Robotically adjustable kinematics in a wrist-driven orthosis eases grasping across tasks

TL;DR

This work tackles the challenge of enabling easier, more natural grasping for individuals with C5-7 SCI by introducing a motorized wrist-driven orthosis (MWDO) that continuously modulates the wrist setpoint during reach-to-grasp. By comparing fixed-setpoint modes (S1 and S2) with a continuously adjustable VS mode, and benchmarking against an Unlinked condition, the study demonstrates that allowing dynamic wrist posture selection reduces task difficulty and perceived exertion across two everyday tasks (a plug and a deodorant stick). The results show task-dependent wrist postures and reduced upper-body compensation with VS, suggesting that continuous setpoint control can improve device versatility and user comfort. These findings point to a practical route for integrating robotic automation with body-powered grips to enhance long-term adoption, while also highlighting the need for home-environment validation and more intuitive VS switching mechanisms. The work lays groundwork for wearable assistive devices that adapt to task demands through seamless motor-assisted posture adaptation, potentially improving daily-life utility for people with SCI.

Abstract

Without finger function, people with C5-7 spinal cord injury (SCI) regularly utilize wrist extension to passively close the fingers and thumb together for grasping. Wearable assistive grasping devices often focus on this familiar wrist-driven technique to provide additional support and amplify grasp force. Despite recent research advances in modernizing these tools, people with SCI often abandon such wearable assistive devices in the long term. We suspect that the wrist constraints imposed by such devices generate undesirable reach and grasp kinematics. Here we show that using continuous robotic motor assistance to give users more adaptability in their wrist posture prior to wrist-driven grasping reduces task difficulty and perceived exertion. Our results demonstrate that more free wrist mobility allows users to select comfortable and natural postures depending on task needs, which improves the versatility of the assistive grasping device for easier use across different hand poses in the arm's workspace. This behavior holds the potential to improve ease of use and desirability of future device designs through new modes of combining both body-power and robotic automation.

Figures (8)

• Figure 1: Assistive devices used in this study. The Tenodesis Grasp Emulator (TGE) Chang2022 was worn by subjects with normative hand function and the Motorized Wrist Driven Orthosis (MWDO) McPherson2020 was worn by the subject with SCI. Images adapted from these prior publications.
• Figure 2: Actuation modes implemented into the assistive devices. The motor in S1 mode and S2 mode limits the wrist's ROM to 40 deg during wrist-driven grasping, and the setpoint of the wrist at the closed hand posture ($\alpha$) stays fixed at 50 deg and 30 deg, respectively. One link is disconnected in the Unlinked mode such that grasping is accomplished with finger flexion. The motor in VS mode takes on dual roles: in State 1 it allows the wrist to move freely without affecting hand posture, and in State 2 it assists with wrist-driven grasping by limiting the wrist's ROM to 40 deg. $\alpha$ in VS mode is determined by the wrist posture selected by the user at the end of State 1.
• Figure 3: Task specifications: a) Subjects grasped a plug over the top with their hand closing around the z axis, and a stick of deodorant, from the side with their hand closing around the y axis. b) Subjects moved objects between two target locations: one near the body and one far from the body.
• Figure 4: Subjects' mean (color) and standard deviation (shaded) of wrist flexion (+) and extension (-) angles in degrees over normalized task time.
• Figure 5: Subjects' wrist flexion (+) and extension (-) angles in degrees, when manipulating each object at each location. (*p$<$0.05, empty circles = outliers)