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Session Schedule & Abstracts

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Wednesday 29th June, 2016

Symposium: Muscle functional morphology: beyond gross anatomy 2

Room: Salon F   4:30 pm–5:30 pm

Moderator(s): A. Hartstone-Rose & D. Marchi
MFM2-1  4:30 pm  Leg muscle architecture in primates and its correlation with locomotion petterns. Marchi D*, University of Pisa; Mikes AE, University of South Carolina School of Medicine; Leischner CL, University of South Carolina School of Medicine; Pastor F, Universidad de Valladolid; Hartstone-Rose A, University of South Carolina School of Medicine
Abstract: Understanding the osteological correlates of the different locomotion patterns of extant primates is important to understanding locomotion of extinct primates. Bone biomechanical studies indicate that more arboreal primates have relative (to the tibia) more robust fibula than more terrestrial ones. Here, we test if the same pattern is seen in the differences in leg muscular architecture. Muscle mass, fascicle lengths (FL), physiological cross-sectional area (PCSA) and reduced PCSA (RPCSA) were studied in 34 primate species (6 strepsirrhines, 13 platyrrhines and 15 catarrhines). Muscles were grouped into toe and ankle flexors and extensors and studied for phylogenetic and functional signals. All variables strongly correlate with body mass: strength variables (mass, PCSA and RPCSA) scale with positive allometry, and speed/stretch measure (FL) scales with isometry across the whole sample. Thus, larger primates are relatively stronger though not faster or more flexible than smaller species. Residuals of the regressions show that strepsirrhines, catarrhines and platyrrhines are statistically indistinguishable. Surprisingly, the only variable that statistically distinguishes the arboreal vs. terrestrial species is total flexor/extensor RPCSA, with arboreal primates relatively stronger in this measure. The strongest functional signal emerged when comparing suspensory and vertical clinging and leaping (VCL) to more quadrupedal primates (QUAD). QUAD have statistically heavier leg mm. though they are not significantly greater in cross-sectional area or reduced in FL. Thus, although QUAD taxa are neither stronger nor slower than suspensory/VCL species, their muscles are more massive. Perhaps this is because of a mass constraint on suspensory/VCL locomotion. These results show the complex relation between leg bone biomechanics and muscle architecture and demand for further studies on this topic. Research funded by NSF grant BCS-14-40599.

MFM2-2  4:45 pm  From bone to behavior: reconstructing habitual activity from muscle attachment site morphology. Turcotte CM*, George Washington University; Rabey KN, Midwestern University; Green DJ, Midwestern University; Arbenz-Smith K, George Washington University; McFarlin SC, George Washington University
Abstract: Muscle attachment sites (entheses) are skeletal features frequently used to infer species-level locomotor patterns and individual activity patterns in the human archaeological and fossil record. Macroscopic surface analyses characterizing enthesis size and complexity offer the opportunity to reconstruct behavior non-destructively under the assumption that increased habitual use results in larger and/or more rugose entheses. However, the precise relationship between soft tissue, bone and behavior remains unclear. This study uses a sample of sedentary, control laboratory mice (n=33) and others with experimentally increased (n=32) activity regimes to quantify activity-mediated effects on entheseal morphology and better understand this complex structure for use in paleontological research. We investigated cross-sectional geometry of the radius at two levels: radial tuberosity (the fibrocartilaginous biceps brachii insertion) and midshaft (a non-enthesis site). We used histological sections and microCT scans to investigate total subperiosteal (TSA), cortical (CA) and medullary area (MA) between midshaft and entheseal regions, and between entheses of control and experimental groups. In the activity group, TSA was significantly greater at the enthesis than at the midshaft. Additionally, though TSA and CA did not vary significantly between the exercised and control groups, MA was significantly smaller at the entheseal level in the exercised group (p<.05). This reduction in MA suggests that bone morphology is responsive to the mechanical environment, particularly at the enthesis. Thus, while recent experimental studies have questioned the reliability of entheseal surface morphology, there is potential for internal bone architecture to be an important tool in behavioral reconstruction from muscle attachment sites. This study was funded by the National Science Foundation [Graduate Research Fellowship DGE-1246908 to CT; DDIG BCS-0824552 to DG] and GWU's Lewis N. Cotlow Fund.

MFM2-3  5:00 pm  Beyond function: muscle energetic and brain evolution. Hemingway H*, University of Kentucky, Lexington; Muchlinski MN, University of Kentucky, Lexington
Abstract: Typically skeletal muscle fiber composition is used to evaluate functional differences in locomotion. However, because there are energetic differences among muscle fiber cells, muscle fiber composition could be used to address energetic questions. Muscle is composed of two main types of fibers: type I ("slow twitch") and type II ("fast twitch"). The difference between the two types can be reduced to differences in how these muscles cells use oxygen and glucose. Type I muscle fibers use oxygen to convert glucose to ATP, while type II fibers rely primarily on anaerobic metabolic processes. The expensive tissue hypothesis (ETH) could be improved upon by taking into consideration metabolic differences in skeletal muscle cells. The ETH proposes that the energetic demands imposed on the body by the brain result in a reduction in other expensive tissues (e.g., gastrointestinal tract). The original ETH dismisses the energetic demands of skeletal muscle because of its low metabolic demands at rest. However, its energetic demands can increase 100 fold when active. Furthermore, skeletal muscle is in direct competition for glucose with the brain. Because of skeletal muscle and the brain are competing for resources, we predict that larger brained primates will have relatively less muscle mass and that they will show a decrease in type I fibers. To test our predictions, we dissected 37 species of primates and obtain body mass and muscle mass values. We collected endocranial volumes from the literature. We also sampled pectoralis major, deltoid, gastrocnemius, and soleus for a subset of the primate sample (n=14). Using immunohistochemistry, a muscle fiber composition profile was created for each species sampled. Results show the percent muscle mass and type I muscle fiber [r(14) = -0.74, p = .02] negatively correlate with brain mass. Results clarify the relationship between muscle mass and brain mass and illustrate how muscle mass can be used to address energetic questions.

MFM2-4  5:15 pm  Old meets new: combining traditional and modern tools in the study of jaw adductor morphology and function. Santana SE*, University of Washington
Abstract: The use of modern computational and imaging methods is significantly advancing the field of functional morphology by allowing scientists to more accurately document, measure, model and map the evolution of morphological complexes. In particular, X-ray Computed Tomography (CT) has opened a new niche in the study of anatomy; this imaging technique not only enables the observation and measurement of anatomical structures while they are still intact, but also the creation of three-dimensional renditions that can be used in modeling. Recently, and thanks to the development of diffusible iodine-based contrast enhanced CT (diceCT) protocols, these imaging tools have also started to show great promise for non-destructive imaging of soft tissues. Here, I illustrate the advantages and difficulties of using diceCT methods in the study of mammal jaw adductor anatomy and function. I use a morphologically and ecologically diverse sample of bats to highlight the utility of diceCT in uncovering interspecific variation in the compartments and attachments of jaw adductors. I also show how diceCT data can, and perhaps may always need to, be integrated with data produced via traditional approaches (e.g., dissections) to quantitatively study muscle function and evolution. By combining these traditional and modern tools into three-dimensional biomechanical models of biting function, I further demonstrate how they can complement each other for accurate predictions of interspecific variation in bite performance.

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