Design of a Variable Constraint Hip Mechanism for a Hybrid Neuroprosthesis to Restore Gait After Spinal Cord Injury

A variable constraint hip mechanism (VCHM) has been developed for a hybrid neuroprosthesis system (HNP) to provide postural stability and uninhibited sagittal hip rotation throughout the gait of individuals with paraplegia. This paper describes the design concepts used in the development of the VCHM. The VCHM utilizes a hydraulic system to reciprocally couple the hips or individually lock and/or free a hip to rotate in one or both sagittal directions. Bench testing results show the feasibility of utilizing a portable hydraulic system in controlling hip joint kinematics. The passive resistive torques of the VCHM against user hip rotation at hip angular velocities typical of gait does not exceed 10% of the achievable hip torque generated by functional neuromuscular stimulation of paralyzed muscle. With the state of the VCHM configured to reciprocally couple the hips, the normalized mechanical efficiency of the VCHM was determined to be 0.7. Since each hip will be independently driven by the FNS of muscle, high torque transfer efficiency between the hips is not essential for successful operation of the VCHM. Future work will focus on the development of a sensor-based feedback controller to modulate the hip constraints of the VCHM and validation of the VCHM as part of a HNP for paraplegic individuals implanted with FNS systems.