Direction-dependent differences in temporal kinematics for vertical prehension movements.

In our daily lives, we can appropriately perform movements on the earth, suggesting that the central nervous system takes into account gravitational forces that act on our bodies during the movements. Recently, gravitational forces have been observed to generate the direction-dependent differences in the spatial properties of the kinematics of prehension movements. However, little is known about how gravitational forces affect the temporal properties of the kinematics of these movements. In this study, we tried to elucidate the gravitational effects on the temporal properties of the kinematics of movements by comparing upward (against gravity) and downward (with gravity) movements. As a result, we found the direction-dependent differences in temporal kinematics in both the reaching and grasping components of movements. For the reaching component, a shorter acceleration time was observed for the upward movements compared to the downward movements. For the grasping component, participants opened their hands earlier and faster for the upward movements than for the downward movements. These direction-dependent differences in the temporal kinematics suggested that the central nervous system takes into account and takes advantage of gravitational effects in the motor plans and controls of vertical prehension movements.