title: Neuromechanical adaptation to hopping with an elastic ankle-foot orthosis.
abstract: When humans hop or run on different surfaces, they adjust their effective leg stiffness to offset changes in surface stiffness. As a result, the overall stiffness of the leg-surface series combination remains independent of surface stiffness. The purpose of this study was to determine whether humans make a similar adjustment when springs are placed in parallel with the leg via a lower limb orthosis. We studied seven human subjects hopping in place on one leg while wearing an ankle-foot orthosis. We used an ankle-foot orthosis because the ankle joint is primarily responsible for leg stiffness during hopping. A spring was added to the ankle-foot orthosis so that it increased orthosis stiffness by providing plantar flexor torque during ankle dorsiflexion. We hypothesized that subjects would decrease their biological ankle stiffness when the spring was added to the orthosis, keeping total ankle stiffness constant. We collected kinematic, kinetic, and electromyographic data during hopping with and without the spring on the orthosis. We found that total ankle stiffness and leg stiffness did not change across the two orthosis conditions (ANOVA, P > 0.05). This was possible because subjects decreased their biological ankle stiffness to offset the orthosis spring stiffness (P < 0.0001). The reduction in biological ankle stiffness was accompanied by decreases in soleus, medial gastrocnemius, and lateral gastrocnemius muscle activation (P < 0.0002). These results suggest that an elastic exoskeleton might improve human running performance by reducing muscle recruitment.
authors.affiliation: Division of Kinesiology, 401 Washtenaw Ave., University of Michigan, Ann Arbor, Michigan 48109-2214, USA. ferrisdp@umich.edu
authors.forename: Daniel P
authors.initials: DP
authors.lastname: Ferris
authors.forename: Zaineb A
authors.initials: ZA
authors.lastname: Bohra
authors.forename: Jamie R
authors.initials: JR
authors.lastname: Lukos
authors.forename: Catherine R
authors.initials: CR
authors.lastname: Kinnaird
mesh_terms: D000222:Adaptation, Physiological / Q000502:physiology; D000328:Adult; D000843:Ankle Joint / Q000502:physiology*; D001696:Biomechanical Phenomena / Q000379:methods; D003198:Computer Simulation; D004548:Elasticity; D005260:Female; D005528:Foot / Q000502:physiology*; D005684:Gait / Q000502:physiology*; D006801:Humans; D007866:Leg / Q000502:physiology*; D008297:Male; D008954:Models, Biological; D009473:Neuronal Plasticity / Q000502:physiology*; D009989:Orthotic Devices; D013314:Stress, Mechanical