The maximum cooperative grasping mass and diameter of the human thumb and index
finger were investigated by 7560 grasp-release trials on various masses of solid cylinders
and various sizes of rings. The maximum grasping mass of the participants’ thumbindex finger depended on gender, age and the sum of thumb-index finger lengths
(P < 0.05), but not on the hand-used and ratio of index finger to thumb length (P > 0.05).
The maximum grasping diameter of the participants’ thumb-index finger depended on
the age, sum of thumb-index finger lengths and ratio of index finger to thumb length
(P < 0.05), but not on the gender and hand-used (P > 0.05). There was a non-linear
regression model for the dependence of the maximum grasping mass on gender, age
and the sum of thumb-index finger lengths and another non-linear regression model for
the dependence of the maximum grasping diameter on the age, sum of thumb-index
finger lengths and ratio of index finger to thumb length. Two regression models were
useful in the optimal size design of robotic hands intending to replicate thumb-index
finger grasping ability. This research can help to define not only a reasonable grasp mass
and size for a bionic robotic hand, but also the requirements for hand rehabilitation.