Influence of object texture on grasping behaviour

When picking up objects using a pinch-grip there are often numerous places at which one could place the thumb and index finger, yet people seem to be consistent in where they place them. They grasp objects in such a manner that a line connecting the fingers would pass through or above the object's centre-of-mass (COM), presumably in order to minimize torque and therefore the required grip force. However, the required grip force does not only depend on the torque but also on the object's shape, its weight and the surface friction at the points at which it is grasped. Here we investigate whether participants adjust their grasping points if the surface near the COM is slippery while off-centre areas are not. Doing so would increase the torque but decrease the grip force that is required to prevent slipping. Participants were asked to lift polished aluminum bars, while their grasping points were recorded. The bars were oriented horizontally, with their center aligned with the participants' body midline. Two different bar lengths were used: 26cm and 13cm. One end of each bar was covered with anti-slip tape. The bars varied in the horizontal offset between the COM and the edge of this high friction area with offsets of 0, 1 and 2cm. Fully covered bars and bars without any anti-slip tape served as control conditions. We examined whether participants grasp further off-centre in the direction of the high friction area. The slipperiness of the surface affected the height of the grasping points, indicating that participants were aware of the difference in friction. Nevertheless, the influence on the primary measure of interest, the horizontal grasping location, was minimal. This shows that the judged COM largely determines how an object is grasped, with limited importance given to surface friction.