In previous research on finger coordination involving the holding of hand-held objects, the main object of investigation was planar static tasks with the vertical orientation of the handle. In the proposed project, these delimitations will be removed. We propose to study: (a) dynamic (manipulation) tasks, (b) arbitrarily oriented handles and (c) prehensile tasks in three dimensions. We will perform five sets of experiments (12 total). Twelve testable hypotheses will be explored. In all experiments, the forces and moments exerted by individual digits in three directions, as well as the points of digit force application, will be recorded. The quest of the research is to discover the main strategies used by the central controller to manipulate the hand-held objects. Experimental set 1 (3 experiments) will test a hypothesis that people adjust their grip force to the force of gravity and inertial forces differently. Set 2 (3 experiments) will deal with mechanisms of tangential force sharing. In experiment 2-3, the data from experiments 2-1 and 2-2 will be used to test a hypothesis that tangential force sharing is determined by the mechanical properties of the fingers and their maximal forces. Set 3 (2 experiments) will address prehension synergies during non-vertical grasping. We expect to show that a popular concept, safety margin (SM), is not applicable during non-vertical grasping. Instead, two newly suggested measures, """"""""generalized safety margin"""""""" and """"""""operational friction coefficient"""""""", describe the experimental facts obtained in non-vertical grasping with good accuracy. Prehension synergies in three dimensions will be explored in experimental set 4 (3 experiments). One hypothesis is that if a torque plane does not coincide with the grasp plane (grasp is not collinear), the CNS solves the planar tasks in three dimensions. A second hypothesis is that the principle of superposition is valid in 3-D grasps for the planes that are orthogonal to the torque plane. Set 5 will further explore the superposition effects in control of digit forces. The subjects will manipulate a motorized handle that will either expand or collapse during object manipulation.
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