Why are the digits' paths curved vertically in human grasping movements?

When humans grasp an object off a table, their digits generally move higher than the line between their starting positions and the positions at which they end on the target object, so that the digits' paths are curved when viewed from the side. We hypothesized that this curvature is caused by limitations imposed by the environment. We distinguish between local constraints that act only at the very beginning or the very end of the movement, and global constraints that act during the movement. In order to find out whether the table causes this vertical curvature by acting as a global constraint, we compared grasping a target object positioned on a table with the same task without the table. The presence of the table did not affect the vertical curvature. To find out whether constraints at the beginning and end of the movement cause the vertical curvature, we manipulated the constraints locally at those positions by letting the subject start with his digits either above or below the end of a rod and by attaching the target object either to the top or to the bottom of another rod. The local constraints at the start of the movement largely explain the vertically curved shape of the digits' paths.