Publications

2020
Richard W Nuckols, Kota Z. Takahashi, Dominic J. Farris, Sarai Mizrachi, Raziel Riemer, and Gregory S. Sawicki. 8/28/2020. “Mechanics and energetics of walking and running up and downhill: A joint-level perspective to guide wearable robot design.” PLoS One. Publisher's Version
Richard W. Nuckols and Gregory S. Sawicki. 6/2020. “Impact of elastic ankle exoskeleton stiffness on neuromechanics and energetics of human walking across multiple speeds.” J Neuroeng Rehabil. Article
Richard W. Nuckols, Krithika Swaminathan, Sangjun Lee, Louis Awad, Conor J. Walsh, and Robert D. Howe. 2020. “Automated detection of soleus concentric contraction in variable gait conditions for improved exosuit control.” In IEEE International Conference on Robotics and Automation (ICRA).Abstract
Exosuits can reduce metabolic demand and improve gait. Controllers explicitly derived from biological mechanisms that reflect the user's joint or muscle dynamics should in theory allow for individualized assistance and enable adaptation to changing gait. With the goal of developing an exosuit control strategy based on muscle power, we present an approach for estimating, at real time rates, when the soleus muscle begins to generate positive power. A low-profile ultrasound system recorded B-mode images of the soleus in walking individuals. An automated routine using optical flow segmented the data to a normalized gait cycle and estimated the onset of concentric contraction at real-time rates (~130Hz). Segmentation error was within 1% of the gait cycle compared to using ground reaction forces. Estimation of onset of concentric contraction had a high correlation (R2=0.92) and an RMSE of 2.6% gait cycle relative to manual estimation. We demonstrated the ability to estimate the onset of concentric contraction during fixed speed walking in healthy individuals that ranged from 39.3% to 45.8% of the gait cycle and feasibility in two persons post-stroke walking at comfortable walking speed. We also showed the ability to measure a shift in onset timing to 7% earlier when the biological system adapts from level to incline walking. Finally, we provided an initial evaluation for how the onset of concentric contraction might be used to inform exosuit control in level and incline walking.
nuckols_et_al-2020-ICRA.pdf
Evelyn J Park, Tunc Akbas, Asa Eckert-Erdheim, Lizeth H Sloot, Richard W Nuckols, Dorothy Orzel, Lexine Schumm, Terry D Ellis, Louis N Awad, and Conor J Walsh. 2020. “A Hinge-Free, Non-Restrictive, Lightweight Tethered Exosuit for Knee Extension Assistance During Walking.” IEEE Transactions on Medical Robotics and Bionics, 2, 2, Pp. 165–175. park_et_al-2020-ieee_tmrb.pdf
RW Nuckols, TJM Dick, ON Beck, and GS Sawicki. 2020. “Ultrasound imaging links soleus muscle neuromechanics and energetics during human walking with elastic ankle exoskeletons.” Scientific reports, 10, 1, Pp. 1–15. Publisher's VersionAbstract

Unpowered exoskeletons with springs in parallel to human plantar flexor muscle-tendons can reduce the metabolic cost of walking. We used ultrasound imaging to look ‘under the skin’ and measure how exoskeleton stiffness alters soleus muscle contractile dynamics and shapes the user’s metabolic rate during walking. Eleven participants (4F, 7M; age: 27.7 ± 3.3 years) walked on a treadmill at 1.25 m s-1 and 0% grade with elastic ankle exoskeletons (rotational stiffness: 0-250 Nm rad-1) in one training and two testing days. Metabolic savings were maximized (4.2%) at a stiffness of 50 Nm rad-1. As exoskeleton stiffness increased, the soleus muscle operated at longer lengths and improved economy (force/activation) during early stance, but this benefit was offset by faster shortening velocity and poorer economy in late stance. Changes in soleus activation rate correlated with changes in users’ metabolic rate (p = 0.038, R2 = 0.44), highlighting a crucial link between muscle neuromechanics and exoskeleton performance; perhaps informing future ‘muscle-in-the loop’ exoskeleton controllers designed to steer contractile dynamics toward more economical force production.

nuckols_et_al-2020-scientific_reports.pdf nuckols-2020-supplementary-analysis.pdf
2019
Owen N Beck, Laksh Kumar Punith, Richard W Nuckols, and Gregory S Sawicki. 2019. “Exoskeletons Improve Locomotion Economy by Reducing Active Muscle Volume.” Exercise and sport sciences reviews, 47, 4, Pp. 237–245.Abstract

Exoskeletons that improve locomotion economy typically are engineered to reduce users’ limb joint mechanical work or moments. Yet, limb joint dynamics do not necessarily reflect muscle dynamics, which dictate whole-body metabolic energy expenditure. Here, we hypothesize that exoskeletons primarily reduce user metabolic energy expenditure across locomotion conditions by reducing active muscle volume.

Key Points

  • Center of mass, limb joint, and muscle mechanical do not explain well how exoskeletons alter locomotion economy.
  • Limb joint dynamics do not necessarily reflect the underlying muscle dynamics across locomotion conditions.
  • Active muscles are the primary drivers of whole-body metabolic energy expenditure during locomotion. Consequently, exoskeletons likely need to consider muscle dynamics to optimize locomotion economy.
  • During walking and hopping with an exoskeleton, muscle force generation is a better correlate to locomotion economy than previously measured mechanical work parameters.
  • Tracking muscle length changes in vivo may help provide reasonably accurate active muscle volume calculations.
  • Future exoskeleton controllers may incorporate real-time muscle physiology measures to update device characteristics and maintain minimal active muscle volume and metabolic energy expenditure across locomotion conditions.
exoskeletons_improve_locomotion_economy_by.pdf
Emily M McCain, Taylor JM Dick, Tracy N Giest, Richard W Nuckols, Michael D Lewek, Katherine R Saul, and Gregory S Sawicki. 2019. “Mechanics and energetics of post-stroke walking aided by a powered ankle exoskeleton with speed-adaptive myoelectric control.” Journal of neuroengineering and rehabilitation, 16, 1, Pp. 57. mccain-et-al.-2019-JNER.pdf
2018
F Porciuncula, RW Nuckols, N Karavas, C Chang, TC Baker, D Orzel, D Perry, T Ellis, L Awad, and CJ Walsh. 10/2018. “Assisting Limb Advancement During Walking in Stroke Using a Wearable Soft Hip Exosuit: A Proof-of-Concept.” International Conference on NeuroRehabilitation. Pisa, Italy.
K Swaminathan, S Lee, RW Nuckols, D Arumukhom Revi, P Singh, RD Howe, R Smith, and CJ Walsh. 10/2018. “Biomechanics Underlying Subject-Dependent Variability in Motor Adaptation to Soft Exosuit Assistance.” International Conference on NeuroRehabilitation. Pisa, Italy.
RW Nuckols, K Swaminathan, S Lee, D Arumukhom Revi, P Singh, CJ Walsh, and RD Howe. 7/8/2018. “Role of Muscle Dynamics on Subject-Specific Response to Exosuit Assistance.” World Congress of Biomechanics. Dublin, IE.
LH Sloot, J Bae, LM Baker, C Siviy, RW Nuckols, K O'Donnel, N Menard, F Porciuncula, T Baker, R Sloutsky, DK Choe, BF Clement, T Ellis, LN Awad, and CJ Walsh. 2018. “A lightweight and portable soft exosuit for paretic ankle assistance after stroke.” World Congress of Biomechanics. Dublin, IE.
2017
RW Nuckols and GS Sawicki. 8/2017. “Effect of speed on the mechanics and energetics of walking with an elastic ankle exoskeleton.” 41st Annual Meeting of American Society of Biomechanics. Boulder, CO.
T Dick, RW Nuckols, and GS Sawicki. 8/2017. “Tuned or not? Ultrasound measurements of soleus fascicle dynamics during human walking with elastic ankle exoskeletons.” 41st Annual Meeting of American Society of Biomechanics. Boulder, CO.
RW Nuckols, TJ Dick, JR Franz, and GS Sawicki. 8/2017. “Using elastic ankle exoskeletons to counteract age-related structure-function deficits.” 41st Annual Meeting of American Society of Biomechanics. Boulder, CO. asb_2017_aging_submitted_231.pdf aging_poster_asb.pdf
2016
RW Nuckols, TN Giest, S Philius, and GS Sawicki. 8/2016. “Embodying human plantarflexor muscle-tendon physiology for neuromuscular model-based control of a powered ankle exoskeleton.” 40th Annual Meeting of American Society of Biomechanics. Raleigh, NC.
TN Giest, RW Nuckols, and GS Sawicki. 8/2016. “Speed-dependent, proportional myoelectric exoskeleton controller with adaptive gains.” 40th Annual Meeting of American Society of Biomechanics. Raleigh, NC.
RW Nuckols and GS Sawicki. 2/2016. “Evaluation of exoskeleton assistance on the mechanics and metabolic cost of human gait (*Best Poster Award).” WeaRAcon 16, Wearable Robotic Association Conference. Phoenix, Az.
RW Nuckols and GS Sawicki. 2016. “The goldilocks zone: interplay of elastic exoskeleton assistance and walking speed on the mechanics and energetics of walking.” 40th Annual Meeting of American Society of Biomechanics. Raleigh, NC.
2015
Richard W Nuckols, Steven H Collins, and Gregory S Sawicki. 2015. “An emulator system to characterize optimal elastic ankle exoskeleton stiffness during human walking and running.” Dynamic Walking. Columbus, Ohio. dw15abstract_nuckolsrichard.pdf