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

Abstract

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...

Impact of elastic ankle exoskeleton stiffness on neuromechanics and energetics of human walking across multiple speeds

“…Using . We used the rate of the metric of interest (average per second) for a more apt comparison to metabolic rate (J s -1 ) (See [43] for a similar approach).…”

“…When exoskeleton assistance is applied, biological moment and thus muscle-tendon force decreases, while strain on the tendon decreases. Modeling and experimental studies of hopping [45,48,59] and walking [43,47,50] with ankle exoskeletons suggest that muscle lengths are in fact longer and undergo increased excursion with increased exoskeleton assistance, and this could limit muscle force capacity and metabolic economy. As well, the biological system may be resistant to increased muscle strain to avoid injury and compensations may arise to limit range of motion [60][61][62].…”

Development of an Unpowered Energy Recycling Exoskeleton for Walking Assist

“…The improvement of the walking economy could thus be bene cial for both healthy individuals and those with reduced mobility [2]. Many attempts have been made to assist walking especially unpowered assistive devices have demonstrated the capacity to capture and release energy throughout the gait cycle, and have shown the ability to achieve a net reduction in energy consumption [3,4].…”

“…In humans, the power for walk is mainly provided by the positive work of the ankle and hip [12]. The greatest portion of energy waste occurs in muscles such as soleus muscle and gastrocnemius muscle during the push-off phase of the walking cycle [4,13,14].…”

From a biological template model to gait assistance with an exosuit

“…Moderating optimal values, found from modeling analyses for the developed 636 assistive devices was found in other studies. For example, in a recent study, Nucklos et 637 al., showed that the optimal ankle stiffness from the assistive device should not be more 638 than (around) 50% of the quasi stiffness of the ankle joint [77] which could be found by 639 neuromuscular models. 640 3) Feedback signal: The last novelty of this study is introducing the ground 641 reaction force (GRF) as a useful feedback signal.…”