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Collision-Avoidance Characteristics of Grasping. Early Signs in Hand and Arm Kinematics

Grasping an object successfully implies avoiding colliding into it before the hand is closed around the object. The present study fo- cuses on prehension kinematics that typically reflect collision-avoidance characteristics of grasping movements. Twelve participants repeatedly grasped vertically-oriented cylinders of various heights, starting from two starting positions and performing the task at two different speeds. Movements of trunk, arm and hand were recorded by means of a 3D motion-tracking system. The results show that cylinder-height moder- ated the approach phase as expected: small cylinders induced grasps from above whereas large cylinders elicited grasps from the side. The collision-avoidance constraint proved not only to be accommodated by aperture overshoots but its effects already showed up early on as differ- ential adaptations of the distal upper limb parameters. We discuss some implications of the present analysis of grasping movements for designing anthropomorphic robots.