Closed-loop wrist stabilization in C4 and C5 tetraplegia

The authors investigated the feasibility of using functional neuromuscular stimulation (PNS) to stabilize wrist flexion/extension angle in individuals with tetraplegia at C4 and C5. Three wrist position controllers were evaluated experimentally and in simulation. Closed-loop feedback regulation increased wrist stability in the presence of wrist moment disturbances, using less wrist muscle activation than an open-loop cocontraction system. However, if the disturbances were large compared to the available wrist muscle moment, controller saturation made the open-loop system more economical, even though the feedback controllers still performed better. The simulations also showed that stimulating the finger flexors can induce a negative stiffness: load at the wrist, which destabilizes wrist position. The destabilizing effects of the negative stiffness were reduced if the passive wrist moment model included nonlinear damping instead of linear damping.