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

Samuel Galle, Philippe Malcolm, Steven Hartley Collins, Dirk De Clercq

Research output: Contribution to journalArticle

30 Citations (Scopus)

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 plantarflexion during 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.

Original languageEnglish (US)
Article number35
JournalJournal of NeuroEngineering and Rehabilitation
Volume14
Issue number1
DOIs
StatePublished - Apr 27 2017

Fingerprint

Ankle
Walking
Costs and Cost Analysis
Foot
Leg
Electromyography
Locomotion
Biomechanical Phenomena
Healthy Volunteers
Muscles

Keywords

  • Augmentation
  • Human locomotion
  • Lower-limb exoskeletons
  • Metabolic cost
  • Optimal assistance

ASJC Scopus subject areas

  • Rehabilitation
  • Health Informatics

Cite this

Reducing the metabolic cost of walking with an ankle exoskeleton : interaction between actuation timing and power. / Galle, Samuel; Malcolm, Philippe; Collins, Steven Hartley; De Clercq, Dirk.

In: Journal of NeuroEngineering and Rehabilitation, Vol. 14, No. 1, 35, 27.04.2017.

Research output: Contribution to journalArticle

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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 plantarflexion during 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.",
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