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

Abstract

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...

Human myoelectric spatial patterns differ among lower limb muscles and locomotion speeds

“…Future research should examine the relative contributions from each of these factors, as the cause of variations in lower limb spatial EMG amplitudes during locomotion may be multifactorial. Ultrasonic recordings of lower limb muscles during locomotion can provide direct measurements of changes in muscle architecture throughout the gait cycle (Nuckols et al, 2020). This may be useful to distinguish how neuromuscular recruitment and architectural changes affect regional variations in the spatial EMG patterns.…”

Mechanics of walking and running up and downhill: A joint-level perspective to guide design of lower-limb exoskeletons

“…This mechanistic approach has been previously applied to exoskeleton development and logically helps explain why the field has so heavily focused on the ankle as a target for assistance in level walking [3,26,27]. The ankle provides the majority of power on level ground [28] and disrupted ankle mechanics common in clinical populations make it a good target for assistance [29,30].…”

“…Lower-limb robotic exoskeletons can apply assistive torque to reduce the metabolic energy used by biological muscles to produce the force and work for locomotion [1]. A majority of these successful exoskeletons have focused on providing assistance at the ankle within a laboratory setting [2][3][4][5][6][7][8][9][10]. More recently, devices have begun to move outside of laboratory confinement.…”

“…Guidance from baseline human gait data has motivated a bioinspired approach to borrow 'best' concepts from the biological system to guide design of wearable devices. For example, our previous work to design and test a clutch-spring ankle 'exo-tendon' [23,26,27] was inspired by the efficient interaction between the triceps surae muscles and the Achilles tendon within the ankle plantarflexors during walking [31,32].…”

Machine Learning to Extract Muscle Fascicle Length Changes from Dynamic Ultrasound Images in Real-Time

“…To reduce computational requirements and increase processing speed, we cropped and down-sampled B-mode images into smaller matrices (S1 File I). First, we cropped them to focus on the soleus muscle, the largest of the calf muscles and often a target for clinical intervention [26], and eliminated both superficial (i.e., gastrocnemius muscle) and deep regions ( Fig. 2D) of the image.…”