferred start grip for successful task completion. We examined successful or unsuccessful task completion, reaction time (RT), and movement time (MT) as a function of task demands (i.e., rotation angle) and age. Results showed that most children of all ages were able to successfully complete the task in the 180° rotation condition. In the most demanding 240° rotation condition, many children had difficulty in completing the task, but successful task completion increased with age. Time course analysis showed increased RT and MT with increasing task demands. Furthermore, whereas RT decreased with age for each rotation angle, MT remained stable with the exception of an increase in MT for the most demanding rotation condition. Together, these results exemplify that children aged 5-12 years are indeed able to engage in forward planning. With development, second-order motor planning proficiency increases, especially for more demanding movements, and the process becomes more efficient.