Non-Destructive Determination of Muscle Architectural Variables Through the Use of DiceCT

Estimated and in vivo measurements of bite force demonstrate exceptionally large bite forces in parrots (Psittaciformes)

Jaw morphology and function determine the range of dietary items that an organism can consume. Bite force is a function of the force exerted by the jaw musculature and applied via the skeleton. Bite force has been studied in a wide range of taxa using various methods, including direct measurement, or calculation from skulls or jaw musculature. Data for parrots (Psittaciformes), considered to have strong bites, are rare. This study calculated bite force for a range of parrot species of differing sizes using a novel method that relied on forces calculated using the area of jaw muscles measured in situ and their masses. The values for bite force were also recorded in vivo using force transducers, allowing for a validation of the dissection-based models. The analysis investigated allometric relationships between measures of body size and calculated bite force. Additionally, the study examined whether a measure of a muscle scar could be a useful proxy to estimate bite force in parrots. Bite force was positively allometric relative to body and skull mass, with macaws having the strongest bite recorded to date for a bird. Calculated values for bite force were not statistically different from measured values. Muscle scars from the adductor muscle attachment on the mandible can be used to accurately predict bite force in parrots. These results have implications for how parrots process hard food items and how bite forces are estimated in other taxa using morphological characteristics of the jaw musculature.

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

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 (Lf) and physiological cross-sectional area (PCSA). It has recently been proposed that Lf estimates are systematically shorter and hence less accurate using in situ sectioning. Here we addressed this hypothesis by comparing Lf estimates between the two methods for the superficial masseter and temporalis muscles in a sample of strepsirrhine and platyrrhine primates. Means or single-specimen Lf estimates 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 of Lf and 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 of Lf and 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 estimating Lf and 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.

Ecological correlates of three-dimensional muscle architecture within the dietarily diverse Strepsirrhini

Analysis of muscle architecture, traditionally conducted via gross dissection, has been used to evaluate adaptive relationships between anatomical form and behavioral function. However, gross dissection cannot preserve three-dimensional relationships between myological structures for analysis. To analyze such data, we employ diffusible, iodine-based contrast-enhanced computed tomography (DiceCT) to explore the relationships between feeding ecology and masticatory muscle microanatomy in eight dietarily diverse strepsirrhines: allowing, for the first time, preservation of three-dimensional fascicle orientation and tortuosity across a functional comparative sample. We find that fascicle properties derived from these digital analyses generally agree with those measured from gross-dissected conspecifics. Physiological cross-sectional area was greatest in species with mechanically challenging diets. Frugivorous taxa and the wood-gouging species all exhibit long jaw adductor fascicles, while more folivorous species show the shortest relative jaw adductor fascicle lengths. Fascicle orientation in the parasagittal plane also seems to have a clear dietary association: most folivorous taxa have masseter and temporalis muscle vectors that intersect acutely while these vectors intersect obliquely in more frugivorous species. Finally, we observed notably greater magnitudes of fascicle tortuosity, as well as greater interspecific variation in tortuosity, within the jaw adductor musculature than in the jaw abductors. While the use of a single specimen per species precludes analysis of intraspecific variation, our data highlight the diversity of microanatomical variation that exists within the strepsirrhine feeding system and suggest that muscle architectural configurations are evolutionarily labile in response to dietary ecology-an observation to be explored across larger samples in the future.

Computational Approaches and Observer Variation in the 3D Musculoskeletal Modeling of the Heads of Anolis

High-resolution imaging, 3D modeling, and quantitative analyses are equipping evolutionary biologists with new approaches to understanding the variation and evolution of the musculoskeletal system. However, challenges with interpreting DiceCT data and higher order use of modeled muscles have not yet been fully explored, and the error in and accuracy of some digital methods remain unclear. West Indian Anolis lizards are a model clade for exploring patterns in functional adaptation, ecomorphology, and sexual size dimorphism in vertebrates. These lizards possess numerous jaw muscles with potentially different anatomies that sculpt the adductor chamber of the skull. Here we test approaches to quantifying the musculoskeletal shape of the heads of two species of Anolis: A. pulchellus and A. sagrei. We employ comparative approaches such as DiceCT segmentation of jaw muscles, 3D surface attachment mapping, and 3D landmarking with the aim of exploring muscle volumes, 3D muscle fiber architecture, and sexual dimorphism of the skull. We then compare sources of measurement error in these 3D analyses while also presenting new 3D musculoskeletal data from the Anolis feeding apparatus. These findings demonstrate the accessibility and repeatability of these emerging techniques as well as provide details regarding the musculoskeletal anatomy of the heads of A. pulchellus and A. sagrei which show potential for further research of comparative biomechanics and evolution in the clade.

Variation in the cranial osteology of the amphisbaenian genus Zygaspis based on high-resolution x-ray computed tomography

Amphisbaenians are a specialized fossorial group of reptiles, having developed head-first burrowing, a specialized skull architecture, and an elongated body. This group is generally small-bodied, with some species possessing skulls only a few millimeters long. In this study, we used high-resolution x-ray computed tomography to compare the skulls of 15 specimens from seven of the eight species in the amphisbaenian genus Zygaspis (Zygaspis dolichomenta, Zygaspis ferox, Zygaspis quadrifrons, Zygaspis kafuensis, Zygaspis nigra, Zygaspis vandami, and Zygaspis violacea). Both interspecific and intraspecific variation, including asymmetry, is observed among the cranial bones of the specimens. There are unique morphological features on some cranial bones, including the premaxilla and ectopterygoid of Z. quadrifrons, the pterygoid and vomer of Z. kafuensis, and the extracolumella of Z. nigra. Sexual dimorphism has been previously reported for the species Z. quadrifrons and is observed here as well.

GoodFibes: An R Package for The Detection of Muscle Fibers from diceCT Scans

Contrast enhanced computed-tomography imaging like diffusible iodine-based contrast-enhanced computed tomography (diceCT) can provide detailed information on muscle architecture important to comparative analyses of functional morphology, using non-destructive approaches. However, manual segmentation of muscle fascicles/fibers is time-consuming, and automated approaches are at times inaccessible and unaffordable. Here, we introduce GoodFibes, an R package for reconstructing muscle architecture in 3D from diceCT image stacks. GoodFibes uses textural analysis of image grayscale values to track straight or curved fiber paths through a muscle image stack. Accessory functions provide quality checking, fiber merging, and 3D visualization and export capabilities. We demonstrate the utility and effectiveness of GoodFibes using two datasets, from an ant and bat diceCT scans. In both cases, GoodFibes provides reliable measurements of mean fiber length compared to traditional approaches, and is as effective as currently available software packages. This open-source, free to use software package will help to improve access to tools in the analysis of muscle fiber anatomy using diceCT scans. The flexible and transparent R-language environment allows other users to build on the functions described here and permits direct statistical analysis of the resulting fiber metrics. We hope that this will increase the number of comparative and evolutionary studies incorporating these rich and functionally important datasets.

Behavioral correlates of fascicular organization: The confluence of muscle architectural anatomy and function

Muscle is a complex tissue that has been studied on numerous hierarchical levels: from gross descriptions of muscle organization to cellular analyses of fiber profiles. In the middle of this space between organismal and cellular biology lies muscle architecture, the level at which functional correlations between a muscle's internal fiber organization and contractile abilities are explored. In this review, we summarize this relationship, detail recent advances in our understanding of this form-function paradigm, and highlight the role played by The Anatomical Record in advancing our understanding of functional morphology within muscle over the past two decades. In so doing, we honor the legacy of Editor-in-Chief Kurt Albertine, whose stewardship of the journal from 2006 through 2020 oversaw the flourishing of myological research, including numerous special issues dedicated to exploring the behavioral correlates of myology across diverse taxa. This legacy has seen the The Anatomical Record establish itself as a preeminent source of myological research, and a true leader within the field of comparative anatomy and functional morphology.

Cryogenic contrast-enhanced microCT enables nondestructive 3D quantitative histopathology of soft biological tissues

Biological tissues comprise a spatially complex structure, composition and organization at the microscale, named the microstructure. Given the close structure-function relationships in tissues, structural characterization is essential to fully understand the functioning of healthy and pathological tissues, as well as the impact of possible treatments. Here, we present a nondestructive imaging approach to perform quantitative 3D histo(patho)logy of biological tissues, termed Cryogenic Contrast-Enhanced MicroCT (cryo-CECT). By combining sample staining, using an X-ray contrast-enhancing staining agent, with freezing the sample at the optimal freezing rate, cryo-CECT enables 3D visualization and structural analysis of individual tissue constituents, such as muscle and collagen fibers. We applied cryo-CECT on murine hearts subjected to pressure overload following transverse aortic constriction surgery. Cryo-CECT allowed to analyze, in an unprecedented manner, the orientation and diameter of the individual muscle fibers in the entire heart, as well as the 3D localization of fibrotic regions within the myocardial layers. We foresee further applications of cryo-CECT in the optimization of tissue/food preservation and donor banking, showing that cryo-CECT also has clinical and industrial potential.

From fibre to function: are we accurately representing muscle architecture and performance?

The size and arrangement of fibres play a determinate role in the kinetic and energetic performance of muscles. Extrapolations between fibre architecture and performance underpin our understanding of how muscles function and how they are adapted to power specific motions within and across species. Here we provide a synopsis of how this 'fibre to function' paradigm has been applied to understand muscle design, performance and adaptation in animals. Our review highlights the widespread application of the fibre to function paradigm across a diverse breadth of biological disciplines but also reveals a potential and highly prevalent limitation running through past studies. Specifically, we find that quantification of muscle architectural properties is almost universally based on an extremely small number of fibre measurements. Despite the volume of research into muscle properties, across a diverse breadth of research disciplines, the fundamental assumption that a small proportion of fibre measurements can accurately represent the architectural properties of a muscle has never been quantitatively tested. Subsequently, we use a combination of medical imaging, statistical analysis, and physics-based computer simulation to address this issue for the first time. By combining diffusion tensor imaging (DTI) and deterministic fibre tractography we generated a large number of fibre measurements (>3000) rapidly for individual human lower limb muscles. Through statistical subsampling simulations of these measurements, we demonstrate that analysing a small number of fibres (n < 25) typically used in previous studies may lead to extremely large errors in the characterisation of overall muscle architectural properties such as mean fibre length and physiological cross-sectional area. Through dynamic musculoskeletal simulations of human walking and jumping, we demonstrate that recovered errors in fibre architecture characterisation have significant implications for quantitative predictions of in-vivo dynamics and muscle fibre function within a species. Furthermore, by applying data-subsampling simulations to comparisons of muscle function in humans and chimpanzees, we demonstrate that error magnitudes significantly impact both qualitative and quantitative assessment of muscle specialisation, potentially generating highly erroneous conclusions about the absolute and relative adaption of muscles across species and evolutionary transitions. Our findings have profound implications for how a broad diversity of research fields quantify muscle architecture and interpret muscle function.

New frontiers in imaging, anatomy, and mechanics of crocodylian jaw muscles

New imaging and biomechanical approaches have heralded a renaissance in our understanding of crocodylian anatomy. Here, we review a series of approaches in the preparation, imaging, and functional analysis of the jaw muscles of crocodylians. Iodine-contrast microCT approaches are enabling new insights into the anatomy of muscles, nerves, and other soft tissues of embryonic as well as adult specimens of alligators. These imaging data and other muscle modeling methods offer increased accuracy of muscle sizes and attachments without destructive methods like dissection. 3D modeling approaches and imaging data together now enable us to see and reconstruct 3D muscle architecture which then allows us to estimate 3D muscle resultants, but also measurements of pennation in ways not seen before. These methods have already revealed new information on the ontogeny, diversity, and function of jaw muscles and the heads of alligators and other crocodylians. Such approaches will lead to enhanced and accurate analyses of form, function, and evolution of crocodylians, their fossil ancestors and vertebrates in general.

Three-dimensional polygonal muscle modelling and line of action estimation in living and extinct taxa

Biomechanical models and simulations of musculoskeletal function rely on accurate muscle parameters, such as muscle masses and lines of action, to estimate force production potential and moment arms. These parameters are often obtained through destructive techniques (i.e., dissection) in living taxa, frequently hindering the measurement of other relevant parameters from a single individual, thus making it necessary to combine multiple specimens and/or sources. Estimating these parameters in extinct taxa is even more challenging as soft tissues are rarely preserved in fossil taxa and the skeletal remains contain relatively little information about the size or exact path of a muscle. Here we describe a new protocol that facilitates the estimation of missing muscle parameters (i.e., muscle volume and path) for extant and extinct taxa. We created three-dimensional volumetric reconstructions for the hindlimb muscles of the extant Nile crocodile and extinct stem-archosaur Euparkeria, and the shoulder muscles of an extant gorilla to demonstrate the broad applicability of this methodology across living and extinct animal clades. Additionally, our method can be combined with surface geometry data digitally captured during dissection, thus facilitating downstream analyses. We evaluated the estimated muscle masses against physical measurements to test their accuracy in estimating missing parameters. Our estimated muscle masses generally compare favourably with segmented iodine-stained muscles and almost all fall within or close to the range of observed muscle masses, thus indicating that our estimates are reliable and the resulting lines of action calculated sufficiently accurately. This method has potential for diverse applications in evolutionary morphology and biomechanics.

A Roadmap to Reconstructing Muscle Architecture from CT Data

Skeletal muscle is responsible for voluntary force generation across animals, and muscle architecture largely determines the parameters of mechanical output. The ability to analyze muscle performance through muscle architecture is thus a key step towards better understanding the ecology and evolution of movements and morphologies. In pennate skeletal muscle, volume, fiber lengths, and attachment angles to force transmitting structures comprise the most relevant parameters of muscle architecture. Measuring these features through tomographic techniques offers an alternative to tedious and destructive dissections, particularly as the availability of tomographic data is rapidly increasing. However, there is a need for streamlined computational methods to access this information efficiently. Here, we establish and compare workflows using partially automated image analysis for fast and accurate estimation of animal muscle architecture. After isolating a target muscle through segmentation, we evaluate freely available and proprietary fiber tracing algorithms to reconstruct muscle fibers. We then present a script using the Blender Python API to estimate attachment angles, fiber lengths, muscle volume, and physiological cross-sectional area. We apply these methods to insect and vertebrate muscle and provide guided workflows. Results from fiber tracing are consistent compared to manual measurements but much less time-consuming. Lastly, we emphasize the capabilities of the open-source three-dimensional software Blender as both a tool for visualization and a scriptable analytic tool to process digitized anatomical data. Across organisms, it is feasible to extract, analyze, and visualize muscle architecture from tomography data by exploiting the spatial features of scans and the geometric properties of muscle fibers. As digital libraries of anatomies continue to grow, the workflows and approach presented here can be part of the open-source future of digital comparative analysis.

The influence of jaw-muscle fibre-type phenotypes on estimating maximum muscle and bite forces in primates

Numerous anthropological studies have been aimed at estimating jaw-adductor muscle forces, which, in turn, are used to estimate bite force. While primate jaw adductors show considerable intra- and intermuscular heterogeneity in fibre types, studies generally model jaw-muscle forces by treating the jaw adductors as either homogeneously slow or homogeneously fast muscles. Here, we provide a novel extension of such studies by integrating fibre architecture, fibre types and fibre-specific tensions to estimate maximum muscle forces in the masseter and temporalis of five anthropoid primates: Sapajus apella (N = 3), Cercocebus atys (N = 4), Macaca fascicularis (N = 3), Gorilla gorilla (N = 1) and Pan troglodytes (N = 2). We calculated maximum muscle forces by proportionally adjusting muscle physiological cross-sectional areas by their fibre types and associated specific tensions. Our results show that the jaw adductors of our sample ubiquitously express MHC α-cardiac, which has low specific tension, and hybrid fibres. We find that treating the jaw adductors as either homogeneously slow or fast muscles potentially overestimates average maximum muscle forces by as much as approximately 44%. Including fibre types and their specific tensions is thus likely to improve jaw-muscle and bite force estimates in primates.

Anthroengineering: an independent interdisciplinary field

In recent decades, funding agencies, institutes and professional bodies have recognized the profound benefits of transdisciplinarity in tackling targeted research questions. However, once questions are answered, the previously abundant support often dissolves. As such, the long-term benefits of these transdisciplinary approaches are never fully achieved. Over the last several decades, the integration of anthropology and engineering through inter- and multidisciplinary work has led to advances in fields such as design, human evolution and medical technologies. The lack of formal recognition, however, of this transdisciplinary approach as a unique entity rather than a useful tool or a subfield makes it difficult for researchers to establish laboratories, secure permanent jobs, fund long-term research programmes and train students in this approach. To facilitate the growth and development and witness the long-term benefits of this approach, we propose the integration of anthropology and engineering be recognized as a new, independent field known as anthroengineering. We present a working definition for anthroengineering and examples of how anthroengineering has been used. We discuss the necessity of recognizing anthroengineering as a unique field and explore potential novel applications. Finally, we discuss the future of anthroengineering, highlighting avenues for moving the field forward.

Architectural properties of the musculoskeletal system in the shoulder of two callitrichid primate species derived from virtual dissection

Callitrichidae are small, arboreal New World primates that utilize a variety of locomotor behaviors including trunk-to-trunk leaping (TTL) and horizontal locomotion which involve differential functional demands. Little is known about the relationship between the preferred locomotor behavior and musculoskeletal architecture of these primates. In this study, we compared the musculoskeletal architecture of selected shoulder muscles in two cadavers each of the trunk-to-trunk leaper Cebuella pygmaea and the mainly pronograde quadrupedally moving Saguinus imperator subgrisescens. Contrast-enhanced microfocus computed tomography (µCT) was used to virtually dissect the cadavers, produce muscle maps, and create 3D reconstructions for an image-based analysis of the muscles. Muscle lengths, muscle volumes, and osteological muscle moment arms were measured, and the anatomical cross-sectional areas (ACSA) were calculated. We expected the muscles of the forelimb of S. imperator to be larger in volume and to be relatively shorter with a larger ACSA due to a higher demand for powerful extension in the forelimbs of this horizontally locomoting species. For C. pygmaea, we expected relatively larger moment arms for the triceps brachii, supraspinatus, infraspinatus and subscapularis, as larger moment arms present an advantage for extensive vertical clinging on the trunk. The muscles of S. imperator were relatively larger in volume than in C. pygmaea and had a relatively larger ACSA. Thus, the shoulder muscles of S. imperator were suited to generate relatively larger forces than those of C. pygmaea. Contrary to our expectations, there were only slight differences between species in regard to muscle lengths and moment arms, which suggests that these properties are not dependent on the preferred locomotor mode. The study of this limited dataset demonstrates that some but not all properties of the musculoskeletal architecture reflect the preferred locomotor behavior in the two species of Callitrichidae examined.

Discovering fiber type architecture over the entire muscle using data-driven analysis

Skeletal muscle function is inferred from the spatial arrangement of muscle fiber architecture, which corresponds to myofiber molecular and metabolic features. Myofiber features are often determined using immunofluorescence on a local sampling, typically obtained from a median region. This median region is assumed to represent the entire muscle. However, it remains largely unknown to what extent this local sampling represents the entire muscle. We present a pipeline to study the architecture of muscle fiber features over the entire muscle, including sectioning, staining, imaging to image quantification and data‐driven analysis with Myofiber type were identified by the expression of myosin heavy chain (MyHC) isoforms, representing contraction properties. We reconstructed muscle architecture from consecutive cross‐sections stained for laminin and MyHC isoforms. Examining the entire muscle using consecutive cross‐sections is extremely laborious, we provide consideration to reduce the dataset without loosing spatial information. Data‐driven analysis with over 150,000 myofibers showed spatial variations in myofiber geometric features, myofiber type, and the distribution of neuromuscular junctions over the entire muscle. We present a workflow to study histological changes over the entire muscle using high‐throughput imaging, image quantification, and data‐driven analysis. Our results suggest that asymmetric spatial distribution of these features over the entire muscle could impact muscle function. Therefore, instead of a single sampling from a median region, representative regions covering the entire muscle should be investigated in future studies.

Tracking tendon fibers to their insertion - a 3D analysis of the Achilles tendon enthesis in mice

Tendon insertions to bone are heavily loaded transitions between soft and hard tissues. The fiber courses in the tendon have profound effects on the distribution of stress along and across the insertion. We tracked fibers of the Achilles tendon in mice in micro-computed tomographies and extracted virtual transversal sections. The fiber tracks and shapes were analyzed from a position in the free tendon to the insertion. Mechanically relevant parameters were extracted. The fiber number was found to stay about constant along the tendon. But the fiber cross-sectional areas decrease towards the insertion. The fibers mainly interact due to tendon twist, while branching only creates small branching clusters with low levels of divergence along the tendon. The highest fiber curvatures were found within the unmineralized entheseal fibrocartilage. The fibers inserting at a protrusion of the insertion area form a distinct portion within the tendon. Tendon twist is expected to contribute to a homogeneous distribution of stress among the fibers. According to the low cross-sectional areas and the high fiber curvatures, tensile and compressive stress are expected to peak at the insertion. These findings raise the question whether the insertion is reinforced in terms of fiber strength or by other load-bearing components besides the fibers. STATEMENT OF SIGNIFICANCE: The presented study is the first analysis of the 3D fiber tracks in macroscopic tendon samples as determined by a combination of cell-maceration, phase-contrast µCT and template-based tracking. The structural findings change the understanding of the tendon-bone insertion and its biomechanics: (1) The insertion is not reinforced in terms of fiber numbers or sizes. Its robustness remains unexplained. (2) The orientation of fibers in the tendon center is higher than in the margins. This arrangement could inspire material development. (3) Fibers inserting at a protrusion of the insertion area stem from a distinct portion within the tendon. The results show that fibrous structure analysis can link macro- to micromechanics and that it is ready for the application to complete muscle-tendon units.

Cross-Sectional Area Measurement Techniques of Soft Tissue: A Literature Review

Evaluation of the biomechanical properties of soft tissues by measuring the stress-strain relationships has been the focus of numerous investigations. The accuracy of stress depends, in part, upon the determination of the cross-sectional area (CSA). However, the complex geometry and pliability of soft tissues, especially ligaments and tendons, make it difficult to obtain accurate CSA, and the development of CSA measurement methods of soft tissues continues. Early attempts to determine the CSA of soft tissues include gravimetric method, geometric approximation technique, area micrometer method, and microtomy technique. Since 1990, a series of new methods have emerged, including medical imaging techniques (e.g. magnetic resonance imaging (MRI), computed tomography (CT), and ultrasound imaging (USI)), laser techniques (e.g. the laser micrometer method, the linear laser scanner (LLS) technique, and the laser reflection system (LRS) method), molding techniques, and three-dimensional (3D) scanning techniques.

Assaying three-dimensional cellular architecture using X-ray tomographic and correlated imaging approaches

Much of our understanding of the spatial organization of and interactions between cellular organelles and macromolecular complexes has been the result of imaging studies utilizing either light- or electron-based microscopic analyses. These classical approaches, while insightful, are nonetheless limited either by restrictions in resolution or by the sheer complexity of generating multidimensional data. Recent advances in the use and application of X-rays to acquire micro- and nanotomographic data sets offer an alternative methodology to visualize cellular architecture at the nanoscale. These new approaches allow for the subcellular analyses of unstained vitrified cells and three-dimensional localization of specific protein targets and have served as an essential tool in bridging light and electron correlative microscopy experiments. Here, we review the theory, instrumentation details, acquisition principles, and applications of both soft X-ray tomography and X-ray microscopy and how the use of these techniques offers a succinct means of analyzing three-dimensional cellular architecture. We discuss some of the recent work that has taken advantage of these approaches and detail how they have become integral in correlative microscopy workflows.

Visualization and quantification of mimetic musculature via DiceCT

The muscles of facial expression are of significant interest to studies of communicative behaviors. However, due to their small size and high integration with other facial tissues, the current literature is largely restricted to descriptions of the presence or absence of specific muscles. Using diffusible iodine-based contrast-enhanced computed tomography (DiceCT) to stain and digitally image the mimetic mask of Eulemur flavifrons (the blue-eyed black lemur), we demonstrate-for the first time-the ability to visualize these muscles in three-dimensional space and to measure their relative volumes. Comparing these data to earlier accounts of mimetic organization with the face of lemuroidea, we demonstrate several novel configurations within this taxon, particularly in the superior auriculolabialis and the posterior auricularis. We conclude that DiceCT facilitates the study these muscles in closer detail than has been previously possible, and offers significant potential for future studies of this anatomy.

Review of the methods used for calculating physiological cross-sectional area (PCSA) for ecological questions

This review examines literature that used physiological cross-sectional area (PCSA) as a representative measure of an individual muscle's maximal isometric force production. PCSA is used to understand the muscle architecture and how a trade-off between muscle force and muscle contractile velocity reflect adaptations of the musculoskeletal system as a reflection of functional demands. Over the decades, methods have been developed to measure muscle volume, fascicle lengths, and pennation angle to calculate PCSA. The advantages and limitations of these methods (especially the inclusion/elimination of pennation angle) are discussed frequently; however, these method descriptions are scattered throughout the literature. Here, we reviewed and summarised the different approaches to collecting and recording muscle architectural properties to subsequently calculate PCSA. By critically discussing the advantages and limitations of each methodology, we aim to provide readers with an overview of repeatable methods to assess muscle architecture. This review may serve as a guide to facilitate readers searching for the appropriate techniques to calculate PCSA and measure muscle architecture to be applied in ecomorphology research. RESEARCH HIGHLIGHTS: Discuss the theories behind PCSA in a synthesised review to inform researchers about PCSA methodology.

3D Digitization in Functional Morphology: Where is the Point of Diminishing Returns?

Modern computational and imaging methods are revolutionizing the fields of comparative morphology, biomechanics, and ecomorphology. In particular, imaging tools such as X-ray micro computed tomography (µCT) and diffusible iodine-based contrast enhanced CT allow observing and measuring small and/or otherwise inaccessible anatomical structures, and creating highly accurate three-dimensional (3D) renditions that can be used in biomechanical modeling and tests of functional or evolutionary hypotheses. But, do the larger datasets generated through 3D digitization always confer greater power to uncover functional or evolutionary patterns, when compared with more traditional methodologies? And, if so, why? Here, we contrast the advantages and challenges of using data generated via (3D) CT methods versus more traditional (2D) approaches in the study of skull macroevolution and feeding functional morphology in bats. First, we test for the effect of dimensionality and landmark number on inferences of adaptive shifts during cranial evolution by contrasting results from 3D versus 2D geometric morphometric datasets of bat crania. We find sharp differences between results generated from the 3D versus some of the 2D datasets (xy, yz, ventral, and frontal), which appear to be primarily driven by the loss of critical dimensions of morphological variation rather than number of landmarks. Second, we examine differences in accuracy and precision among 2D and 3D predictive models of bite force by comparing three skull lever models that differ in the sources of skull and muscle anatomical data. We find that a 3D model that relies on skull µCT scans and muscle data partly derived from diceCT is slightly more accurate than models based on skull photographs or skull µCT and muscle data fully derived from dissections. However, the benefit of using the diceCT-informed model is modest given the effort it currently takes to virtually dissect muscles from CT scans. By contrasting traditional and modern tools, we illustrate when and why 3D datasets may be preferable over 2D data, and vice versa, and how different methodologies can complement each other in comparative analyses of morphological function and evolution.

Unique skull network complexity of Tyrannosaurus rex among land vertebrates

Like other diapsids, Tyrannosaurus rex has two openings in the temporal skull region. In addition, like in other dinosaurs, its snout and lower jaw show large cranial fenestrae. In T. rex, they are thought to decrease skull weight, because, unlike most other amniotes, the skull proportion is immense compared to the body. Understanding morphofunctional complexity of this impressive skull architecture requires a broad scale phylogenetic comparison with skull types different to that of dinosaurs with fundamentally diverging cranial regionalization. Extant fully terrestrial vertebrates (amniotes) provide the best opportunities in that regard, as their skull performance is known from life. We apply for the first time anatomical network analysis to study skull bone integration and modular constructions in tyrannosaur and compare it with five representatives of the major amniote groups in order to get an understanding of the general patterns of amniote skull modularity. Our results reveal that the tyrannosaur has the most modular skull organization among the amniotes included in our study, with an unexpected separation of the snout in upper and lower sub-modules and the presence of a lower adductor chamber module. Independent pathways of bone reduction in opossum and chicken resulted in different degrees of cranial complexity with chicken having a typical sauropsidian pattern. The akinetic skull of opossum, alligator, and leatherback turtle evolved in independent ways mirrored in different patterns of skull modularity. Kinetic forms also show great diversity in modularity. The complex tyrannosaur skull modularity likely represents a refined mosaic of phylogenetic and ecological factors with food processing being probably most important for shaping its skull architecture. Mode of food processing primarily shaped skull integration among amniotes, however, phylogenetic patterns of skull integration are low in our sampling. Our general conclusions on amniote skull integrity are obviously preliminary and should be tested in subsequent studies. As such, this study provides a framework for future research focusing on the evolution of modularity on lower taxonomic levels.

3D Muscle Architecture of the Pectoral Muscles of European Starling (Sturnus vulgaris)

Avian flight is achieved through a number of modifications to the body, including the pectoral girdle, yet little is known about the architecture of the pectoral musculature. Muscle architecture is a critical variable in determining the biomechanical function of the vertebrate musculoskeletal system; however, accurate three-dimensional (3D) understanding of muscle architecture has been historically difficult to acquire. Here, we present a musculoskeletal model of a European starling (Sturnus vulgaris) pectoral girdle generated from iodine contrast-enhanced micro-computed-tomography (CT) data and 3D fiber tracking analysis. We used a template-based fiber-tracking algorithm to reconstruct muscle fibers in 3D based on grayscale differences in CT images, which allowed us to estimate fascicle lengths, pennation angles, muscle volumes, and physiological cross-sectional area. Our modeled muscles were qualitatively accurate; however, quantitative muscle architecture data differed between digital and traditional gross-dissection methods reflecting the complex organization of the tissue and differing natures of data collection. We found that model quality is affected by the resolution of CT image data and the fiber-tracking program’s input parameters. Nonetheless, digital fiber tracking offers numerous advantages over gross-dissection methods, most importantly, the ability to visualize and quantify entire muscles in three-dimensions, yielding a much more accurate estimation of whole muscle architecture.

The effect of the masticatory muscle physiological cross-sectional area on the structure of the temporomandibular joint in Carnivora

We compared the temporomandibular joint structure between species of the order Carnivora and investigated its variation among family lineages. We also investigated the effect of the masticatory muscle physiological cross-sectional area (PCSA) on temporomandibular joint structure. The masticatory muscle is composed of multiple muscles, which contract in different directions and exert pressure on the temporomandibular joint. We investigated the effect of the ratio of each muscle’s PCSA—an indicator of muscle force—and muscle size relative to body size on temporomandibular joint structure. The temporalis PCSA relative to body size showed the highest correlation with temporomandibular joint structure. When the temporalis PCSA is large relative to body size, the preglenoid projects caudally, the postglenoid projects rostrally and the pre-postglenoid angle interval is small, indicating that the condyle is locked in the fossa to reinforce the temporomandibular joint. Most Carnivora use blade-like carnassial teeth when slicing food. However, dislocation occurs when the carnassial teeth are used by the temporalis muscle. Our results suggest that the temporomandibular joint is reinforced to prevent dislocation caused by the temporalis muscle. In Mustelidae, the temporomandibular joint with a rostrally projecting postglenoid is suitable for carnassial biting using the temporalis muscle. In Felidae, the force of the masseter onto the carnassial teeth is diverted to the canine by tightening the temporomandibular joint. In Canidae, the masticatory muscle arrangement is well-balanced, enabling combined action. Hence, reinforcement of the temporomandibular joint by bone structure is unnecessary.

Behavioral Correlates of Cranial Muscle Functional Morphology

This issue of the Anatomical Record is the first of a two-volume set that focuses on new investigations into behavioral correlates of muscle functional morphology. Much of the research on functional morphology and adaptation to specific functional niches focuses on the shapes of hard-tissues-bones and teeth. Investigations into soft-tissue anatomy tend to be predominantly descriptive with only brief allusion to ontogenetic or evolutionary origins of structures. When muscles are included in analyses of functional systems, their function tends to be oversimplified-usually considered a simple force vector connecting two osteological points, with the force treated as a constant derived from some simple calculation of muscle size. The goal of these special issues is to present a series of studies that take a more elaborate look at how muscles can be viewed from a functional perspective in studies searching for morphological correlates of behavior. This first volume focuses on the behavioral correlates of cranial muscles-starting with a paper about the mimetic musculature of primates and ending with a series of papers on the masticatory muscles of many lineages of vertebrates. The next issue of the Anatomical Record (March 2018) includes our papers on the behavioral correlates of postcranial muscles. Taken together, we hope you agree that this series presents valuable insights into these form/function relationships using both traditional approaches+ and cutting-edge techniques. Anat Rec, 301:197-201, 2018. © 2018 Wiley Periodicals, Inc.

Functional Morphology and Behavioral Correlates to Postcranial Musculature

In this the second issue of a two-volume set of the Anatomical Record on the relationship between muscle functional morphology and behavior, the focus is on the postcranial musculature. Traditionally, when talking of the postcranium we think of the skeletal parts that primarily provide the lever system necessary for body movements. However, without the force produced by muscle, the postcranial skeleton could not perform these or most other tasks. In this special issue, our colleagues present ten papers that focus on postcranial muscle morphology and function from different perspectives. They include papers on forelimb and hindlimb muscle functional morphology of vertebrates, including lizards, bats, primates, a carnivoran and a rodent, and involved in different substrate use (arboreal, terrestrial, and flying) and locomotion behavior (quadrupedal, leaper, and suspensory) along with a historical overview to help bookend the contextualization of the issues. The picture that these papers provide is one of great liveliness in the field of muscle functional morphology where both young students and affirmed professors continue to contribute with both traditional approaches and new techniques to further our knowledge of muscle morphology and its relationship with animal behavior. Anat Rec, 301:419-423, 2018. © 2018 Wiley Periodicals, Inc.

Dynamic Musculoskeletal Functional Morphology: Integrating diceCT and XROMM

The tradeoff between force and velocity in skeletal muscle is a fundamental constraint on vertebrate musculoskeletal design (form:function relationships). Understanding how and why different lineages address this biomechanical problem is an important goal of vertebrate musculoskeletal functional morphology. Our ability to answer questions about the different solutions to this tradeoff has been significantly improved by recent advances in techniques for quantifying musculoskeletal morphology and movement. Herein, we have three objectives: (1) review the morphological and physiological parameters that affect muscle function and how these parameters interact; (2) discuss the necessity of integrating morphological and physiological lines of evidence to understand muscle function and the new, high resolution imaging technologies that do so; and (3) present a method that integrates high spatiotemporal resolution motion capture (XROMM, including its corollary fluoromicrometry), high resolution soft tissue imaging (diceCT), and electromyography to study musculoskeletal dynamics in vivo. The method is demonstrated using a case study of in vivo primate hyolingual biomechanics during chewing and swallowing. A sensitivity analysis demonstrates that small deviations in reconstructed hyoid muscle attachment site location introduce an average error of 13.2% to in vivo muscle kinematics. The observed hyoid and muscle kinematics suggest that hyoid elevation is produced by multiple muscles and that fascicle rotation and tendon strain decouple fascicle strain from hyoid movement and whole muscle length. Lastly, we highlight current limitations of these techniques, some of which will likely soon be overcome through methodological improvements, and some of which are inherent. Anat Rec, 301:378-406, 2018. © 2018 Wiley Periodicals, Inc.