A hybrid neuroprosthesis (HNP) was developed with the goal of providing improved gait to individuals with paraplegia relative to existing assistive gait systems. The HNP is an approach to restoring gait by combining a lower extremity exoskeleton with functional neuromuscular stimulation (FNS). Individually, exoskeletons apply constraints for support, but provide limited step length and depend on upper extremity actions on a walker for forward propulsion. Conversely, FNS mobilizes the limbs through electrical pulses to paralyzed muscles. However, muscles targeted for stimulation quickly fatigue and provide inadequate postural support. The HNP was designed to functionally combine the supportive features of the exoskeleton and joint mobility of FNS.
Controllable knee and hip joint mechanisms were developed to support the user while allowing for functional motion from FNS for forward progression. These mechanisms were optimized for maximal torque when supporting a joint and minimal resistance when driven by FNS. A closed-loop controller based on sensor measurements of joint dynamics was developed to synchronize exoskeletal operation with muscle stimulus activity. The objectives were to modulate joint constraints to provide continual support to the user while minimizing the deleterious effects of the constraints on joint mobility, deactivate stimulus to target muscles when certain exoskeletal constraints are engaged to allow the target muscles to rest, and modulate FNS from baseline levels to achieve functional joint positions.
The operational response of the controller and mechanisms were characterized through simulation, bench, and able-bodied testing. Implementation of the HNP with an individual with paraplegia respectively showed a 40 % and 16 % reduction in maximum exerted upper extremity forces relative to exoskeleton-only and FNS-only gait. Step lengths were shown to be comparable between HNP and FNS-only gait. When comparing the HNP with and without the FNS modulation, the average gait speed was increased by 16 % with FNS modulation due to a 10 % increase in the hip range of motion. Reductions in muscle activity were feasible when the exoskeletal constraints were enabled.
Future work to optimize joint coordination or apply an active mechanism to the exoskeleton to assist hip extension may improve postural control and forward progression.