title: Effect of timing of hip extension assistance during loaded walking with a soft exosuit.
abstract: BACKGROUND
  Recent advances in wearable robotic devices have demonstrated the ability to reduce the metabolic cost of walking by assisting the ankle joint. To achieve greater gains in the future it will be important to determine optimal actuation parameters and explore the effect of assisting other joints. The aim of the present work is to investigate how the timing of hip extension assistance affects the positive mechanical power delivered by an exosuit and its effect on biological joint power and metabolic cost during loaded walking. In this study, we evaluated 4 different hip assistive profiles with different actuation timings: early-start-early-peak (ESEP), early-start-late-peak (ESLP), late-start-early-peak (LSEP), late-start-late-peak (LSLP).
  
  
  METHODS
  Eight healthy participants walked on a treadmill at a constant speed of 1.5 m · s-1while carrying a 23 kg backpack load. We tested five different conditions: four with the assistive profiles described above and one unpowered condition where no assistance was provided. We evaluated participants' lower limb kinetics, kinematics, metabolic cost and muscle activation.
  
  
  RESULTS
  The variation of timing in the hip extension assistance resulted in a different amount of mechanical power delivered to the wearer across conditions; with the ESLP condition providing a significantly higher amount of positive mechanical power (0.219 ± 0.006 W · kg-1) with respect to the other powered conditions. Biological joint power was significantly reduced at the hip (ESEP and ESLP) and at the knee (ESEP, ESLP and LSEP) with respect to the unpowered condition. Further, all assistive profiles significantly reduced the metabolic cost of walking compared to the unpowered condition by 5.7 ± 1.5 %, 8.5 ± 0.9 %, 6.3 ± 1.4 % and 7.1 ± 1.9 % (mean ± SE for ESEP, ESLP, LSEP, LSLP, respectively).
  
  
  CONCLUSIONS
  The highest positive mechanical power delivered by the soft exosuit was reported in the ESLP condition, which showed also a significant reduction in both biological hip and knee joint power. Further, the ESLP condition had the highest average metabolic reduction among the powered conditions. Future work on autonomous hip exoskeletons may incorporate these considerations when designing effective control strategies.
authors.affiliation: John A. Paulson School of Engineering and Applied Sciences, Harvard University, 29 Oxford Street, Cambridge, MA, 02138, USA.|Wyss Institute for Biologically Inspired Engineering at Harvard, 3 Blackfan Circle, Boston, MA, 02115, USA.
authors.forename: Ye
authors.initials: Y
authors.lastname: Ding
authors.affiliation: John A. Paulson School of Engineering and Applied Sciences, Harvard University, 29 Oxford Street, Cambridge, MA, 02138, USA.|Wyss Institute for Biologically Inspired Engineering at Harvard, 3 Blackfan Circle, Boston, MA, 02115, USA.
authors.forename: Fausto A
authors.initials: FA
authors.lastname: Panizzolo
authors.affiliation: John A. Paulson School of Engineering and Applied Sciences, Harvard University, 29 Oxford Street, Cambridge, MA, 02138, USA.|Wyss Institute for Biologically Inspired Engineering at Harvard, 3 Blackfan Circle, Boston, MA, 02115, USA.
authors.forename: Christopher
authors.initials: C
authors.lastname: Siviy
authors.affiliation: John A. Paulson School of Engineering and Applied Sciences, Harvard University, 29 Oxford Street, Cambridge, MA, 02138, USA.|Wyss Institute for Biologically Inspired Engineering at Harvard, 3 Blackfan Circle, Boston, MA, 02115, USA.
authors.forename: Philippe
authors.initials: P
authors.lastname: Malcolm
authors.affiliation: John A. Paulson School of Engineering and Applied Sciences, Harvard University, 29 Oxford Street, Cambridge, MA, 02138, USA.|Wyss Institute for Biologically Inspired Engineering at Harvard, 3 Blackfan Circle, Boston, MA, 02115, USA.
authors.forename: Ignacio
authors.initials: I
authors.lastname: Galiana
authors.affiliation: Sargent College of Health and Rehabilitation Science, Boston University, 635 Commonwealth Ave, Boston, MA, 02215, USA.
authors.forename: Kenneth G
authors.initials: KG
authors.lastname: Holt
authors.affiliation: John A. Paulson School of Engineering and Applied Sciences, Harvard University, 29 Oxford Street, Cambridge, MA, 02138, USA. walsh@seas.harvard.edu.|Wyss Institute for Biologically Inspired Engineering at Harvard, 3 Blackfan Circle, Boston, MA, 02115, USA. walsh@seas.harvard.edu.
authors.forename: Conor J
authors.initials: CJ
authors.lastname: Walsh
mesh_terms: D000328:Adult; D001696:Biomechanical Phenomena; D000067190:Exoskeleton Device; D005260:Female; D006621:Hip Joint / Q000502:physiology*; D006801:Humans; D008297:Male; D012371:Robotics / Q000295:instrumentation*; D016138:Walking / Q000502:physiology