The Role of the Thumb: Study of Finger Motion in Grasping and Reachability Space in Human and Robotic Hands

It is well acknowledged that the opposing thumbgranted humans advanced manipulation capabilities. However,such feature is not statistically quantified and its representationis not formally addressed in robotics yet. This paper studieswhether the displacement of the opposing thumb in humansis a determining factor for shaping the grip. Using statisticalanalysis of the variability of motion capture data from theGRASP database, we found that the displacement of the thumbplays a leading role on the shaping of the grip, independentlyfrom the specific object being grasped. Furthermore, we map andcompare the reachability spaces of the human thumb and twostate-of-the-art robotic thumbs - the Shadow and the iCub hands.We conclude that the kinematics of robotic thumbs does notevenly span the reachability space of the human thumb, favouringprecision grasping motions. Hence our findings contributes to thediscussion of the optimal modelling of robotic hands.