PROJECT TITLE :

A Simple Model to Estimate Plantarflexor Muscle–Tendon Mechanics and Energetics During Walking With Elastic Ankle Exoskeletons

ABSTRACT:

Goal: A recent experiment demonstrated that when humans wear unpowered elastic ankle exoskeletons with intermediate spring stiffness, they will scale back their metabolic energy cost to run by ∼seven%. Springs that are too compliant or too stiff have little benefit. The purpose of this study was to use modeling and simulation to explore the muscle-level mechanisms for the “sweet spot” in stiffness throughout exoskeleton assisted walking. Ways: We have a tendency to developed a simple lumped uniarticular musculoskeletal model of the plantarflexors operating in parallel with an elastic “exo-tendon.” Using an inverse approach with constrained kinematics and kinetics, we tend to rapidly simulated human walking over a vary of exoskeleton stiffness values and examined the underlying neuromechanics and energetics of the biological plantarflexors. Results: Stiffer ankle exoskeleton springs resulted in larger decreases in plantarflexor muscle forces, activations, and metabolic energy consumption. However, in the method of unloading the compliant biological muscle–tendon unit, the muscle fascicles experienced larger excursions that negatively impacted series elastic component recoil that's characteristic of a tuned “catapult mechanism.” Conclusion: The combination of disrupted muscle–tendon dynamics and the requirement to provide compensatory forces/moments to maintain overall net ankle moment invariance may make a case for the “sweet spot” in metabolic performance at intermediate ankle exoskeleton stiffness. Future work will aim to produce experimental proof to support the model predictions presented here using ultrasound imaging of muscle-level dynamics during walking with elastic ankle exoskeletons. Significance: Engineers should account for the muscle-level effects of exoskeleton styles in order to realize maximal performance objectives.