Mouse shoulder morphology responds to locomotor activity and the kinematic differences of climbing and running

Macroscopic differences in adult human femora are linked to body mass index

Bone functional adaptation is routinely invoked to interpret skeletal morphology despite ongoing debate regarding the limits of the bone response to mechanical stimuli. The wide variation in human body mass presents an opportunity to explore the relationship between mechanical load and skeletal response in weight-bearing elements. Here, we examine variation in femoral macroscopic morphology as a function of body mass index (BMI), which is used as a metric of load history. A sample of 80 femora (40 female; 40 male) from recent modern humans was selected from the Texas State University Donated Skeletal Collection. Femora were imaged using x-ray computed tomography (voxel size ~0.5 mm), and segmented to produce surface models. Landmark-based geometric morphometric analyses based on the Coherent Point Drift algorithm were conducted to quantify shape. Principal components analyses were used to summarize shape variation, and component scores were regressed on BMI. Within the male sample, increased BMI was associated with a mediolaterally expanded femoral shaft, as well as increased neck-shaft angle and decreased femoral neck anteversion angle. No statistically significant relationships between shape and BMI were found in the female sample. While mechanical stimulus has traditionally been applied to changes in long bong diaphyseal shape it appears that bone functional adaptation may also result in fundamental changes in the shape of skeletal elements.

Muscle attachment sites and behavioral reconstruction: An experimental test of muscle-bone structural response to habitual activity

OBJECTIVE

Behavioral reconstruction from muscle attachment sites (entheses) is a common practice in anthropology. However, experimental evidence provides mixed support for the assumed association between enthesis size and shape with changes in habitual activity. In this study, a laboratory mouse model was used to experimentally test whether activity level and type alters muscle architecture and the underlying bone cross-sectional geometry of entheses in order to assess the underlying assumption that behavioral changes lead to quantifiable differences in both muscle and enthesis morphology.

MATERIALS AND METHODS

Female wild-type mice were separated into one control group and two experimentally increased activity groups (running, climbing) over an 11-week study period. At the start of the experiment, half of the mice were 4 weeks and half were 7 weeks of age. The postmortem deltoideus and biceps brachii muscles were measured for potential force production (physiological cross-sectional area) and potential muscle excursion (fiber length). Bone cross-sectional geometry variables were measured from microCT scans of the humerus and radius at the enthesis and non-enthesis regions of interest across activity groups.

RESULTS

Activity level and type altered potential force production and potential muscle excursion of both muscles in the younger cohort. We observed differences in cortical bone geometry in both the humerus enthesis and radius non-enthesis region driven exclusively among the younger wheel-running mice.

DISCUSSION

These results indicate that in addition to muscle architectural changes, bone structural properties at the enthesis do show an adaptive response to increased activity, such as running but only during earlier development. However, further research is required in order to apply these findings to the reconstruction of living behavior from anthropological specimens.

Development of a comparative chimpanzee musculoskeletal glenohumeral model: implications for human function

Modern human shoulder function is affected by the evolutionary adaptations that have occurred to ensure survival and prosperity of the species. Robust examination of behavioral shoulder performance and injury risk can be holistically improved through an interdisciplinary approach that integrates anthropology and biomechanics. Coordination of these fields can allow different perspectives to contribute to a more complete interpretation of biomechanics of the modern human shoulder. The purpose of this study was to develop a novel biomechanical and comparative chimpanzee glenohumeral model, designed to parallel an existing human glenohumeral model, and compare predicted musculoskeletal outputs between the two models. The chimpanzee glenohumeral model consists of three modules - an external torque module, a musculoskeletal geometric module and an internal muscle force prediction module. Together, these modules use postural kinematics, subject-specific anthropometrics, a novel shoulder rhythm, glenohumeral stability ratios, hand forces, musculoskeletal geometry and an optimization routine to estimate joint reaction forces and moments, subacromial space dimensions, and muscle and tissue forces. Using static postural data of a horizontal bimanual suspension task, predicted muscle forces and subacromial space were compared between chimpanzees and humans. Compared with chimpanzees, the human model predicted a 2 mm narrower subacromial space, deltoid muscle forces that were often double those of chimpanzees and a strong reliance on infraspinatus and teres minor (60-100% maximal force) over other rotator cuff muscles. These results agree with previous work on inter-species differences that inform basic human rotator cuff function and pathology.

Postcranial Skeletal Differences in Free-Range and Captive-Born Primates

Skeletal morphology is important in evolutionary, genetic, developmental, physiological, and functional studies. Although samples from free-ranging individuals may be preferable, constraints of sample size, demography, or conservation status may necessitate the inclusion of captive-born individuals. Captivity may be associated with physical, physiological, or behavioral differences that may affect skeletal form. This study assesses differences in postcranial skeletal form between free-range and captive-born Macaca mulatta and Saguinus oedipus. Samples included free-range M. mulatta from Cayo Santiago (Caribbean Primate Research Center) and captive-born macaques from the Wisconsin National Primate Research Center. S. oedipus samples included free-range born and captive-born individuals from the Oak Ridge Associated Universities Marmoset Research Center. Twenty-four dimensions of various bones, including the scapula, upper limb, innominate and lower limb, were recorded for adults. Age of epiphyseal closure was recorded for immature captive-born M. mulatta. Analysis of variance and principal component analyses tested significant differences between free-range born and captive-born individuals in each species. Significant differences were present in size and shape of postcrania between free-range and captive-born within taxa. Free-range macaques were larger than captive-born macaques, but this pattern did not consistently carry over to the Saguinus samples. Shape differences, while present throughout the skeleton, were especially prominent in the scapula. Differences in developmental timing, nutrition, and physical activity can be expected to contribute to the observed differences in postcranial skeletal form. These differences should be considered when captive-born primates are included in morphological or evolutionary studies. Anat Rec, 2018. © 2018 Wiley Periodicals, Inc. Anat Rec, 302:761-774, 2019. © 2018 Wiley Periodicals, Inc.

Effects of dietary fatty acid content on humeral cartilage and bone structure in a mouse model of diet-induced obesity

Obesity is a primary risk factor for osteoarthritis (OA), and previous studies have shown that dietary content may play an important role in the pathogenesis of cartilage and bone in knee OA. Several previous studies have shown that the ratio of ω-3 polyunsaturated fatty acids (PUFAs), ω-6 PUFAs, and saturated fatty acids can significantly influence bone structure and OA progression. However, the influence of obesity or dietary fatty acid content on shoulder OA is not well understood. The goal of this study was to investigate the role of dietary fatty acid content on bone and cartilage structure in the mouse shoulder in a model of diet-induced obesity. For 24 weeks, mice were fed control or high-fat diets supplemented with ω-3 PUFAs, ω-6 PUFAs, or saturated fatty acids. The humeral heads were analyzed for bone morphometry and mineral density by microCT. Cartilage structure and joint synovitis were determined by histological grading, and microscale mechanical properties of the cartilage extracellular and pericellular matrices were quantified using atomic force microscopy. Diet-induced obesity significantly altered bone morphology and mineral density in a manner that was dependent on dietary free fatty acid content. In general, high-fat diet groups showed decreased bone quality, with the ω-3 diet being partially protective. Cartilage mechanical properties and OA scores showed no changes with obesity or diet. These findings are consistent with clinical literature showing little if any relationship between obesity and shoulder OA (unlike knee OA), but suggest that diet-induced obesity may influence other joint tissues. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res.

Out on a limb: bandicoot limb co-variation suggests complex impacts of development and adaptation on marsupial forelimb evolution

Marsupials display far less forelimb diversity than placentals, possibly because of the laborious forelimb-powered climb to the pouch performed by most marsupial neonates. This is thought to result in stronger morphological integration (i.e., higher co-variance) within the marsupial forelimb skeleton, and lower integration between marsupial fore- and hind limbs, compared to other mammals. Possible mechanisms for this constraint are a fundamental developmental change in marsupial limb patterning, or alternatively more immediate perinatal biomechanical and metabolic requirements. In the latter case, peramelid marsupials (bandicoots), which have neonates that climb very little, should show lower within-limb and higher between-limb integration, compared to other marsupials. We tested this in four peramelid species and the related bilby, using partial correlation analyses of between-landmark linear measurements of limb bones, and Procrustes-based two-block partial least-squares analysis (2B-PLS) of limb bone shapes using the same landmarks. We find extensive between-limb integration in partial correlation analyses of only bone lengths, consistent with a reduction of a short-term biomechanical/allocation constraint in peramelid forelimbs. However, partial correlations of bone proportions and 2B-PLS reveal extensive shape divergence between correlated bone pairs. This result contradicts expectations of developmental constraints or serial homology, instead suggesting a function-driven integration pattern. Comparing visualizations from cross-species principal components analysis and 2B-PLS, we tentatively identify selection for digging and half-bounding as the main driver of bandicoot limb integration patterning. This calls for further assessments of functional versus developmental limb integration in marsupials with a more strenuous neonatal climb to the pouch.

Locomotor activity influences muscle architecture and bone growth but not muscle attachment site morphology

The ability to make behavioural inferences from skeletal remains is critical to understanding the lifestyles and activities of past human populations and extinct animals. Muscle attachment site (enthesis) morphology has long been assumed to reflect muscle strength and activity during life, but little experimental evidence exists to directly link activity patterns with muscle development and the morphology of their attachments to the skeleton. We used a mouse model to experimentally test how the level and type of activity influences forelimb muscle architecture of spinodeltoideus, acromiodeltoideus, and superficial pectoralis, bone growth rate and gross morphology of their insertion sites. Over an 11-week period, we collected data on activity levels in one control group and two experimental activity groups (running, climbing) of female wild-type mice. Our results show that both activity type and level increased bone growth rates influenced muscle architecture, including differences in potential muscular excursion (fibre length) and potential force production (physiological cross-sectional area). However, despite significant influences on muscle architecture and bone development, activity had no observable effect on enthesis morphology. These results suggest that the gross morphology of entheses is less reliable than internal bone structure for making inferences about an individual's past behaviour.

Geometric morphometrics of hominoid infraspinous fossa shape

Recent discoveries of early hominin scapulae from Ethiopia (Dikika, Woranso-Mille) and South Africa (Malapa) have motivated new examinations of the relationship between scapular morphology and locomotor function. In particular, infraspinous fossa shape has been shown to significantly differ among hominoids. However, this region presents relatively few homologous landmarks, such that traditional distance and angle-based methods may oversimplify this three-dimensional structure. To more thoroughly assess infraspinous fossa shape variation as it relates to function among adult hominoid representatives, we considered two geometric morphometric (GM) approaches--one employing five homologous landmarks ("wireframe") and another with 83 sliding semilandmarks along the border of the infraspinous fossa. We identified several differences in infraspinous fossa shape with traditional approaches, particularly in superoinferior fossa breadth and scapular spine orientation. The wireframe analysis reliably captured the range of shape variation in the sample, which reflects the relatively straightforward geometry of the infraspinous fossa. Building on the traditional approach, the GM results highlighted how the orientation of the medial portion of the infraspinous fossa differed relative to both the axillary border and spine. These features distinguished Pan from Gorilla in a way that traditional analyses had not been able to discern. Relative to the wireframe method, the semilandmark approach further distinguished Pongo from Homo, highlighting aspects of infraspinous fossa morphology that may be associated with climbing behaviors in hominoid taxa. These results highlight the ways that GM methods can enhance our ability to evaluate complex aspects of shape for refining and testing hypotheses about functional morphology.

Ontogeny of the hominoid scapula: The influence of locomotion on morphology

Primate shoulder morphology has been linked with locomotor habits, oftentimes irrespective of phylogenetic heritage. Among hominoids, juvenile African apes are known to climb more frequently than adults, while orangutans and gibbons maintain an arboreal lifestyle throughout ontogeny. This study examined if these ontogenetic locomotor differences carry a morphological signal, which should be evident in the scapulae of chimpanzees and gorillas but absent in taxa that do not display ontogenetic behavioral shifts. The scapular morphology of five hominoid primates and one catarrhine outgroup was examined throughout ontogeny to evaluate if scapular traits linked with arboreal activities are modified in response to ontogenetic behavioral shifts away from climbing. Specifically, the following questions were addressed: 1) which scapular characteristics distinguish taxa with different locomotor habits; and 2) do these traits show associated changes during development in taxa known to modify their behavioral patterns? Several traits characterized suspensory taxa from nonsuspensory forms, such as cranially oriented glenohumeral joints, obliquely oriented scapular spines, relatively narrow infraspinous fossae, and inferolaterally expanded subscapularis fossae. The relative shape of the dorsal scapular fossae changed in Pan, Gorilla, and also Macaca in line with predictions based on reported ontogenetic changes in locomotor behavior. These morphological changes were mostly distinct from those seen in Pongo, Hylobates, and Homo and imply a unique developmental pattern, possibly related to ontogenetic locomotor shifts. Accordingly, features that sorted taxa by locomotor habits and changed in concert with ontogenetic behavioral patterns should be particularly useful for reconstructing the locomotor habits of fossil forms.