Designing Active Assistance for Gait with Hip Abductors Weakness Using Predictive Simulation

Individuals with weak hip abductors commonly exhibit compensatory gait patterns and excessive lateral center of mass excursions, which may reduce walking efficiency and increase the risk of falling. In this study, we investigated the influence of hip abductor weakness and of ideal hip abduction assistive torque on gait patterns through an established predictive simulation framework and a full-body musculoskeletal model. Using this model and simulation framework, hip abductor muscle strength was reduced in several increments, and gait patterns were predicted and qualitatively compared to reported gait patterns in persons with weak hip abductors. Hip abductor assistive torque was then added to the model as ideal joint torques with three distinct time series profiles (step pattern, trapezoid pattern, and biological pattern). For each assistive torque pattern, timing and magnitude parameters were optimized in a bilevel optimization such that the predicted gait was normalized as much as possible. The key features of the predicted compensatory gait patterns were successfully captured in the simulation. All assistive torque profiles normalized gait kinematics relatively well, wherein the trapezoid and biological profiles normalized kinematics and reduced estimated costs of transport nearly equally well. Due to its simplicity and performance, the trapezoid profile can be considered as the most preferable. Our findings provide valuable insights into the impact of hip abductor weakness on compensatory gait, the potential benefits of hip abduction assistive torque, and the use of bilevel optimization to identify parameters that characterize optimal assistive torque, and can be useful in the exoskeleton control design for individuals with hip abductor weakness.