Design and validation of 3D printed hand interfaces for wrist stiffness assessment

Estimating the joint impedance in humans is crucial for better understanding human motor control and assessing neuromuscular diseases. The static passive component of the impedance, i.e. passive stiffness, is particularly relevant for the evaluation of neurological disorders, such as Parkinson’s Disease. In this paper we focus on the estimation of passive wrist stiffness, which is commonly done by moving the subject’s wrist through hand interfaces- manually or with robotic devices. However, handles employed so far often use not ergonomic designs or may request subjects to grasp the handle, causing muscle contraction which, in turn, affects wrist stiffness. We designed, 3D-printed and evaluated three different designs for ergonomic hand interfaces (H1, H2, H3) to passively move the hand while avoiding grasping or undesired muscle contraction. Performing features were evaluated on four healthy volunteers, during passive wrist manipulation performed by three different experimenters, in terms of: i) relative motion between actual hand movements and movements imposed by the experimenters; ii) activation of wrist muscles in the forearm; iii) subjects’ perceived comfort. All three devices showed good performance in terms of comfort, minimization of muscular activity and relative motion, which on average was around 0.010 m for the relative translations and $8.5 \deg$ for the relative rotations. However, H2 and H3 showed slightly higher performance compared to H1 in terms of lower relative motion, while H2 was also rated as the most comfortable.