Center or side: biases in selecting grasp points on small bars.

Choosing appropriate grasp points is necessary for successfully interacting with objects in our environment. We brought two possible determinants of grasp point selection into conflict: the attempt to grasp an object near its center of mass to minimize torque and ensure stability and the attempt to minimize movement distance. We let our participants grasp two elongated objects of different mass and surface friction that were approached from different distances to both sides of the object. Maximizing stability predicts grasp points close to the object's center, while minimizing movement costs predicts a bias of the grasp axis toward the side at which the movement started. We found smaller deviations from the center of mass for the smooth and heavy object, presumably because the larger torques and more slippery surface for the heavy object increase the chance of unwanted object rotation. However, our right-handed participants tended to grasp the objects to the right of the center of mass, irrespective of where the movement started. The rightward bias persisted when vision was removed once the hand was half way to the object. It was reduced when the required precision was increased. Starting the movement above the object eliminated the bias. Grasping with the left hand, participants tended to grasp the object to the left of its center. Thus, the selected grasp points seem to reflect a compromise between maximizing stability by grasping near the center of mass and grasping on the side of the acting hand, perhaps to increase visibility of the object.