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1. Effect of Joint Friction Compensation on a “Muscle-First” Motor-Assisted Hybrid Neuroprosthesis

   https://doi.org/10.3389/fnbot.2020.588950  

   Reyes, Ryan-David ; Kobetic, Rudolf ; Nandor, Mark ; Makowski, Nathaniel ; Audu, Musa ; Quinn, Roger ; Triolo, Ronald ( December 2020 , Frontiers in Neurorobotics)

   null (Ed.)

   This study assessed the metabolic energy consumption of walking with the external components of a “Muscle-First” Motor Assisted Hybrid Neuroprosthesis (MAHNP), which combines implanted neuromuscular stimulation with a motorized exoskeleton. The “Muscle-First” approach prioritizes generating motion with the wearer's own muscles via electrical stimulation with the actuators assisting on an as-needed basis. The motorized exoskeleton contributes passive resistance torques at both the hip and knee joints of 6Nm and constrains motions to the sagittal plane. For the muscle contractions elicited by neural stimulation to be most effective, the motorized joints need to move freely when not actively assisting the desired motion. This study isolated the effect of the passive resistance or “friction” added at the joints by the assistive motors and transmissions on the metabolic energy consumption of walking in the device. Oxygen consumption was measured on six able-bodied subjects performing 6 min walk tests at three different speeds (0.4, 0.8, and 1.2 m/s) under two different conditions: one with the motors producing no torque to compensate for friction, and the other having the motors injecting power to overcome passive friction based on a feedforward friction model. Average oxygen consumption in the uncompensated condition across all speeds, measured in Metabolic Equivalent of Task (METs), was statistically different than the friction compensated condition. There was an average decrease of 8.8% for METs and 1.9% for heart rate across all speeds. While oxygen consumption was reduced when the brace performed friction compensation, other factors may have a greater contribution to the metabolic energy consumption when using the device. Future studies will assess the effects of gravity compensation on the muscular effort required to lift the weight of the distal segments of the exoskeleton as well as the sagittal plane constraint on walking motions in individuals with spinal cord injuries (SCI). 

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2. Generalizability of muscle synergies in isometric force generation versus point-to-point reaching in the human upper extremity workspace

   https://doi.org/10.3389/fnhum.2023.1144860  

   Pham, Katherine ; Portilla-Jiménez, Manuel ; Roh, Jinsook ( July 2023 , Frontiers in Human Neuroscience)

   Isometric force generation and kinematic reaching in the upper extremity has been found to be represented by a limited number of muscle synergies, even across task-specific variations. However, the extent of the generalizability of muscle synergies between these two motor tasks within the arm workspace remains unknown. In this study, we recorded electromyographic (EMG) signals from 13 different arm, shoulder, and back muscles of ten healthy individuals while they performed isometric and kinematic center-out target matches to one of 12 equidistant directional targets in the horizontal plane and at each of four starting arm positions. Non-negative matrix factorization was applied to the EMG data to identify the muscle synergies. Five and six muscle synergies were found to represent the isometric force generation and point-to-point reaches. We also found that the number and composition of muscle synergies were conserved across the arm workspace per motor task. Similar tuning directions of muscle synergy activation profiles were observed at different starting arm locations. Between the isometric and kinematic motor tasks, we found that two to four out of five muscle synergies were common in the composition and activation profiles across the starting arm locations. The greater number of muscle synergies that were involved in achieving a target match in the reaching task compared to the isometric task may explain the complexity of neuromotor control in arm reaching movements. Overall, our results may provide further insight into the neuromotor compartmentalization of shared muscle synergies between two different arm motor tasks and can be utilized to assess motor disabilities in individuals with upper limb motor impairments.

    

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3. Non-cross Bridge Viscoelastic Elements Contribute to Muscle Force and Work During Stretch-Shortening Cycles: Evidence From Whole Muscles and Permeabilized Fibers

   https://doi.org/10.3389/fphys.2021.648019  

   Hessel, Anthony L. ; Monroy, Jenna A. ; Nishikawa, Kiisa C. ( March 2021 , Frontiers in Physiology)

   null (Ed.)

   The sliding filament–swinging cross bridge theory of skeletal muscle contraction provides a reasonable description of muscle properties during isometric contractions at or near maximum isometric force. However, it fails to predict muscle force during dynamic length changes, implying that the model is not complete. Mounting evidence suggests that, along with cross bridges, a Ca 2+ -sensitive viscoelastic element, likely the titin protein, contributes to muscle force and work. The purpose of this study was to develop a multi-level approach deploying stretch-shortening cycles (SSCs) to test the hypothesis that, along with cross bridges, Ca 2+ -sensitive viscoelastic elements in sarcomeres contribute to force and work. Using whole soleus muscles from wild type and mdm mice, which carry a small deletion in the N2A region of titin, we measured the activation- and phase-dependence of enhanced force and work during SSCs with and without doublet stimuli. In wild type muscles, a doublet stimulus led to an increase in peak force and work per cycle, with the largest effects occurring for stimulation during the lengthening phase of SSCs. In contrast, mdm muscles showed neither doublet potentiation features, nor phase-dependence of activation. To further distinguish the contributions of cross bridge and non-cross bridge elements, we performed SSCs on permeabilized psoas fiber bundles activated to different levels using either [Ca 2+ ] or [Ca 2+ ] plus the myosin inhibitor 2,3-butanedione monoxime (BDM). Across activation levels ranging from 15 to 100% of maximum isometric force, peak force, and work per cycle were enhanced for fibers in [Ca 2+ ] plus BDM compared to [Ca 2+ ] alone at a corresponding activation level, suggesting a contribution from Ca 2+ -sensitive, non-cross bridge, viscoelastic elements. Taken together, our results suggest that a tunable viscoelastic element such as titin contributes to: (1) persistence of force at low [Ca 2+ ] in doublet potentiation; (2) phase- and length-dependence of doublet potentiation observed in wild type muscles and the absence of these effects in mdm muscles; and (3) increased peak force and work per cycle in SSCs. We conclude that non-cross bridge viscoelastic elements, likely titin, contribute substantially to muscle force and work, as well as the phase-dependence of these quantities, during dynamic length changes. 

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4. Loss of variable fascicle gearing during voluntary isometric contractions of paretic medial gastrocnemius muscles in male chronic stroke survivors

   https://doi.org/10.1113/JP280126  

   Son, Jongsang ; Rymer, William Zev ( September 2020 , The Journal of Physiology)

   Maximum fascicle shortening/rotation was significantly decreased in paretic medial gastrocnemius (MG) muscles compared to non‐paretic MG muscles.

   The fascicle gear ratio on both sides decreased as the ankle became dorsiflexed, but the slope of the fascicle gear ratio over ankle joint angle was significantly lower on the paretic side.

   The side‐to‐side slope difference was strongly correlated with the relative maximum joint torque and with the relative shear wave speed, suggesting that variable gearing may explain muscle weakness after stroke.

   The present study aimed to understand variable fascicle gearing during voluntary isometric contractions of the medial gastrocnemius (MG) muscle in chronic stroke survivors. Using ultrasonography, we characterized fascicle behaviour on both paretic and non‐paretic sides during plantarflexion contractions at different intensities and at different ankle joint angles. Shear wave speed was also recorded from the MG muscle belly under passive conditions. Fascicle gear ratios were then calculated as the ratio of muscle belly shortening velocity to fascicle shortening velocity, and variable fascicle gearing was quantified from the slope of gear ratiovs. joint angle relations. This slope was used to establish associations with maximum joint torques and with shear wave speeds. At all measured angles, we found a significant reduction in both maximum fascicle shortening and maximum fascicle rotation on the paretic side compared to the non‐paretic side on our stroke survivor cohort. The fascicle rotation per fascicle shortening on the paretic side was also significantly smaller than on the non‐paretic side, especially at plantarflexed positions. Furthermore, the fascicle gear ratio on both sides decreased as the ankle became dorsiflexed, but the change in the fascicle gear ratio was significantly lower on the paretic side. The side‐to‐side difference in the gear ratio slope was also strongly correlated with the relative maximum joint torque and with the relative shear wave speed, suggesting that variable gearing may explain muscle weakness after stroke. Further studies are needed to investigate how muscular changes after stroke may impede variable gearing and adversely impact muscle performance.

    

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5. A three‐dimensional musculoskeletal model of the pelvis and lower limb of Australopithecus afarensis

   https://doi.org/10.1002/ajpa.24845  

   O'Neill, Matthew C. ; Nagano, Akinori ; Umberger, Brian R. ( September 2023 , American Journal of Biological Anthropology)

   Musculoskeletal modeling is a powerful approach for studying the biomechanics and energetics of locomotion.Australopithecus (A.) afarensisis among the best represented fossil hominins and provides critical information about the evolution of musculoskeletal design and locomotion in the hominin lineage. Here, we develop and evaluate a three‐dimensional (3‐D) musculoskeletal model of the pelvis and lower limb ofA. afarensisfor predicting muscle‐tendon moment arms and moment‐generating capacities across lower limb joint positions encompassing a range of locomotor behaviors.

   A 3‐D musculoskeletal model of an adultA. afarensispelvis and lower limb was developed based primarily on the A.L. 288‐1 partial skeleton. The model includes geometric representations of bones, joints and 35 muscle‐tendon units represented using 43 Hill‐type muscle models. Two muscle parameter datasets were created from human and chimpanzee sources. 3‐D muscle‐tendon moment arms and isometric joint moments were predicted over a wide range of joint positions.

   Predicted muscle‐tendon moment arms generally agreed with skeletal metrics, and corresponded with human and chimpanzee models. Human and chimpanzee‐based muscle parameterizations were similar, with some differences in maximum isometric force‐producing capabilities. The model is amenable to size scaling from A.L. 288‐1 to the larger KSD‐VP‐1/1, which subsumes a wide range of size variation inA. afarensis.

   This model represents an important tool for studying the integrated function of the neuromusculoskeletal systems inA. afarensis. It is similar to current human and chimpanzee models in musculoskeletal detail, and will permit direct, comparative 3‐D simulation studies.

    

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