Abstract
When we grasp an object, we must select appropriate contact points on the surface of the object to accomplish a comfortable and stable grasp. Previous studies have identified multiple key factors that influence the selection of grasp points during precision grip grasps. The natural grip axis is one such constraint that human participants are typically unwilling to oppose. In this study, we aimed to challenge the preference for the natural grip axis by selecting objects for which grasps aligned with the natural grip axis would result in differing levels of torque. We tracked participants’ index finger and thumb while they lifted L-shaped cuboids that were presented in various orientations (-45°, 0°, 45°, 90° or 180° counter-clockwise) with respect to the natural grasp axis. The cuboids were either made purely out of wood (light weight) or brass and wood (high weight) such that the objects’ weight distribution would produce high torques if the grasp axis and surface normals at the contact points with the object were misaligned. Analyses of the grasp patterns showed that participants had strong perceptual intuitions regarding the torque produced by different grasps. Specifically, for the brass object participants consistently counter-balanced torque by selecting grasp points such that the thumb and index finger placements required low forces to lift and manipulate the objects. These results suggest that humans are able to make physically accurate predictions about the behavior of complex objects.