Characterization of Coupled Wrist and Forearm Stiffness

Wrist stiffness has been shown to have a greater effect on wrist dynamics than inertia and damping for all but the fastest wrist rotations [1]. Therefore, making coordinated wrist rotations requires that humans predict and compensate for the effects of wrist stiffness. Because wrist stiffness is anisotropic (with greater stiffness in radial-ulnar deviation than flexion-extension), some movement directions require more torque than others. We suspect that humans may use their forearm to allow their wrist to rotate along “paths of least resistance.” Evaluating this hypothesis requires knowledge of the stiffness encountered during wrist and forearm movements. Previous research has only measured the passive stiffness of the wrist or forearm separately. While these measurements inform us of the stiffness encountered during wrist or forearm movements, humans rarely move in pure wrist or pure forearm movements—upper limb movements usually involve combinations of wrist and forearm movements. The present study characterizes the passive stiffness of the wrist and forearm throughout their combined three degrees of freedom in flexion/extension (FE), radial/ulnar deviation (RUD), and pronation/supination (PS).