More is not always better: modeling the effects of elastic exoskeleton compliance on underlying ankle muscle-tendon dynamics

Robertson, Benjamin D., Farris, Dominic J. and Sawicki, Gregory S. (2014) More is not always better: modeling the effects of elastic exoskeleton compliance on underlying ankle muscle-tendon dynamics. Bioinspiration and Biomimetics, 9 4: 046018.1-046018.11. doi:10.1088/1748-3182/9/4/046018


Author Robertson, Benjamin D.
Farris, Dominic J.
Sawicki, Gregory S.
Title More is not always better: modeling the effects of elastic exoskeleton compliance on underlying ankle muscle-tendon dynamics
Journal name Bioinspiration and Biomimetics   Check publisher's open access policy
ISSN 1748-3190
1748-3182
Publication date 2014-11-24
Sub-type Article (original research)
DOI 10.1088/1748-3182/9/4/046018
Open Access Status
Volume 9
Issue 4
Start page 046018.1
End page 046018.11
Total pages 11
Place of publication Bristol, United Kingdom
Publisher Institute of Physics Publishing
Collection year 2015
Language eng
Formatted abstract
Development of robotic exoskeletons to assist/enhance human locomotor performance involves lengthy prototyping, testing, and analysis. This process is further convoluted by variability in limb/body morphology and preferred gait patterns between individuals. In an attempt to expedite this process, and establish a physiological basis for actuator prescription, we developed a simple, predictive model of human neuromechanical adaptation to a passive elastic exoskeleton applied at the ankle joint during a functional task. We modeled the human triceps surae–Achilles tendon muscle tendon unit (MTU) as a single Hill-type muscle, or contractile element (CE), and series tendon, or series elastic element (SEE). This modeled system was placed under gravitational load and underwent cyclic stimulation at a regular frequency (i.e. hopping) with and without exoskeleton (Exo) assistance. We explored the effect that both Exo stiffness (kExo ) and muscle activation (Astim ) had on combined MTU and Exo (MTU + Exo), MTU, and CE/SEE mechanics and energetics. Model accuracy was verified via qualitative and quantitative comparisons
between modeled and prior experimental outcomes. We demonstrated that reduced Astim can be traded for increased kExo to maintain consistent MTU + Exo mechanics (i.e. average positive power + (P ) ¯ mech output) from an unassisted condition (i.e. = − kExo 0 kN · m 1). For these regions of parameter space, our model predicted a reduction in MTU force, SEE energy cycling, and
metabolic rate (P ) ¯ met , as well as constant CE + P¯ mech output compared to unassisted conditions. This agreed with previous experimental observations, demonstrating our model’s predictive ability. Model predictions also provided insight into mechanisms of metabolic cost minimization, and/or enhanced mechanical performance, and we concluded that both of these outcomes cannot be achieved simultaneously, and that one must come at the detriment of the other in a spring-assisted compliant MTU.
Keyword Elastic ankle exoskeleton
Muscle–tendon interaction
Mechanics
Energetics
Hopping
Ultrasound
Q-Index Code C1
Q-Index Status Confirmed Code
Institutional Status UQ

Document type: Journal Article
Sub-type: Article (original research)
Collections: Official 2015 Collection
School of Human Movement and Nutrition Sciences Publications
 
Versions
Version Filter Type
Citation counts: TR Web of Science Citation Count  Cited 4 times in Thomson Reuters Web of Science Article | Citations
Scopus Citation Count Cited 2 times in Scopus Article | Citations
Google Scholar Search Google Scholar
Created: Fri, 05 Dec 2014, 10:48:21 EST by Dominic James Farris on behalf of School of Human Movement and Nutrition Sciences