Human-in-the-loop Bayesian optimization of wearable device parameters

J NeuroEngineering Rehabil

Kang

et al. 2020

292

201

Since the early 2000s, researchers have been trying to develop lower-limb exoskeletons that augment human mobility by reducing the metabolic cost of walking and running versus without a device. In 2013, researchers finally broke this 'metabolic cost barrier'. We analyzed the literature through December 2019, and identified 23 studies that demonstrate exoskeleton designs that improved human walking and running economy beyond capable without a device. Here, we reviewed these studies and highlighted key innovations and techniques that enabled these devices to surpass the metabolic cost barrier and steadily improve user walking and running economy from 2013 to nearly 2020. These studies include, physiologically-informed targeting of lower-limb joints; use of off-board actuators to rapidly prototype exoskeleton controllers; mechatronic designs of both active and passive systems; and a renewed focus on human-exoskeleton interface design. Lastly, we highlight emerging trends that we anticipate will further augment wearable-device performance and pose the next grand challenges facing exoskeleton technology for augmenting human mobility.

Section: Leading Approaches and Technologies For Advancing Exoskeletonsmentioning

confidence: 99%

The exoskeleton expansion: improving walking and running economy

Sawicki

J NeuroEngineering Rehabil

Kang

et al. 2020

292

201

“…However, there is potential to use advanced tools from image processing and machine-learning to extract real-time muscle length and velocity samples from ultrasound images 82 , and then use them in combination with EMG or a musculoskeletal model 83,84 as input to an exoskeleton controller. Access to muscle dynamics in real-time would enable application of state-of-the-art human in the loop optimization techniques 17,85,86 to individual or groups of target muscles. The possibility of 'muscle in the loop' exoskeleton control strategies opens the door to devices capable of steering neuromechanical structure and function over both short and long timescales during locomotion 87 .…”

mentioning

confidence: 99%

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

Nuckols

Dick

et al. 2020

Sci Rep

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...

“…However, there is potential to use advanced tools from image processing and machine-learning to extract real-time muscle length and velocity samples from ultrasound images 82 , and then use them in combination with EMG or a musculoskeletal model 83,84 as input to an exoskeleton controller. Access to muscle dynamics in real-time would enable application of state-of-the-art human in the loop optimization techniques 17,85,86 to individual or groups of target muscles. The possibility of ‘muscle in the loop’ exoskeleton control strategies opens the door to devices capable of steering neuromechanical structure and function over both short and long timescales during locomotion 87,88 .…”

Section: Discussionmentioning

confidence: 99%

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

Nuckols

Dick

et al. 2020

Preprint

26Unpowered exoskeletons with springs in parallel to human plantar flexor muscle-tendons can reduce the 27 metabolic cost of walking. We used ultrasound imaging to look 'under the skin' and measure how 28 exoskeleton stiffness alters soleus muscle contractile dynamics and shapes the user's metabolic rate 29 during walking. Eleven participants (4F, 7M; age: 27.7 ± 3.3 years) walked on a treadmill at 1.25 m s -1 30 and 0% grade with elastic ankle exoskeletons (rotational stiffness: 0-250 Nm rad -1 ) in one training and 31 two testing days. Metabolic savings were maximized (4.2%) at a stiffness of 50 Nm rad -1 . As 32 exoskeleton stiffness increased, the soleus muscle operated at longer lengths and improved economy 33 (force/activation) during early stance, but this benefit was offset by faster shortening velocity and poorer 34 economy in late stance. Changes in soleus activation rate correlated with changes in users' metabolic 35 rate (p = 0.038, R 2 = 0.44), highlighting a crucial link between muscle neuromechanics and exoskeleton 36 performance; perhaps informing future 'muscle-in-the loop' exoskeleton controllers designed to steer 37 contractile dynamics toward more economical force production. 38 39 40 41 48the plantar flexors and Achilles tendon is tuned. Throughout early stance, 0% (heel strike) to 40% 49 stride, plantar flexor muscle fascicles generate force isometrically while the Achilles tendon stretches 50 and stores mechanical energy 6 . In late stance, 40% to 60% (toe-off) stride, the plantar flexors shorten 51 and the tendon rapidly recoils, providing a burst of positive mechanical power 1,4,7 . This coordinated MT 52 interaction permits the plantar flexor muscle fascicles to operate over a narrow region of their force-53 length (F-L) curve and remain at slow shortening velocities which are favorable contractile conditions 54 for economical force production 7-12 . Any disruption to this 'catapult' like mechanism may decrease the 55 ankle's efficient mechanical power production and worsen walking efficiency. 57Exoskeletons are a class of wearable devices that often act in parallel with human MTs to restore or 58 augment human movement. An increasing number of studies 13 are establishing that both tethered 14-18 59 and portable 19-21 lower-limb exoskeletons can deliver mechanical power to the body to reduce metabolic 60 demand during walking in young healthy individuals 14-22 , individuals post-stroke 23 , and older adults 24,25 . 61 Recently, our group has demonstrated that exoskeletons need not deliver net external mechanical power 62 to the body to reduce the metabolic rate of walking 26 . We showed that elastic exoskeletons that place a 63 tension spring in parallel with the human plantar flexor-Achilles tendon complex can reduce the net 64 metabolic rate of walking by an average of 7.2% 26 versus not using a device. The relationship between 65 the user's net metabolic rate and the rotational stiffness of the exoskeleton is bowl-shaped, with a 66 'sweet-spot' that maximizes metabolic benefit...