Difficulty-dependent trajectory planning during target-reaching movements.

This study explored how the difficulty of a task influenced motor control during target-reaching movements. During the experiment, twelve healthy subjects were recruited to perform target-reaching tasks with three different target sizes over three distances as quickly and accurately as possible using their index fingers. There were nine levels of difficulty of the tasks, with a combination of three target sizes and three distances, and the difficulty of the tasks could be measured by Fitts' law in terms of the index of difficulty (ID). The kinematic variables to represent movement performance were peak velocity (Vpeak), percentage time to peak velocity (PTPV), normalized jerk score (NJS) and fApEn (fuzzy approximate entropy). The results showed both distance and target size significantly influenced these parameters with the exception of the effect of the target size on Vpeak. Vpeak and fApEn were only linearly related to the ID when the individual target size across movement distances was considered. And a linear relationship between PTPV or NJS and ID was found. The increase in the difficulty of the task could lead to a shift from feedforward to feedback control by the central nerve system. The findings in this study contribute to an understanding of the underlying motor control during target reaching movements and can be applied as a quantitative method of evaluation in the clinic in the future.