Selection Criteria for Preparatory Object Rotation in Manual Lifting Actions

Participants lifted a canister by its handle while balancing a ball on the lid. Experiment 1 allowed object rotation prior to lifting. A lifting comfort zone was measured by the variability in object orientation at lift; its size depended on the object mass and required task precision. The amount of pre-lift rotation correlated with the resulting change in lifting capability, as measured for different object orientations. Experiment 2 required direct grasping, without preparatory rotation. Task completion time and success rate decreased, and initial object orientation affected pre-lift time. Results suggest that lifting from the comfort zone produces more robust performance at a cost of slower completion; moreover, physical rotation could be replaced by mental planning when direct grasping is enforced.