Studies have suggested that the proficiency of an end effector is the primary factor that defines kinematics of reach-to-grasp movements across the types of effectors, such as the hand or a tool. In particular, the duration of the plateau, or the time of static open aperture (i.e., the distance between tips of effectors), is typically longer for tool use compared with natural grasping with a hand. This study investigated how improvement in the proficiency of tool use modifies the kinematics of reach-to-grasp movements. To clarify the effects of required accuracy on the kinematics in tool-use grasping, movement speed and difficulty of grasping were manipulated. The results showed that plateau duration, the length of which indicates that reaching and grasping components are temporally dissociated, shortened as tool-use practice proceeded. These results indirectly support the idea that shortened plateau duration was induced by improvement in the proficiency of tool use. Moreover, plateau duration was shortened at faster movement speeds or under conditions not requiring accurate grasping, even without any practice of tool-use grasping. Additional analyses found that plateau duration did not scale with movement time. These results suggest that the kinematic features supposed to be characteristic of tool-use grasping are not inevitable but are greatly influenced by a strategy that is not intentionally but rather automatically implemented to compensate for the lack of proficiency of end effectors, in agreement with the idea that the brain focuses on the tips of an end effector regardless of its effector type in reach-to-grasp movements. NEW & NOTEWORTHY This study is the first reporting the relation between characteristic aperture time profile, called plateau duration, and movement time of tool-use grasping. The results suggest that improved coordination between reaching and grasping components was induced by improvement in the proficiency of tool use but not by just shortened movement time. The results also indicate the possibility that the constraints for calculations in motor planning are essentially the same between hand-use grasping and tool-use grasping.
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