Hand orientation for grasping depends on the direction of the reaching movement

The 3D orientation of the hand for grasping was studied while subjects reached for objects placed at several locations on a horizontal board, with movements starting from three initial hand positions. The hand movements were recorded with electromagnetic sensors giving 3D position and orientation information. The study focused on the azimuth, which is the projection of the hand orientation in a horizontal plane. The hand azimuth for grasping was linearly correlated with the direction of the reaching movement and not with the object direction in head- or shoulder-centered coordinates. This relationship was valid regardless of the initial hand position. A control experiment with constant movement direction showed a weaker, probably postural, effect of object direction in shoulder-centered coordinates. We suggest that hand orientation for grasping is mainly controlled in relation to the reaching movement direction.