Kinematic Modeling of Human Forearm Rotation With Distal Radio-Ulnar Circumduction

A circumduction of the distal ulna as a complex movement has been observed at the distal radio-ulnar joint during forearm rotation and it has been reported that a stationary ulna makes postures of the hand to be transposed or tilted in the pro-supination. In this paper, a new forearm model reflecting the ulnar circumduction is proposed by changing the joint types of a spatial linkage mechanism. For this purpose, a conventional forearm model with R-R-U-S joint types (established by Whiteley in 2000) which generates the horizontal movement of the distal ulna is firstly analyzed to figure out a kinematic feature for its joint motions. And then this model is reconstructed as an equivalent model with R-R-U-U joints. At the same time, its joint types are realigned as U-R-U-R joints to realize the ulnar circumduction. Finally, a structural design based on the proposed forearm model with the U-R-U-R joints is presented, and its overall movements are described.