title: Musculoskeletal modelling deconstructs the paradoxical effects of elastic ankle exoskeletons on plantar-flexor mechanics and energetics during hopping.
abstract: Experiments have shown that elastic ankle exoskeletons can be used to reduce ankle joint and plantar-flexor muscle loading when hopping in place and, in turn, reduce metabolic energy consumption. However, recent experimental work has shown that such exoskeletons cause less favourable soleus (SO) muscle-tendon mechanics than is observed during normal hopping, which might limit the capacity of the exoskeleton to reduce energy consumption. To directly link plantar-flexor mechanics and energy consumption when hopping in exoskeletons, we used a musculoskeletal model of the human leg and a model of muscle energetics in simulations of muscle-tendon dynamics during hopping with and without elastic ankle exoskeletons. Simulations were driven by experimental electromyograms, joint kinematics and exoskeleton torque taken from previously published data. The data were from seven males who hopped at 2.5 Hz with and without elastic ankle exoskeletons. The energetics model showed that the total rate of metabolic energy consumption by ankle muscles was not significantly reduced by an ankle exoskeleton. This was despite large reductions in plantar-flexor force production (40-50%). The lack of larger metabolic reductions with exoskeletons was attributed to increases in plantar-flexor muscle fibre velocities and a shift to less favourable muscle fibre lengths during active force production. This limited the capacity for plantar-flexors to reduce activation and energy consumption when hopping with exoskeleton assistance.
authors.affiliation: School of Human Movement Studies, University of Queensland, St Lucia, QLD 4072, Australia Joint Department of Biomedical Engineering, University of North Carolina-Chapel Hill and North Carolina State University, Raleigh, NC 27606, USA d.farris@uq.edu.au.
authors.forename: Dominic James
authors.initials: DJ
authors.lastname: Farris
authors.affiliation: Department of Bioengineering, Stanford University, Stanford, CA 94305-4125, USA.
authors.forename: Jennifer L
authors.initials: JL
authors.lastname: Hicks
authors.affiliation: Department of Bioengineering, Stanford University, Stanford, CA 94305-4125, USA Department of Mechanical Engineering, Stanford University, Stanford, CA 94305-3030, USA.
authors.forename: Scott L
authors.initials: SL
authors.lastname: Delp
authors.affiliation: Joint Department of Biomedical Engineering, University of North Carolina-Chapel Hill and North Carolina State University, Raleigh, NC 27606, USA.
authors.forename: Gregory S
authors.initials: GS
authors.lastname: Sawicki
mesh_terms: D000328:Adult; D000843:Ankle Joint / Q000502:physiology*; D001696:Biomechanical Phenomena; D004576:Electromyography; D004734:Energy Metabolism / Q000502:physiology*; D005684:Gait / Q000502:physiology*; D006801:Humans; D008297:Male; D008962:Models, Theoretical; D018482:Muscle, Skeletal / Q000502:physiology*; D009989:Orthotic Devices; D013710:Tendons / Q000502:physiology
keywords: Assistive robotics; Dynamics simulation; Energetics; Locomotion; Metabolic energy; Muscle mechanics