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1. Direct intraoperative measurement of isometric contractile properties in living human muscle

   https://doi.org/10.1113/JP284092  

   Binder‐Markey, Benjamin I. ; Persad, Lomas S. ; Shin, Alexander Y. ; Litchy, William J. ; Kaufman, Kenton R. ; Lieber, Richard L. ( April 2023 , The Journal of Physiology)

   Skeletal muscle's isometric contractile properties are one of the classic structure–function relationships in all of biology allowing for extrapolation of single fibre mechanical properties to whole muscle properties based on the muscle's optimal fibre length and physiological cross‐sectional area (PCSA). However, this relationship has only been validated in small animals and then extrapolated to human muscles, which are much larger in terms of length and PCSA. The present study aimed to measure directly thein situproperties and function of the human gracilis muscle to validate this relationship. We leveraged a unique surgical technique in which a human gracilis muscle is transferred from the thigh to the arm, restoring elbow flexion after brachial plexus injury. During this surgery, we directly measured subject specific gracilis muscle force–length relationshipin situand propertiesex vivo. Each subject's optimal fibre length was calculated from their muscle's length‐tension properties. Each subject's PCSA was calculated from their muscle volume and optimal fibre length. From these experimental data, we established a human muscle fibre‐specific tension of 171 kPa. We also determined that average gracilis optimal fibre length is 12.9 cm. Using this subject‐specific fibre length, we observed an excellent fit between experimental and theorical active length‐tension curves. However, these fibre lengths were about half of the previously reported optimal fascicle lengths of 23 cm. Thus, the long gracilis muscle appears to be composed of relatively short fibres acting in parallel that may not have been appreciated based on traditional anatomical methods.image

   Skeletal muscle's isometric contractile properties represent one of the classic structure–function relationships in all of biology and allow scaling single fibre mechanical properties to whole muscle properties based on the muscle's architecture.

   This physiological relationship has only been validated in small animals but is often extrapolated to human muscles, which are orders of magnitude larger.

   We leverage a unique surgical technique in which a human gracilis muscle is transplanted from the thigh to the arm to restore elbow flexion after brachial plexus injury, aiming to directly measure muscles propertiesin situand test directly the architectural scaling predictions.

   Using these direct measurements, we establish human muscle fibre‐specific tension of ∼170 kPa.

   Furthermore, we show that the gracilis muscle actually functions as a muscle with relatively short fibres acting in parallelvs. long fibres as previously assumed based on traditional anatomical models.

    

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2. Monitoring muscle over three orders of magnitude: Widespread positive allometry among locomotor and body support musculature in the pectoral girdle of varanid lizards ( Varanidae )

   https://doi.org/10.1111/joa.13273  

   Cieri, Robert L. ; Dick, Taylor J. M. ; Clemente, Christofer J. ( July 2020 , Journal of Anatomy)

   There is a functional trade‐off in the design of skeletal muscle. Muscle strength depends on the number of muscle fibers in parallel, while shortening velocity and operational distance depend on fascicle length, leading to a trade‐off between the maximum force a muscle can produce and its ability to change length and contract rapidly. This trade‐off becomes even more pronounced as animals increase in size because muscle strength scales with area (length²) while body mass scales with volume (length³). In order to understand this muscle trade‐off and how animals deal with the biomechanical consequences of size, we investigated muscle properties in the pectoral girdle of varanid lizards. Varanids are an ideal group to study the scaling of muscle properties because they retain similar body proportions and posture across five orders of magnitude in body mass and are highly active, terrestrially adapted reptiles. We measured muscle mass, physiological cross‐sectional area, fascicle length, proximal and distal tendon lengths, and proximal and distal moment arms for 27 pectoral girdle muscles in 13 individuals across 8 species ranging from 64 g to 40 kg. Standard and phylogenetically informed reduced major axis regression was used to investigate how muscle architecture properties scale with body size. Allometric growth was widespread for muscle mass (scaling exponent >1), physiological cross‐sectional area (scaling exponent >0.66), but not tendon length (scaling exponent >0.33). Positive allometry for muscle mass was universal among muscles responsible for translating the trunk forward and flexing the elbow, and nearly universal among humeral protractors and wrist flexors. Positive allometry for PCSA was also common among trunk translators and humeral protractors, though less so than muscle mass. Positive scaling for fascicle length was not widespread, but common among humeral protractors. A higher proportion of pectoral girdle muscles scaled with positive allometry than our previous work showed for the pelvic girdle, suggesting that the center of mass may move cranially with body size in varanids, or that the pectoral girdle may assume a more dominant role in locomotion in larger species. Scaling exponents for physiological cross‐sectional area among muscles primarily associated with propulsion or with a dual role were generally higher than those associated primarily with support against gravity, suggesting that locomotor demands have at least an equal influence on muscle architecture as body support. Overall, these results suggest that larger varanids compensate for the increased biomechanical demands of locomotion and body support at higher body sizes by developing larger pectoral muscles with higher physiological cross‐sectional areas. The isometric scaling rates for fascicle length among locomotion‐oriented pectoral girdle muscles suggest that larger varanids may be forced to use shorter stride lengths, but this problem may be circumvented by increases in limb excursion afforded by the sliding coracosternal joint.

    

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3. Upper Extremity Joint Torque Estimation Through an Electromyography-Driven Model

   https://doi.org/10.1115/1.4056255  

   Tahmid, Shadman ; Font-Llagunes, Josep M. ; Yang, James ( June 2023 , Journal of Computing and Information Science in Engineering)

   Abstract Cerebrovascular accidents like a stroke can affect the lower limb as well as upper extremity joints (i.e., shoulder, elbow, or wrist) and hinder the ability to produce necessary torque for activities of daily living. In such cases, muscles’ ability to generate forces reduces, thus affecting the joint’s torque production. Understanding how muscles generate forces is a key element to injury detection. Researchers have developed several computational methods to obtain muscle forces and joint torques. Electromyography (EMG) driven modeling is one of the approaches to estimate muscle forces and obtain joint torques from muscle activity measurements. Musculoskeletal models and EMG-driven models require necessary muscle-specific parameters for the calculation. The focus of this study is to investigate the EMG-driven approach along with an upper extremity musculoskeletal model to determine muscle forces of two major muscle groups, biceps brachii and triceps brachii, consisting of seven muscle-tendon units. Estimated muscle forces are used to determine the elbow joint torque. Experimental EMG signals and motion capture data are collected for a healthy subject. The musculoskeletal model is scaled to match the geometric parameters of the subject. Then, the approach calculates muscle forces and joint moment for two tasks: simple elbow flexion extension and triceps kickback. Individual muscle forces and net joint torques for both tasks are estimated. The study also has compared the effect of muscle-tendon parameters (optimal fiber length and tendon slack length) on the estimated results. 

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4. Corn Snakes Show Consistent Sarcomere Length Ranges Across Muscle Groups and Ontogeny

   https://doi.org/10.1093/iob/obac040  

   Jurestovsky, Derek J. ; Tingle, Jessica L. ; Astley, Henry C. ( September 2022 , Integrative Organismal Biology)

   The force-generating capacity of muscle depends upon many factors including the actin-myosin filament overlap due to the relative length of the sarcomere. Consequently, the force output of a muscle may vary throughout its range of motion, and the body posture allowing maximum force generation may differ even in otherwise similar species. We hypothesized that corn snakes would show an ontogenetic shift in sarcomere length range from being centered on the plateau of the length-tension curve in small individuals to being on the descending limb in adults. Sarcomere lengths across the plateau would be advantageous for locomotion, while the descending limb would be advantageous for constriction due to the increase in force as the coil tightens around the prey. To test this hypothesis, we collected sarcomere lengths from freshly euthanized corn snakes, preserving segments in straight and maximally curved postures, and quantifying sarcomere length via light microscopy. We dissected 7 muscles (spinalis, semispinalis, multifidus, longissimus dorsi, iliocostalis (dorsal and ventral), and levator costae) in an ontogenetic series of corn snakes (mass = 80–335 g) at multiple regions along the body (anterior, middle, and posterior). Our data shows all of the muscles analyzed are on the descending limb of the length-tension curve at rest across all masses, regions, and muscles analyzed, with muscles shortening onto or past the plateau when flexed. While these results are consistent with being advantageous for constriction at all sizes, there could also be unknown benefits of this sarcomere arrangement for locomotion or striking.

    

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5. Visualization and Quantification of Digitally Dissected Muscle Fascicles in the Masticatory Muscles of Callithrix jacchus Using Nondestructive DiceCT

   https://doi.org/10.1002/ar.24212  

   Dickinson, Edwin ; Basham, Colin ; Rana, Avadh ; Hartstone‐Rose, Adam ( July 2019 , The Anatomical Record)

   The organization and length of a muscle's fascicles imparts its contractile properties. Longer fascicles permit increased muscle excursion, whereas changes in fascicle orientation relate to the overall vector of contractile force. Collecting data on fascicle architecture has traditionally involved destructive and irreversible gross dissection. In recent years, however, new imaging modalities have permitted muscles and their fascicles to be visualized nondestructively. Here, we present data from a primate (Callithrix jacchus), in which, for the first time, individual muscle fascicles are digitally “dissected” (segmented and reconstructed) using nondestructive, high‐resolution diffusible iodine‐based contrast‐enhanced computed tomography (DiceCT) techniques. We also present quantitative data on the length and orientation of these fascicles within 10 muscle divisions of the jaw adductor and abductor musculature (superficial, deep, and zygomatic portions of temporalis and masseter; medial and lateral pterygoid; anterior and posterior digastric) and compare these digitally measured lengths to fascicular lengths measured using traditional gross and chemical dissection. Digitally derived fascicle lengths correspond well to their dissection‐derived counterparts. Moreover, our analyses of changes in fascicle orientation across the adductor complex enable us to visualize previously uncharacterized levels of detail and highlight significant variation between adjacent muscle layers within muscle groups (e.g., between superficial, deep, and zygomatic portions of masseter and temporalis). We conclude that this technique offers great potential to future research, particularly for questions centered around the visualization and quantification of obscured and often‐overlooked muscles such as the pterygoid and digastric muscles, and for deriving more accurate models of the masticatory system as a whole. Anat Rec, 302:1891–1900, 2019. © 2019 American Association for Anatomy

    

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