The wrist brace in the video employs a function generator to map the constant stiffness of a standard torsion spring to the angle-dependent stiffness of the wrist of a stroke victim suffering from spasticity. Those suffering from this condition have a difficult time controlling the angle of their wrist, which tends to settle at the extremes of flexion.
Related publication:
M. Plecnik and J. M. McCarthy, 2016. “Kinematic Synthesis of Stephenson III Six-bar Function Generators,” Mechanism and Machine Theory, 97:112-126. (link)
Related patent:
M. Plecnik, D. Bissell, D. J. Reinkensmeyer, and J. M. McCarthy. Torque-Compensating Assistive Wrist Braces. U.S. Patent US20200046537A1. Priority 2015. Published 2020. (link)