Closed-loop wrist stabilization in C4 and C5 tetraplegia.

We investigated the feasibility of using functional neuromuscular stimulation (FNS) 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.