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1. The impact of measurement technique and sampling on estimates of skeletal muscle fibre architecture

   https://doi.org/10.1002/ar.25415  

   Taylor, Andrea B. ; Terhune, Claire E. ; Ross, Callum F. ; Vinyard, Christopher J. ( February 2024 , The Anatomical Record)

   Skeletal muscle fibre architecture provides important insights into performance of vertebrate locomotor and feeding behaviours. Chemical digestion and in situ sectioning of muscle bellies along their lengths to expose fibres, fibre orientation and intramuscular tendon, are two classical methods for estimating architectural variables such as fibre length (L_f) and physiological cross‐sectional area (PCSA). It has recently been proposed thatL_festimates are systematically shorter and hence less accurate using in situ sectioning. Here we addressed this hypothesis by comparingL_festimates between the two methods for the superficial masseter and temporalis muscles in a sample of strepsirrhine and platyrrhine primates. Means or single‐specimenL_festimates using chemical digestion were greater in 17/32 comparisons (53.13%), indicating the probability of achieving longer fibres using chemical digestion is no greater than chance in these taxonomic samples. We further explored the impact of sampling on scaling ofL_fand PCSA in platyrrhines applying a bootstrapping approach. We found that sampling—both numbers of individuals within species and representation of species across the clade significantly influence scaling results ofL_fand PCSA in platyrrhines. We show that intraspecific and clade sampling strategies can account for differences between previously published platyrrhine scaling studies. We suggest that differences in these two methodological approaches to assessing muscle architecture are relatively less consequential when estimatingL_fand PCSA for comparative studies, whereas achieving more reliable estimates within species through larger samples and representation of the full clade space are important considerations in comparative studies of fibre architecture and scaling.

    

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2. 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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3. Borehole fibre-optic seismology inside the Northeast Greenland Ice Stream

   https://doi.org/10.1093/gji/ggad344  

   Fichtner, Andreas ; Hofstede, Coen ; Gebraad, Lars ; Zunino, Andrea ; Zigone, Dimitri ; Eisen, Olaf ( October 2023 , Geophysical Journal International)

   Ice streams are major contributors to ice sheet mass loss and sea level rise. Effects of their dynamic behaviour are imprinted into seismic properties, such as wave speeds and anisotropy. Here, we present results from a distributed acoustic sensing (DAS) experiment in a deep ice-core borehole in the onset region of the Northeast Greenland Ice Stream, with focus on phenomenological and methodological aspects. A series of active seismic surface sources produced clear recordings of the P and S wavefield, including internal reflections, along a 1500 m long fibre-optic cable that was placed into the borehole. The combination of nonlinear traveltime tomography with a firn model constrained by multimode surface wave data, allows us to invert for P and S wave speeds with depth-dependent uncertainties on the order of only 10 m s−1, and vertical resolution of 20–70 m. The wave speed model in conjunction with the regularly spaced DAS data enable a straightforward separation of internal upward reflections followed by a reverse-time migration that provides a detailed reflectivity image of the ice. While the differences between P and S wave speeds hint at anisotropy related to crystal orientation fabric, the reflectivity image seems to carry a pronounced climatic imprint caused by rapid variations in grain size. Further improvements in resolution do not seem to be limited by the DAS channel spacing. Instead, the maximum frequency of body waves below ∼200 Hz, low signal-to-noise ratio caused by poor coupling, and systematic errors produced by the ray approximation, appear to be the leading-order issues. Among these, only the latter has a simple existing solution in the form of full-waveform inversion. Improving signal bandwidth and quality, however, will likely require a significantly larger effort in terms of both sensing equipment and logistics.

    

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4. Regularized tomographic PIV for incompressible flows based on conservation of mass

   https://doi.org/10.1364/AO.380720  

   Liu, Ning ; Ma, Lin ( February 2020 , Applied Optics)

   Three-dimensional and three-component (3D3C) velocity measurements have long been desired to resolve the 3D spatial structures of turbulent flows. Recent advancements have demonstrated tomographic particle image velocimetry (tomo-PIV) as a powerful technique to enable such measurements. The existing tomo-PIV technique obtains 3D3C velocity field by cross-correlating two frames of 3D tomographic reconstructions of the seeding particles. A most important issue in 3D3C velocity measurement involves uncertainty, as the derivatives of the measurements are usually of ultimate interest and uncertainties are amplified when calculating derivatives. To reduce the uncertainties of 3D3C velocity measurements, this work developed a regularized tomo-PIV method. The new method was demonstrated to enhance accuracy significantly by incorporating the conservation of mass into the tomo-PIV process. The new method was demonstrated and validated both experimentally and numerically. The results illustrated that the new method was able to enhance the accuracy of 3D3C velocity measurements by 40%–50% in terms of velocity magnitude and by 0.6°–1.1° in terms of velocity orientation, compared to the existing tomo-PIV technique. These improvements brought about by the new method are expected to expand the application of tomo-PIV techniques when accuracy and quantitative 3D flow properties are required.

    

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5. A sensor-based calibration system for three-dimensional digital image correlation

   https://doi.org/10.1117/12.2612106  

   Bottalico, Fabio ; Valente, Nicholas A. ; Dabetwar, Shweta ; Jerath, Kshitij ; Luo, Yan ; Niezrecki, Christopher ; Sabato, Alessandro ( April 2022 , Health Monitoring of Structural and Biological Systems XVI)

   Fromme, Paul ; Su, Zhongqing (Ed.)

   Three-dimensional digital image correlation (3D-DIC) has become a strong alternative to traditional contact-based techniques for structural health monitoring. 3D-DIC can extract the full-field displacement of a structure from a set of synchronized stereo images. Before performing 3D-DIC, a complex calibration process must be completed to obtain the stereovision system’s extrinsic parameters (i.e., cameras’ distance and orientation). The time required for the calibration depends on the dimensions of the targeted structure. For example, for large-scale structures, the calibration may take several hours. Furthermore, every time the cameras’ position changes, a new calibration is required to recalculate the extrinsic parameters. The approach proposed in this research allows determining the 3D-DIC extrinsic parameters using the data measured with commercially available sensors. The system utilizes three Inertial Measurement Units with a laser distance meter to compute the relative orientation and distance between the cameras. In this paper, an evaluation of the sensitivity of the newly developed sensor suite is provided by assessing the errors in the measurement of the extrinsic parameters. Analytical simulations performed on a 7.5 x 5.7 m field of view using the data retrieved from the sensors show that the proposed approach provides an accuracy of ~10-6 m and a promising way to reduce the complexity of 3D-DIC calibration. 

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