A Variable Impedance Hybrid Neuroprosthesis for Enhanced Locomotion after Spinal Cord Injury

A hybrid neuroprosthesis combines functional neuromuscular stimulation with controllable bracing to restore walking function after paralysis from spinal cord injury. This approach confines stimulation to driving the limbs forward while utilizing bracing to lock joints for postural support during stance. However, a stiff leg during stance is inefficient and can limit the range of achievable walking tasks. We hypothesize that a hybrid neuroprosthesis which regulates joint motion under loading will augment stimulation driven gait to create better walking performance.;A novel variable impedance mechanism was developed to control the knee joint during stance phase of locomotion. This mechanism was optimized to provide adequate torque for regulation of knee motion under load at all joint angles, creating an orthosis capable of achieving power absorption which is not possible through stimulation of muscles or lockable orthotics. A finite state, closed loop control system based on sensor feedback was developed to coordinate activation of the variable impedance knee mechanism with electrical stimulation to create a new hybrid neuroprosthesis for walking (VIKM-HNP). The goal of the VIKM-HNP was to allow knee extensor muscles to rest while providing power absorption for stance phase knee flexion and enabling unencumbered swing phase motion.;The new VIKM-HNP was evaluated during level walking in two individuals with thoracic level spinal cord injury. The results showed a significant increase in stance knee flexion and power absorption while reducing knee extensor stimulation duty cycle by up to 40%. The altered knee behavior created more steady forward progression, increased minimum instantaneous gait speed, and reduced impulsive ground reaction forces compared to stimulation alone.;A second finite state machine was developed to enable step-by-step forward stair descent using the VIKM-HNP. Knee extensor stimulation of the trailing limb was deactivated while the orthosis was activated to regulate lowering speed. The system was evaluated in one participant. Regulation of lowering speed during descent was possible with minimal upper extremity effort during the initial parts of lowering and a maximum upper extremity force of approximately 45% body weight, constituting a significant improvement over previously published values for stimulation only systems.