Development of a Biomimetic Extensor Mechanism for Restoring Normal Kinematics of Finger Movements Post-Stroke

Cable-driven devices for hands allow compact and lightweight design that could provide various functional movements. However, for many patients post-stroke, cable-driven devices produce nonphysiologic movements, such as metacarpophalangeal joint hyperextension, due to their abnormal passive joint impedance. In this study, we developed a novel bio-inspired device mimicking the anatomy of the extensor mechanism of the human finger, which can be tuned for individuals to provide 'subject-specific' assistance to achieve physiological movement patterns. We first evaluated the proposed design via mathematical modeling and computer simulation. Its performance was then tested experimentally with twenty-four subjects, including six healthy and eighteen chronic stroke survivors. We determined the loading condition of the device from the experimental identification of passive joint impedance of each subject before device use. Our results showed that the proposed design could achieve improved spatiotemporal coordination of finger movements compared to conventional cable-driven design by providing 'subject-specific' assistance based on identified passive stiffness values of each subject. We also identified a significant (negative) correlation between the metacarpophalangeal joint stiffness and the intrinsic exotendon loading level across subjects. The proposed system can restore normal movement patterns for patients with different types of impairments, which were previously found important in improving rehabilitative outcomes.