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Reducing the metabolic cost of walking with an ankle exoskeleton : interaction between actuation timing and power

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Abstract
Background: Powered ankle-foot exoskeletons can reduce the metabolic cost of human walking to below normal levels, but optimal assistance properties remain unclear. The purpose of this study was to test the effects of different assistance timing and power characteristics in an experiment with a tethered ankle-foot exoskeleton. Methods: Ten healthy female subjects walked on a treadmill with bilateral ankle-foot exoskeletons in 10 different assistance conditions. Artificial pneumatic muscles assisted plantarflexionduring ankle push-off using one of four actuation onset timings (36, 42, 48 and 54% of the stride) and three power levels (average positive exoskeleton power over a stride, summed for both legs, of 0.2, 0.4 and 0.5 W.kg(-1)). We compared metabolic rate, kinematics and electromyography (EMG) between conditions. Results: Optimal assistance was achieved with an onset of 42% stride and average power of 0.4 W.kg(-1), leading to 21% reduction in metabolic cost compared to walking with the exoskeleton deactivated and 12% reduction compared to normal walking without the exoskeleton. With suboptimal timing or power, the exoskeleton still reduced metabolic cost, but substantially less so. The relationship between timing, power and metabolic rate was well-characterized by a two-dimensional quadratic function. The assistive mechanisms leading to these improvements included reducing muscular activity in the ankle plantarflexors and assisting leg swing initiation. Conclusions: These results emphasize the importance of optimizing exoskeleton actuation properties when assisting or augmenting human locomotion. Our optimal assistance onset timing and average power levels could be used for other exoskeletons to improve assistance and resulting benefits.
Keywords
Biomechanics and Motor control of Human Movement, Augmentation, Human locomotion, Lower-limb exoskeletons, Metabolic cost, Optimal assistance, TO-STEP TRANSITIONS, MECHANICAL WORK, PLANTARFLEXION ASSISTANCE, GAIT, ENERGY, LOCOMOTION, ENERGETICS, FORCE, FOOT, LEG

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Chicago
Galle, Samuel, Philippe Malcolm, Steven Hartley Collins, and Dirk De Clercq. 2017. “Reducing the Metabolic Cost of Walking with an Ankle Exoskeleton : Interaction Between Actuation Timing and Power.” Journal of Neuroengineering and Rehabilitation 14 (1).
APA
Galle, S., Malcolm, P., Collins, S. H., & De Clercq, D. (2017). Reducing the metabolic cost of walking with an ankle exoskeleton : interaction between actuation timing and power. JOURNAL OF NEUROENGINEERING AND REHABILITATION, 14(1).
Vancouver
1.
Galle S, Malcolm P, Collins SH, De Clercq D. Reducing the metabolic cost of walking with an ankle exoskeleton : interaction between actuation timing and power. JOURNAL OF NEUROENGINEERING AND REHABILITATION. 2017;14(1).
MLA
Galle, Samuel, Philippe Malcolm, Steven Hartley Collins, et al. “Reducing the Metabolic Cost of Walking with an Ankle Exoskeleton : Interaction Between Actuation Timing and Power.” JOURNAL OF NEUROENGINEERING AND REHABILITATION 14.1 (2017): n. pag. Print.
@article{8531241,
  abstract     = {Background: Powered ankle-foot exoskeletons can reduce the metabolic cost of human walking to below normal levels, but optimal assistance properties remain unclear. The purpose of this study was to test the effects of different assistance timing and power characteristics in an experiment with a tethered ankle-foot exoskeleton. 
Methods: Ten healthy female subjects walked on a treadmill with bilateral ankle-foot exoskeletons in 10 different assistance conditions. Artificial pneumatic muscles assisted plantarflexionduring ankle push-off using one of four actuation onset timings (36, 42, 48 and 54\% of the stride) and three power levels (average positive exoskeleton power over a stride, summed for both legs, of 0.2, 0.4 and 0.5 W.kg(-1)). We compared metabolic rate, kinematics and electromyography (EMG) between conditions. 
Results: Optimal assistance was achieved with an onset of 42\% stride and average power of 0.4 W.kg(-1), leading to 21\% reduction in metabolic cost compared to walking with the exoskeleton deactivated and 12\% reduction compared to normal walking without the exoskeleton. With suboptimal timing or power, the exoskeleton still reduced metabolic cost, but substantially less so. The relationship between timing, power and metabolic rate was well-characterized by a two-dimensional quadratic function. The assistive mechanisms leading to these improvements included reducing muscular activity in the ankle plantarflexors and assisting leg swing initiation. 
Conclusions: These results emphasize the importance of optimizing exoskeleton actuation properties when assisting or augmenting human locomotion. Our optimal assistance onset timing and average power levels could be used for other exoskeletons to improve assistance and resulting benefits.},
  articleno    = {35},
  author       = {Galle, Samuel and Malcolm, Philippe and Collins, Steven Hartley and De Clercq, Dirk},
  issn         = {1743-0003},
  journal      = {JOURNAL OF NEUROENGINEERING AND REHABILITATION},
  keyword      = {Biomechanics and Motor control of Human Movement,Augmentation,Human locomotion,Lower-limb exoskeletons,Metabolic cost,Optimal assistance,TO-STEP TRANSITIONS,MECHANICAL WORK,PLANTARFLEXION ASSISTANCE,GAIT,ENERGY,LOCOMOTION,ENERGETICS,FORCE,FOOT,LEG},
  language     = {eng},
  number       = {1},
  pages        = {16},
  title        = {Reducing the metabolic cost of walking with an ankle exoskeleton : interaction between actuation timing and power},
  url          = {http://dx.doi.org/10.1186/s12984-017-0235-0},
  volume       = {14},
  year         = {2017},
}

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