Abstract
Choosing appropriate grasp points on objects requires considering several factors. We aim to understand grasp preference when we put natural hand configuration in conflict with grasp stability. Twenty-one participants grasped and lifted objects using their right hand’s thumb and index finger. We rotated three brass cuboids to be either aligned or unaligned with the individual’s natural grasp axis (NGA) to investigate when and how people stray from their natural hand configuration. Grasp stability was manipulated by changing the surface properties on the cuboids: (a) all-brass, (b) two opposing sides covered with thin wooden strips and (c) two opposing brass sides smeared with Vaseline with the remaining two sides covered with sand paper. Analyses of the grasps on the all-brass object revealed the effect of object orientation: Participants grasped the NGA aligned cuboids with both, clockwise and counterclockwise grasps, but the cuboids rotated away from the NGA primarily with clockwise grasps. This behavior avoided extreme joint angles at the moment of the grasp. Grasps on the wood and sand paper objects, however, were significantly attracted toward the higher friction sides: People grasped the wooden sides more often than the brass sides and the sand paper sides more often that the Vaseline covered brass. The attraction towards the higher friction surfaces was significantly stronger in the sand paper than wood conditions. Our findings show that participants do not tolerate grasp locations on lower friction surfaces to maintain their usual grasp configuration; they instead favor grasps that ensure stability over hand configurations that may minimize energy expenditures. A simple model in which individual participants’ NGAs are shifted proportionally to the visually-perceived friction of the cuboid surfaces neatly accounts for our results. These findings suggest a crucial role of surface material properties in grasping.