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The influence of push-off timing in a robotic ankle-foot prosthesis on the energetics and mechanics of walking

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Abstract
Background: Robotic ankle-foot prostheses that provide net positive push-off work can reduce the metabolic rate of walking for individuals with amputation, but benefits might be sensitive to push-off timing. Simple walking models suggest that preemptive push-off reduces center-of-mass work, possibly reducing metabolic rate. Studies with bilateral exoskeletons have found that push-off beginning before leading leg contact minimizes metabolic rate, but timing was not varied independently from push-off work, and the effects of push-off timing on biomechanics were not measured. Most lower-limb amputations are unilateral, which could also affect optimal timing. The goal of this study was to vary the timing of positive prosthesis push-off work in isolation and measure the effects on energetics, mechanics and muscle activity. Methods: We tested 10 able-bodied participants walking on a treadmill at 1.25 m.s(-1). Participants wore a tethered ankle-foot prosthesis emulator on one leg using a rigid boot adapter. We programmed the prosthesis to apply torque bursts that began between 46% and 56% of stride in different conditions. We iteratively adjusted torque magnitude to maintain constant net positive push-off work. Results: When push-off began at or after leading leg contact, metabolic rate was about 10% lower than in a condition with Spring-like prosthesis behavior. When push-off began before leading leg contact, metabolic rate was not different from the Spring-like condition. Early push-off led to increased prosthesis-side vastus medialis and biceps femoris activity during push-off and increased variability in step length and prosthesis loading during push-off. Prosthesis push-off timing had no influence on intact-side leg center-of-mass collision work. Conclusions: Prosthesis push-off timing, isolated from push-off work, strongly affected metabolic rate, with optimal timing at or after intact-side heel contact. Increased thigh muscle activation and increased human variability appear to have caused the lack of reduction in metabolic rate when push-off was provided too early. Optimal timing with respect to opposite heel contact was not different from normal walking, but the trends in metabolic rate and center-of-mass mechanics were not consistent with simple model predictions. Optimal push-off timing should also be characterized for individuals with amputation, since meaningful benefits might be realized with improved timing.
Keywords
ORTHOSIS, AMPUTATION, POWER, WORK, GAIT, ENERGY-COST, KNEE AMPUTEES, METABOLIC COST, BIPEDAL WALKING, Bionics, Gait, Collision, Push-off, Timing, Metabolic, Walking, Prosthesis, Ankle, Biomechatronics

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Chicago
Malcolm, Philippe, Roberto E Quesada, Joshua Caputo, and Steven H Collins. 2015. “The Influence of Push-off Timing in a Robotic Ankle-foot Prosthesis on the Energetics and Mechanics of Walking.” Journal of Neuroengineering and Rehabilitation 12.
APA
Malcolm, P., Quesada, R. E., Caputo, J., & Collins, S. H. (2015). The influence of push-off timing in a robotic ankle-foot prosthesis on the energetics and mechanics of walking. JOURNAL OF NEUROENGINEERING AND REHABILITATION, 12.
Vancouver
1.
Malcolm P, Quesada RE, Caputo J, Collins SH. The influence of push-off timing in a robotic ankle-foot prosthesis on the energetics and mechanics of walking. JOURNAL OF NEUROENGINEERING AND REHABILITATION. 2015;12.
MLA
Malcolm, Philippe, Roberto E Quesada, Joshua Caputo, et al. “The Influence of Push-off Timing in a Robotic Ankle-foot Prosthesis on the Energetics and Mechanics of Walking.” JOURNAL OF NEUROENGINEERING AND REHABILITATION 12 (2015): n. pag. Print.
@article{6872160,
  abstract     = {Background: Robotic ankle-foot prostheses that provide net positive push-off work can reduce the metabolic rate of walking for individuals with amputation, but benefits might be sensitive to push-off timing. Simple walking models suggest that preemptive push-off reduces center-of-mass work, possibly reducing metabolic rate. Studies with bilateral exoskeletons have found that push-off beginning before leading leg contact minimizes metabolic rate, but timing was not varied independently from push-off work, and the effects of push-off timing on biomechanics were not measured. Most lower-limb amputations are unilateral, which could also affect optimal timing. The goal of this study was to vary the timing of positive prosthesis push-off work in isolation and measure the effects on energetics, mechanics and muscle activity. 
Methods: We tested 10 able-bodied participants walking on a treadmill at 1.25 m.s(-1). Participants wore a tethered ankle-foot prosthesis emulator on one leg using a rigid boot adapter. We programmed the prosthesis to apply torque bursts that began between 46\% and 56\% of stride in different conditions. We iteratively adjusted torque magnitude to maintain constant net positive push-off work. 
Results: When push-off began at or after leading leg contact, metabolic rate was about 10\% lower than in a condition with Spring-like prosthesis behavior. When push-off began before leading leg contact, metabolic rate was not different from the Spring-like condition. Early push-off led to increased prosthesis-side vastus medialis and biceps femoris activity during push-off and increased variability in step length and prosthesis loading during push-off. Prosthesis push-off timing had no influence on intact-side leg center-of-mass collision work. 
Conclusions: Prosthesis push-off timing, isolated from push-off work, strongly affected metabolic rate, with optimal timing at or after intact-side heel contact. Increased thigh muscle activation and increased human variability appear to have caused the lack of reduction in metabolic rate when push-off was provided too early. Optimal timing with respect to opposite heel contact was not different from normal walking, but the trends in metabolic rate and center-of-mass mechanics were not consistent with simple model predictions. Optimal push-off timing should also be characterized for individuals with amputation, since meaningful benefits might be realized with improved timing.},
  articleno    = {21},
  author       = {Malcolm, Philippe and Quesada, Roberto E and Caputo, Joshua and Collins, Steven H},
  issn         = {1743-0003},
  journal      = {JOURNAL OF NEUROENGINEERING AND REHABILITATION},
  language     = {eng},
  pages        = {14},
  title        = {The influence of push-off timing in a robotic ankle-foot prosthesis on the energetics and mechanics of walking},
  url          = {http://dx.doi.org/10.1186/s12984-015-0014-8},
  volume       = {12},
  year         = {2015},
}

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