Optimizing Contact Area and Joint Stiffness of a Passive Foot-Ankle Exoskeleton for Locomotion on Deformable Terrain

Designing exoskeletons to improve locomotion energetics on deformable terrain is an engineering challenge. This is due to complex, dissipative boundary conditions at the foot-ground interface that cause unsteady muscle tendon (MT) dynamics. During ground contact, the MT is required to perform net positive work to overcome energy loss to the environment. This creates an opportunity for assistive technology that targets muscle level function to improve economy/efficiency of force/work generation in dissipative environments. As a first step, we developed a musculoskeletal model to understand device-MT interaction in order to predict changes in metabolic energy cost (P_met) on solid ground vs. sand. We modelled an exoskeletal device with variable ankle stiffness and foot contact area coupled to a Hill-Type model representing the calf-Achilles tendon MT. During fixed frequency locomotion cycles, we found terrain-specific exoskeleton parameters that eliminated the P_met penalty incurred during locomotion on sand vs. solid ground. This preliminary work in-silico to optimize exoskeleton parameters offline will drive the development of novel hardware solutions to augment performance at the human-machine interface in unstructured, real world environments.