A simple exoskeleton that assists plantarflexion can reduce the metabolic cost of human walking

PLoS One. 2013;8(2):e56137. doi: 10.1371/journal.pone.0056137. Epub 2013 Feb 13.


Background: Even though walking can be sustained for great distances, considerable energy is required for plantarflexion around the instant of opposite leg heel contact. Different groups attempted to reduce metabolic cost with exoskeletons but none could achieve a reduction beyond the level of walking without exoskeleton, possibly because there is no consensus on the optimal actuation timing. The main research question of our study was whether it is possible to obtain a higher reduction in metabolic cost by tuning the actuation timing.

Methodology/principal findings: We measured metabolic cost by means of respiratory gas analysis. Test subjects walked with a simple pneumatic exoskeleton that assists plantarflexion with different actuation timings. We found that the exoskeleton can reduce metabolic cost by 0.18±0.06 W kg(-1) or 6±2% (standard error of the mean) (p = 0.019) below the cost of walking without exoskeleton if actuation starts just before opposite leg heel contact.

Conclusions/significance: The optimum timing that we found concurs with the prediction from a mathematical model of walking. While the present exoskeleton was not ambulant, measurements of joint kinetics reveal that the required power could be recycled from knee extension deceleration work that occurs naturally during walking. This demonstrates that it is theoretically possible to build future ambulant exoskeletons that reduce metabolic cost, without power supply restrictions.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Ankle Joint / physiology*
  • Biomechanical Phenomena
  • Electromyography
  • Energy Metabolism / physiology*
  • Exercise Test / instrumentation
  • Exercise Test / methods
  • Female
  • Foot / physiology
  • Gait / physiology
  • Heel / physiology
  • Humans
  • Leg / physiology
  • Male
  • Models, Biological
  • Muscle, Skeletal / physiology
  • Orthotic Devices*
  • Oxygen Consumption / physiology
  • Pulmonary Gas Exchange / physiology
  • Time Factors
  • Walking / physiology*
  • Young Adult

Grant support

This research was funded by the Ghent University (it was conducted during the appointment of the corresponding author as an assistant/PhD student). There was no external funding. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.