Reducing the metabolic cost of walking with an ankle exoskeleton: interaction between actuation timing and power

Galle, Samuel; Malcolm, Philippe; Collins, Steven H.; Clercq, Dirk De

doi:10.1186/s12984-017-0235-0

Cited by 174 publications

(186 citation statements)

References 46 publications

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“…; Selinger & Donelan, ; Galle et al . ) designed to reduce metabolic cost by performing mechanical work that would otherwise need to be generated by muscles. However, the degree to which these assistive devices reduce metabolic cost depends not only on the amount of work performed by the device, but also on the individual's ability to exploit adaptive learning processes to take advantage of the external assistance (Zhang et al .…”

Section: Introductionmentioning

confidence: 99%

Taking advantage of external mechanical work to reduce metabolic cost: the mechanics and energetics of split‐belt treadmill walking

Sánchez

Simha

Donelan

et al. 2019

The Journal of Physiology

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Key pointsr The neuromotor system generates flexible motor patterns that can adapt to changes in our bodies or environment and also take advantage of assistance provided by the environment.r We ask how energy minimization influences adaptive learning during human locomotion to improve economy when walking on a split-belt treadmill. We use a model-based approach to predict how people should adjust their walking pattern to take advantage of the assistance provided by the treadmill, and we validate these predictions empirically.r We show that adaptation to a split-belt treadmill can be explained as a process by which people reduce step length asymmetry to take advantage of the work performed by the treadmill to reduce metabolic cost.r Our results also have implications for the evaluation of devices designed to reduce effort during walking, as locomotor adaptation may serve as a model approach to understand how people learn to take advantage of external assistance.Abstract In everyday tasks such as walking and running, we often exploit the work performed by external sources to reduce effort. Recent research has focused on designing assistive devices capable of performing mechanical work to reduce the work performed by muscles and improve walking function. The success of these devices relies on the user learning to take advantage of this Natalia Sánchez holds a BS in Biomedical Engineering from the Universidad EIA-CES, Colombia, and MS and PhD degrees in Biomedical Engineering from Northwestern University, researching the neuropathophysiology of stroke. She completed her postdoctoral training at the Division of Biokinesiology and Physical Therapy at the University of Southern California, where she is now an Assistant Professor of Research. Her research focuses on investigating gait adaptation in healthy and pathological populations using experimental and statistical methods. Her research is funded by the American Heart Association and the Mentored Career Development in Clinical and Translational Science programme (KL2). Surabhi Simha holds a Bachelor of Engineering in Biotechnology from Sri Jayachamarajendra College of Engineering, India. She is currently pursuing her PhD in Biomedical Physiology and Kinesiology at Simon Fraser University, Canada. In her research, she uses experiments and computer simulations to understand the algorithms employed by the human nervous system to control walking. N. Sánchez and S. N. Simha contributed equally to this work. This article was first published as a preprint: Sánchez N, Simha SN, Donelan JM, Finley JM (2018). Taking advantage of external mechanical work to reduce metabolic cost: the mechanics and energetics of split-belt treadmill walking. bioRxiv https://doi.N. Sánchez and others J Physiol 597.15external assistance. Although adaptation is central to this process, the study of adaptation is often done using approaches that seem to have little in common with the use of external assistance. We show in 16 young, healthy participants that a common approach for studying adaptation,...

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Section: Introductionmentioning

confidence: 99%

Taking advantage of external mechanical work to reduce metabolic cost: the mechanics and energetics of split‐belt treadmill walking

Sánchez

Simha

Donelan

et al. 2019

The Journal of Physiology

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“…But our results indicate no relationship between changes in users' metabolic rate and changes in mechanical power of the soleus muscle due to exoskeleton assistance (Table 2). In addition, other lines of evidence suggest that changes in biological mechanical power at the center of mass, joint-, or muscle-level 14,36,56 are unable to explain how exoskeletons alter users metabolic rate. Taken together, these results motivate the need to incorporate muscle-level analyses into the design process for wearable devices intended to influence the dynamic function of musculoskeletal tissues.…”

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confidence: 99%

Ultrasound imaging links soleus muscle neuromechanics and energetics during human walking with elastic ankle exoskeletons

Nuckols

Dick

Beck

et al. 2020

Sci Rep

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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, R 2 = 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. The plantar flexors serve an essential role in human walking by providing more than 50% of the leg's total positive mechanical energy 1 and up to 60% of the mechanical power output for redirecting the body's center of mass during push-off 2. The importance of the ankle plantar flexors in legged locomotion is linked to their morphology. Muscle-tendons (MTs) with short pennate muscle fibers and long compliant in-series tendons are well suited for economical locomotion because the elastic tendons store and return mechanical energy over each step 3-5. During steady-state walking, the interaction of the plantar flexors and Achilles tendon is tuned. Throughout early stance, 0% (heel strike) to 40% stride, plantar flexor muscle fascicles generate force isometrically while the Achilles tendon stretches and stores mechanical energy 6. In late stance, 40% to 60% (toe-off) stride, the plantar flexors shorten and the tendon rapidly recoils, providing a burst of positive mechanical power 1,4,7. This coordinated MT interaction permits the plantar flexor muscle fascicles to operate over a narrow region of their force-length (F-L) curve and remain at slow shortening velocities which are favorable contractile conditions for economical force production 7-12. Any disruption to this 'catapult' like mechanism may decrease the ankle's efficient mechanical power production and worsen walking efficiency. Exoskeletons are a class of wearable devices that often act in parallel with human MTs to restore or augment human movement. An increasing number of studies 13 are establishing that both tethered 14-18 and portable 19-21 lower-limb exoskeletons can deliver mechanical power to the body to reduce metabolic demand during walking in young healthy individuals 14-22 , individuals post-stroke 23 , and older adults 24,25. Recently, our group has demonstrated that exoskeletons need not deli...

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“…Thus, it is possible that wearing a lower limb assistive device that reduces muscle activity has the potential to decrease in series elastic stiffness, and ultimately lead to an increase in collision force and the metabolic cost of transport. The mechanical properties of ankle exoskeletons can be modified to affect push-off timing (Galle et al, 2017), perhaps neutralizing the negative effects of reduced ankle elasticity. Collision forces have been vigorously evaluated in lower limb prostheses (Caputo and Collins, 2014; Herr and Grabowski, 2012; Houdijk et al, 2009) and need to be conducted in exoskeletons.…”

Section: Discussionmentioning

confidence: 99%

Triceps surae torque-length relationships relevant for walking activity levels with and without an ankle exoskeleton

Hessel

Raiteri

Marsh

et al. 2019

Preprint

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Ankle exoskeletons have been developed to assist walking by offloading the plantar flexors work requirements, which reduces muscle activity level. However, reduced muscle activity alters plantar flexor muscle-tendon unit dynamics in a way that is poorly understood. We therefore evaluated torque-fascicle length properties of the soleus and lateral gastrocnemius during voluntary contractions at simulated activity levels typical during late stance with and without an ankle exoskeleton. Soleus activity levels (100, 30, and 22% maximal voluntary activity) were produced by participants via visual electromyography feedback at ankle angles ranging from −10° plantar flexion to 35° dorsiflexion. Using dynamometry and ultrasound imaging, torque-fascicle length data of the soleus and lateral gastrocnemius were produced. The results indicate that muscle activity reductions observed with an exoskeleton shift the torque-angle and torque-fascicle length curves to more dorsiflexed ankle angles and longer fascicle lengths where no descending limb is physiologically possible. This shift is in line with previous simulations that predicted a similar increase in the operating fascicle range when wearing an exoskeleton. These data suggest that a small reduction in muscle activity causes changes to torque-fascicle length properties, which has implications for the design and testing of future ankle exoskeletons for assisted walking.Significance StatementAssistive lower-limb exoskeletons reduce the metabolic cost of walking by reducing the positive work requirements of the plantar flexor muscles. However, if the exoskeleton reduces plantar flexor muscle activity too much, then the metabolic benefit is lost. The biological reasons for this are unclear and hinder further exoskeleton development. This research study is the first to directly evaluate if a reduction in plantar flexor muscle activity similar to that caused by wearing an exoskeleton affects muscle function. We found that reduced muscle activity changes the torque-length properties of two plantar flexors, which could explain why reducing muscle activity too much can increase metabolic cost.

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Reducing the metabolic cost of walking with an ankle exoskeleton: interaction between actuation timing and power

Cited by 174 publications

References 46 publications

Taking advantage of external mechanical work to reduce metabolic cost: the mechanics and energetics of split‐belt treadmill walking

Taking advantage of external mechanical work to reduce metabolic cost: the mechanics and energetics of split‐belt treadmill walking

Ultrasound imaging links soleus muscle neuromechanics and energetics during human walking with elastic ankle exoskeletons

Triceps surae torque-length relationships relevant for walking activity levels with and without an ankle exoskeleton

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