Second-order grasp planning reflects sensitivity to inertial factors

Previous studies have shown that people's grasps of objects are tuned to the objects' inertial properties. In most of those studies, information obtained about the attunement of the grasps to the objects' inertial properties was limited to first-order grasp planning (i.e., planning of grasps based on immediate task demands). We investigated attunement of grasps to an object's inertial properties in the context of second-order grasp planning (i.e., planning of grasps based on subsequent task demands). In Experiment 1, participants grasped a horizontal rod whose right or left end would be brought down onto a target. Consistent with previous findings, participants grasped the rod so as to complete the movement with a thumb-up posture, using an overhand grasp when the right end of the rod was to be brought to the target and an underhand grasp when the left end was to be brought to the target. They also grasped the rod to the right of center, but more so when doing this with an underhand than when with an overhand grasp. In Experiment 2, participants performed the same task with an asymmetrically weighted rod. Changes in subjects' grasps in Experiment 2 compared to Experiment 1 suggested that participants grasped the rod based on the inertial properties of the rod in a way that took advantage of the pendular properties of the hand-plus-object.