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Session Schedule & Abstracts




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Wednesday 29th June, 2016

MFM1
Symposium: Muscle functional morphology: beyond gross anatomy 1

Room: Salon F   2:30 pm–4:00 pm

Moderator(s): A. Hartstone-Rose & D. Marchi
MFM1-1  2:30 pm  Stretch, strength, and speed: functional interpretations of muscle fiber architecture in limbs and the masticatory apparatus. Hartstone-Rose A*, University of South Carolina School of Medicine; Marchi D, University of Pisa   AdamHR@sc.edu
Abstract: Traditional myological approaches to functional morphology focus almost exclusively on the gross anatomy of muscles and their osteological attachments. However, an expanding number of researchers are exploring muscle functional anatomy using new approaches that are enriching our understanding of the adaptations in the soft tissues of the musculoskeletal system—the focus of this symposium. Among these is a growing body of literature that focuses on coupling traditional gross anatomical dissections with chemical dissections resulting in deeper understanding of muscle fiber architecture. Through these techniques, we are learning new information about muscle stretch, strength and speed production abilities. Our muscle fiber architecture research, conducted with our students and numerous collaborators, has focused on primate and carnivore adaptations in masticatory and limb muscles as they correlate with variation in diet and locomotion respectively. Coupling these data with their osteological correlates has allowed us to estimate the masticatory abilities of extinct carnivores and primates and will eventually lead to more accurate analyses of the posture, locomotion and substrate use of extinct primates. In short, muscle fiber analysis has allowed us to expand our understanding of functional myology beyond the level of gross anatomy in ways that are adding depth to conversations about adaptations and niche partitioning in living and extinct species. This research was funded by NSF grant BCS-14-40599.

MFM1-2  2:45 pm  Modelling jaw muscle function in marsupials: from dissection to multibody dynamics analysis. Sharp AC*, University of New England; Graham DF, Griffith University; Trusler PW, Monash University; Crompton AW, Harvard University   asharp6@une.edu.au
Abstract: Multibody dynamics is a powerful modelling tool which is becoming increasingly popular for the simulation and analysis of jaw muscle function. It can be used to apply varying muscle forces to predict joint and bite forces during static and dynamic motions as well as investigating muscle activation patterns and how they vary to produce specific movements. The sequence of activity in the jaw muscles of macropods and wombats vary from those in other mammals including the closely related koala. Jaw movements are divided into a vertical phase and horizontal phase, but the number of muscles involved and the level of activity associated with each phase varies considerably between species. To investigate jaw muscle function in wombats, koalas and kangaroos, multibody dynamics models were constructed by combining data from gross dissection and 3D imaging techniques (Magnetic Resonance Imaging, MRI, and Computed Tomography, CT). MRI allowed 3D visualisation of soft tissues in situ, and through virtual dissection we estimated muscle forces and complex vectors of muscle action, allowing us to combine these data with electromyography data to simulate biting. Results show that the greatly enlarged masseter and medial pterygoid muscles in the wombat reflect their ability to exert very high compressive forces on the tooth row simultaneously with the dominant horizontal movement of the mandible. The unique activation pattern in wombats whereby only working-side muscles are active during the power stroke reduces rotation of the mandible to prevent the balancing-side molars from occluding and allows transverse movement of the jaw. These results vary considerably from those of the kangaroo, which has a dominant vertical phase, and the koala, in which the two phases are more even and muscle force is more equally distributed. Combining gross dissection with imaging techniques and multibody dynamics has given us new insights into how the jaw muscles of marsupials have adapted beyond their simple gross anatomy for various functions.

MFM1-3  3:00 pm  Jaw adductor muscle fiber architecture and estimated bite force in tree shrews (Mammalia: Scandentia). Kristjanson HL*, Johns Hopkins University School of Medicine; Perry JMG, Johns Hopkins University School of Medicine   hkristj1@jhmi.edu
Abstract: The soft tissue of the masticatory apparatus is influenced by the properties of food ingested. Tree shrews (Order Scandentia) are omnivorous, and will eat a variety of leaves, fruit, arthropods and insects. It is hypothesized that tree shrews will reflect these dietary differences in their chewing musculature (soft tissue) and their mandibular morphology (hard tissue). Specifically, we look to see whether tree shrew species with a larger component of insects in their diet (Tupaia tana and Tupaia montana) will exhibit a higher ratio of temporalis to masseter musculature. Here, five species of tree shrew are dissected and their muscles of mastication chemically dissolved in order to measure muscle fascicle lengths and calculate physiological cross-sectional area (PCSA). Bite force is reconstructed for each species at the anterior premolar, first lower molar and posterior third molar from a combination of PCSA, muscle orientation, and fascicle length. Results indicate that bite force scales with positive allometry at each bite point (β=3.07; 3.55; 4.12). In each case, bite force increases when moving posteriorly along the tooth row, indicating increased force closer to the mandibular condyle. A comparison of the temporalis and masseter muscles indicate that neither T. montana nor T. tana had a higher ration of temporalis muscle than masseter. T. nicobarica, on the other hand, did. This suggests a larger proportion of insects in T. nicobarica's diet, or a lesser dependence on insects in the diet of T. montana and T. tana than previously thought. These results suggest the need for more work into the feeding habits of T. nicobarica. In addition, understanding the morphology of tree shrew chewing mechanics could have the potential for reconstructing the same muscles of mastication in morphologically similar and phylogenetically closely related stem primates.

MFM1-4  3:15 pm  Biomechanics of the chewing musculature: osteological correlates of function and inferences from fossils. Perry JMG*, Johns Hopkins University; St Clair EM, Johns Hopkins University; Hartstone-Rose A, University of South Carolina School of Medicine   jperry31@jhmi.edu
Abstract: There are many different approaches to the inference of chewing parameters in fossil mammals. These have included differing sources of data from extant analogues and have incorporated different levels of realism. The practical outcomes of such inferences include (more proximately) the input values for finite element and other biomechanical analyses and (more ultimately) conclusions regarding the diets and oral behaviors of extinct taxa. Here, we evaluate several different methods of inference focusing on the chewing muscles of primates. We compare the benefits of including more or less information on muscle position, orientation, size, and architecture. We generated inferences of muscle size, cross-sectional area, bite force, and gape adaptation in three extinct primate groups: Adapidae (Eocene, Europe), Notharctidae (Eocene, North America), and subfossil Lemuriformes (Pleistocene and Holocene, Madagascar). The data on extinct groups were compared with the corresponding values for extant strepsirrhine primates. Results indicate that adapids had large, powerful chewing muscles and great bite force; notharctids had large, powerful temporalis muscles and moderate bite force; and subfossil lemurs had large, powerful, masseter and medial pterygoid muscles. This suggests that some members of these extinct groups were using powerful bites to break their foods, but that different phylogenetic groups may have been emphasizing different individual chewing muscles. The fossil species have signs of gape limitations, consistent with processing small resistant foods (relative to their own body size). Here we also explore the relative merits of high-tech and low-tech methods for evaluating osteological correlates of muscle dimensions, and discuss some new, promising low-tech measurements. Funding came from Johns Hopkins University, Duke University, and Midwestern University.

MFM1-5  3:30 pm  Preliminary bite force estimations of Miocene giant mustelids (Carnivora, Mustelidae). Valenciano A.*, Departamento de Geología Sedimentaria y Cambio Medioambiental, Instituto de Geociencias (CSIC, UCM), Madrid, Spain and Departamento de Paleontología UCM, Facultad de Ciencias Geologicas UCM, Madrid, Spain; Leischner C. L., Department of Cell Biology and Anatomy, University of South Carolina School of Medicine, Columbia SC, USA; Grant A., Department of Cell Biology and Anatomy, University of South Carolina School of Medicine, Columbia SC, USA; Abella J., Universidad Estatal Península de Santa Elena, and Universitat Autònoma de Barcelona, Spain; Hartstone-Rose A., Department of Cell Biology and Anatomy, University of South Carolina School of Medicine, Columbia SC, USA   alb3rtovv@gmail.com
Abstract: We use a sample of 22 modern carnivorans (Canidae, Felidae, Hyaenidae, Mustelidae and Ursidae) spanning the entire body size range of the order from Mustela erminea to Ursus maritimus to reconstruct the bite force (BF) in the Miocene giant mustelids Megalictis, Eomellivora, Ekorus, Plesiogulo and Enhydritherium. BF of the extant specimens was calculated following methods described by Hartstone-Rose et al., 2012: Specimens are dissected, their masticatory muscle fiber architecture is analyzed and the physiological cross-sectional area (PCSA) of each adductor is multiplied by its leverage and the muscle force constant, and the sum of those forces is divided by the leverage to the bite point. Since muscle origin areas correlate closely with PCSA in the extant sample, these areas were used to estimate PCSA in the fossils. Combining these with the leverage measurements allows us to estimate the BF of the extinct taxa. When regressing BF against a body size proxy—the geometric mean of 8 skull measurements (GM)—carnivorans overall scale with significant positive allometry (larger species have relatively higher BF m=1.50 at the canine bite point). This trend is even more pronounced in extant mustelids (m=2.28) likely influenced by the very high BF of the largest taxon Gulo. As expected, the fossil mustelids also have very high BFs—all their residuals are statistically significantly higher than the extant carnivoran sample. Although their skulls are smaller, Ekorus and Megalictis have higher estimated BFs than Canis lupus, Lycaon pictus, Panthera pardus, P. uncia, Ursus americanus, U. maritimus, and small Crocuta crocuta included in our extant sample. In fact, only the large U. arctos and P. onca and P. tigris exceeded their estimated BF. Also by our estimates Ekorus was able to produce BF ~ twice as powerful as those of the most powerful extant mustelid Gulo.

MFM1-6  3:45 pm  Functional adaptations of primate forearm muscle fiber architecture. Leischner CL*, University of South Carolina School of Medicine; Allen KL, Washington University School of Medicine in St. Louis; Pastor F, Universidad de Valladolid; Marchi D, University of Pisa; Hartstone-Rose A, University of South Carolina School of Medicine   leischne@email.sc.edu
Abstract: Distal humerus morphology is thought to reflect variation in the force production capabilities of the forearm musculature, necessitated by differences in substrate use and locomotion. Previously, we demonstrated that forearm muscle mass scales isometrically with body mass in primates and thus muscle mass alone is not an indicator of locomotor behavior. In preliminary data presented at the last ICVM, we found some functional correlates of muscle architecture, but the biases in the preliminary taxonomic sample precluded strong conclusions. We present new data from a greatly expanded sample including 55 specimens from 44 species: strepsirrhines (n=9), platyrrhines (n=15), and catarrhines (n=20). This final sample spans the entire size range of the order from Microcebus to Gorilla, and includes all major locomotor and substrate use groups. Contrary to our previous findings, forearm muscle mass actually scales with positive allometry across all primates (m=1.12, r2=0.98). In terms of architecture, catarrhines exhibit positive allometry in their physiological (and reduced physiological) cross-sectional areas (PCSA: m=1.28, RPCSA: m=1.39) indicating that larger catarrhines have relatively stronger forearm muscles. In terms of substrate use, while PCSA and RPCSA scale with isometry for terrestrial species, they scale with positive allometry (PCSA: m=1.15, RPCSA: m=1.16) for arboreal ones—thus larger arboreal primates have relatively stronger and faster forearms. Furthermore, terrestrial species have significantly greater PCSA (p=0.0133) and RPCSA (p=0.0011). Thus, terrestrial primates have greater forearm strength. We also studied subsets of these muscles, examining the architecture of wrist and digital flexors and extensors. Surprisingly, there are no statistical differences in the fiber architecture of quadrupedal primates when compared to vertical clinging and leaping or suspensory species. This research was funded by NSF grant BCS-14-40599.



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