title: From reaching to reach-to-grasp: the arm posture difference and its implications on human motion control strategy.
abstract: Reach-to-grasp arm postures differ from those in pure reaching because they are affected by grasp position/orientation, rather than simple transport to a position during a reaching motion. This paper investigates this difference via an analysis of experimental  data collected on reaching and reach-to-grasp motions. A seven-degree-of-freedom (DOFs) kinematic arm model with the swivel angle is used for the motion analysis. Compared to a widely used anatomical arm model, this model distinguishes clearly the four grasping-relevant DOFs (GR-DOFs) that are affected by positions and orientations of the objects to be grasped. These four GR-DOFs include the swivel angle that measures the elbow rotation about the shoulder-wrist axis, and three wrist joint angles. For each GR-DOF, we quantify position vs orientation task-relevance bias that measures how much the DOF is affected by the grasping position vs orientation. The swivel angle and forearm supination have similar bias, and the analysis of their motion suggests two hypotheses regarding the synergistic coordination of the macro- and micro-structures of the human arm (1) DOFs with similar task-relevance are synergistically coordinated; and (2) such synergy breaks when a task-relevant DOF is close to its joint limit without necessarily reaching the limit. This study provides a motion analysis method to reduce the control complexity for reach-to-grasp tasks, and suggests using dynamic coupling to coordinate the hand and arm of upper-limb exoskeletons.
authors.affiliation: Department of Mechanical Engineering, Worcester Polytechnic Institute, Worcester, MA, 01609, USA. zli11@wpi.edu.
authors.forename: Zhi
authors.identifier: 0000-0002-3194-5176
authors.initials: Z
authors.lastname: Li
authors.affiliation: Department of Computer Engineering, University of California, Santa Cruz, Santa Cruz, CA, 95064, USA.
authors.forename: Dejan
authors.initials: D
authors.lastname: Milutinović
authors.affiliation: Department of Mechanical and Aerospace Engineering, University of California, Los Angeles, Los Angeles, CA, 90095, USA.
authors.forename: Jacob
authors.initials: J
authors.lastname: Rosen
mesh_terms: D001132:Arm; D001696:Biomechanical Phenomena; D005260:Female; D018737:Hand Strength / Q000502:physiology*; D006801:Humans; D008297:Male; D008954:Models, Biological; D009038:Motion; D009949:Orientation; D011187:Posture / Q000502:physiology*; D011597:Psychomotor Performance / Q000502:physiology*; D016059:Range of Motion, Articular / Q000502:physiology*; D012399:Rotation; D013223:Statistics as Topic
keywords: Arm motion control; Human-like robot; Task-relevance; Upper limb exoskeleton