Why does an obstacle just below the digits’ paths not influence a grasping movement while an obstacle to the side of their paths does?

When we grasp objects in daily life, they are often surrounded by obstacles. To decrease the chance of colliding with an obstacle, people tend to move in a manner that does not bring body parts too near to the obstacle. However, in a previous study, when we compared moving above empty space and moving above an obstacle (a table), we did not find an effect of the obstacle on the height of the digit’s paths despite the fact that the distance between the final positions of the digits and the obstacle was marginal. This lack of effect seems to be inconsistent with what we know about avoiding obstacles, because we would expect an increase in the height of the digits’ paths when the obstacle is present. We consider four possible explanations for the lack of effect: that people changed movement speed rather than movement path, that the height component is not sensitive to obstacles that do not physically obstruct the movement, that obstacles below the starting position are not taken into account because the digits do not enter the space below the starting position, and that manipulable obstacles interfere with movement planning while a table does not. We found that from these four explanations only not taking obstacles placed below the starting position into account can be responsible for the lack of effect found in our previous study.