The control of arm pointing movements in three dimensions

In this study we investigated pointing movements made with an extended arm. Despite the large number of mechanical degrees of freedom, limb orientation adopted during pointing could be described by rotation axes contained on a two-dimensional curved surface. As a result of the curvature, the orientation of a linear plane approximating a small region of the curved surface was dependent on the location of the movements within the full workspace. These results account for earlier suggestions that limb orientation could be described by coplanar rotation vectors and that the orientation of the plane moved with the workspace. Despite the additional complexity, our results indicate that the number of degrees of freedom used to position the extended forearm is reduced from four to two for normal pointing movements. Contributions to orientation of the wrist and hand by supination/pronation of the forearm were minor for changes in shoulder yaw angle. However, supination/pronation added significantly to orientation of the hand for changes in shoulder pitch angle.