In vivo strains in the femur of river cooter turtles (Pseudemys concinna) during terrestrial locomotion: tests of force-platform models of loading mechanics

Piezoelectric Scaffolds as Smart Materials for Bone Tissue Engineering

Bone repair and regeneration require physiological cues, including mechanical, electrical, and biochemical activity. Many biomaterials have been investigated as bioactive scaffolds with excellent electrical properties. Amongst biomaterials, piezoelectric materials (PMs) are gaining attention in biomedicine, power harvesting, biomedical devices, and structural health monitoring. PMs have unique properties, such as the ability to affect physiological movements and deliver electrical stimuli to damaged bone or cells without an external power source. The crucial bone property is its piezoelectricity. Bones can generate electrical charges and potential in response to mechanical stimuli, as they influence bone growth and regeneration. Piezoelectric materials respond to human microenvironment stimuli and are an important factor in bone regeneration and repair. This manuscript is an overview of the fundamentals of the materials generating the piezoelectric effect and their influence on bone repair and regeneration. This paper focuses on the state of the art of piezoelectric materials, such as polymers, ceramics, and composites, and their application in bone tissue engineering. We present important information from the point of view of bone tissue engineering. We highlight promising upcoming approaches and new generations of piezoelectric materials.

Breed and loading history influence in vivo skeletal strain patterns in pre-pubertal female chickens

The influence of loading history on in vivo strains within a given specie remains poorly understood, and although in vivo strains have been measured at the hindlimb bones of various species, strains engendered during modes of activity other than locomotion are lacking, particularly in non-human species. For commercial egg-laying chickens specifically, there is an interest in understanding their bones' mechanical behaviour, particularly during youth, to develop early interventions to prevent the high incidence of osteoporosis in this population. We measured in vivo mechanical strains at the tibiotarsus midshaft during steady activities (ground, uphill, downhill locomotion) and non-steady activities (perching, jumping, aerial transition landing) in 48 pre-pubescent female (egg-laying) chickens from two breeds that were reared in three different housing systems, allowing varying amounts and types of physical activity. Mechanical strain patterns differed between breeds, and were dependent on the activity performed. Mechanical strains were also affected by rearing environment: chickens that were restricted from performing dynamic load bearing activity due to caged-housing generally exhibited higher mechanical strain levels during steady, but not non-steady activities, compared to chickens with prior dynamic load-bearing activity experience. Among chickens with prior experience of dynamic load bearing activity, those reared in housing systems that allowed more frequent physical activity did not exhibit lower mechanical strains. In all groups, the tibiotarsus was subjected to a loading environment consisting of a combination of axial compression, bending, and torsion, with torsion being the predominant source of strain. Aerial transition landing produced the highest strain levels with unusual strain patterns compared to other activities, suggesting it may produce the strongest anabolic response. These results exemplify how different breeds within a given specie adapt to maintain different patterns of mechanical strains, and how benefits of physical activity in terms of resistance to strain are activity-type dependent and do not necessarily increase with increased physical activity. These findings directly inform controlled loading experiments aimed at studying the bone mechanoresponse in young female chickens and can also be associated to measures of bone morphology and material properties to understand how these features influence bone mechanical properties in vivo.

Anatomy informs geology: Hydrodynamic dispersal of alligator bones, with implications for taphonomic interpretations of fossil deposits of crocodylians, dinosaurs, and other morphologically novel taxa

Distinctive anatomical features of bones can influence not only how these structures perform in living animals but also the tendency of elements to be transported by flowing water after death. Such transport can be critical in the concentration of fossils from animals that live near freshwater habitats, providing important context for interpreting the composition of paleocommunities. Measurements of the tendency of flowing water to disperse skeletal elements have been collected for diverse taxa, including mammals, turtles, and birds. However, these extant models may not be entirely appropriate for many morphologically distinct extinct lineages, such as non-avian dinosaurs. To expand the range of models available for evaluating the influence of hydrodynamic transport on the assembly of fossil deposits, we used a flow tank to measure the water speeds that disperse bones from a subadult American alligator (Alligator mississippiensis), with the skull and mandible tested in multiple starting orientations. Alligator bones are sorted into three main dispersal groups: early (vertebrae, most girdle elements), intermediate (ribs, most limb bones), and late (pubis, femur), with the skull and mandible varying between intermediate and late depending on orientation. Late dispersing elements tended to be heavy or very flat. These results can refine interpretations of the taphonomic context for deposits of fossil crocodylians and morphologically similar taxa (e.g., choristoderes, phytosaurs) and provide an additional comparative model for deposits of non-avian dinosaurs. Moreover, variation in hydrodynamic sorting across lineages highlights how distinctive anatomical features can influence the concentration of fossils, shaping understanding of assemblage composition and paleofaunal evolution.

Determination of muscle strength and function in plesiosaur limbs: finite element structural analyses of Cryptoclidus eurymerus humerus and femur

Background

The Plesiosauria (Sauropterygia) are secondary marine diapsids. They are the only tetrapods to have evolved hydrofoil fore- and hindflippers. Once this specialization of locomotion had evolved, it remained essentially unchanged for 135 Ma. It is still controversial whether plesiosaurs flew underwater, rowed, or used a mixture of the two modes of locomotion. The long bones of Tetrapoda are functionally loaded by torsion, bending, compression, and tension during locomotion. Superposition of load cases shows that the bones are loaded mainly by compressive stresses. Therefore, it is possible to use finite element structure analysis (FESA) as a test environment for loading hypotheses. These include muscle reconstructions and muscle lines of action (LOA) when the goal is to obtain a homogeneous compressive stress distribution and to minimize bending in the model. Myological reconstruction revealed a muscle-powered flipper twisting mechanism. The flippers of plesiosaurs were twisted along the flipper length axis by extensors and flexors that originated from the humerus and femur as well as further distal locations.

Methods

To investigate locomotion in plesiosaurs, the humerus and femur of a mounted skeleton of Cryptoclidus eurymerus (Middle Jurassic Oxford Clay Formation from Britain) were analyzed using FE methods based on the concept of optimization of loading by compression. After limb muscle reconstructions including the flipper twisting muscles, LOA were derived for all humerus and femur muscles of Cryptoclidus by stretching cords along casts of the fore- and hindflippers of the mounted skeleton. LOA and muscle attachments were added to meshed volumetric models of the humerus and femur derived from micro-CT scans. Muscle forces were approximated by stochastic iteration and the compressive stress distribution for the two load cases, "downstroke" and "upstroke", for each bone were calculated by aiming at a homogeneous compressive stress distribution.

Results

Humeral and femoral depressors and retractors, which drive underwater flight rather than rowing, were found to exert higher muscle forces than the elevators and protractors. Furthermore, extensors and flexors exert high muscle forces compared to Cheloniidae. This confirms a convergently evolved myological mechanism of flipper twisting in plesiosaurs and complements hydrodynamic studies that showed flipper twisting is critical for efficient plesiosaur underwater flight.

Secondary ossification center induces and protects growth plate structure

Growth plate and articular cartilage constitute a single anatomical entity early in development but later separate into two distinct structures by the secondary ossification center (SOC). The reason for such separation remains unknown. We found that evolutionarily SOC appears in animals conquering the land - amniotes. Analysis of the ossification pattern in mammals with specialized extremities (whales, bats, jerboa) revealed that SOC development correlates with the extent of mechanical loads. Mathematical modeling revealed that SOC reduces mechanical stress within the growth plate. Functional experiments revealed the high vulnerability of hypertrophic chondrocytes to mechanical stress and showed that SOC protects these cells from apoptosis caused by extensive loading. Atomic force microscopy showed that hypertrophic chondrocytes are the least mechanically stiff cells within the growth plate. Altogether, these findings suggest that SOC has evolved to protect the hypertrophic chondrocytes from the high mechanical stress encountered in the terrestrial environment.

Biomechanical properties of anuran long bones: correlations with locomotor modes and habitat use

Long bones are subjected to mechanical loads during locomotion that will influence their biomechanical properties through a feedback mechanism (the bone mechanostat). This mechanism adapts the spatial distribution of the mineralized tissue to resist compression, bending and torsion. Among vertebrates, anurans represent an excellent group to study long bone properties because they vary widely in locomotor modes and habitat use, which enforce different skeletal loadings. In this study, we hypothesized that (a) the cortical bone mass, density and design of anuran femur and tibiofibula would reflect the mechanical influences of the different locomotor modes and habitat use, and (b) the relationships between the architectural efficiency of cortical design (cross-sectional moments of inertia) and the intrinsic stiffness of cortical tissue [cortical mineral density; the 'distribution/quality' (d/q) relationship] would describe some inter-specific differences in the efficiency of the bone mechanostat to improve bone design under different mechanical loads. To test this hypothesis, we determined tomographic (peripheral quantitative computed tomography) indicators of bone mass, mineralization, and design along the femur and tibiofibula of four anuran species with different modes of locomotion and use of habitat. We found inter-specific differences in all measures between the distal and proximal ends and mid-diaphysis of the bones. In general, terrestrial-hopper species had the highest values. Arboreal-walker species had the lowest values for all variables except for cortical bone mineral density, which was lowest in aquatic-swimmer species. The d/q relationships showed similar responses of bone modeling as a function of cortical stiffness for aquatic and arboreal species, whereas terrestrial-hoppers had higher values for moments of inertia regardless of the tissue compliance to be deformed. These results provide new evidence regarding the significant role of movement and habitat use in addition to the biomechanical properties of long bones within a morpho-functional and comparative context in anuran species.

Bone adaptation: Safety factors and load predictability in shaping skeletal form

Much is known about skeletal adaptation in relation to the mechanical functions that bones serve. This includes how bone adapts to mechanical loading during an individual's lifetime as well as over evolutionary time. Although controlled loading in animal models allows us to observe short-term bone adaptation (epigenetic mechanobiology), examining an assemblage of extant vertebrate bones or a group of fossils' bony structures can reveal the combined effects of long-term trends in loading history and the effects of natural selection. In this survey we examine adaptations that take place over both time scales and highlight a few of the extraordinary insights first published by John Currey. First, we provide a historical perspective on bone adaptation control mechanisms, followed by a discussion of safety factors in bone. We then summarize examples of structural- and material-level adaptations and mechanotransduction, and analyze the relationship between these structural- and material-level adaptations observed in situations where loading modes are either predictable or unpredictable. We argue that load predictability is a major consideration for bone adaptation broadly across an evolutionary timescale, but that its importance can also be seen during ontogenetic growth trajectories, which are subject to natural selection as well. Furthermore, we suggest that bones with highly predictable load patterns demonstrate more precise design with lower safety factors, while bones that experience less predictable loads or those that are less capable of repair and adaptation are designed with a higher safety factor. Finally, exposure to rare loading events with high potential costs of failure leads to design of structures with very high safety factor compared to everyday loading experience. Understanding bone adaptations at the structural and material levels, which take place over an individual's lifetime or over evolutionary time has numerous applications in translational and clinical research to understand and treat musculoskeletal diseases, as well as to permit the furthering of human extraterrestrial exploration in environments with altered gravity.

Humerus osteology, myology, and finite element structure analysis of Cheloniidae

Adaptation of osteology and myology lead to the formation of hydrofoil foreflippers in Cheloniidae (all recent sea turtles except Dermochelys coriacea) which are used mainly for underwater flight. Recent research shows the biomechanical advantages of a complex system of agonistic and antagonistic tension chords that reduce bending stress in bones. Finite element structure analysis (FESA) of a cheloniid humerus is used to provide a better understanding of morphology and microanatomy and to link these with the main flipper function, underwater flight. Dissection of a Caretta caretta gave insights into lines of action, that is, the course that a muscle takes between its origin and insertion, of foreflipper musculature. Lines of action were determined by spanning physical threads on a skeleton of Chelonia mydas. The right humerus of this skeleton was micro-CT scanned. Based on the scans, a finite element (FE) model was built and muscle force vectors were entered. Muscle forces were iteratively approximated until a uniform compressive stress distribution was attained. Two load cases, downstroke and upstroke, were computed. We found that muscle wrappings (m. coracobrachialis magnus and brevis, several extensors, humeral head of m. triceps) are crucial in addition to axial loading to obtain homogenous compressive loading in all bone cross-sections. Detailed knowledge on muscle disposition leads to compressive stress distribution in the FE model which corresponds with the bone microstructure. The FE analysis of the cheloniid humerus shows that bone may be loaded mainly by compression if the bending moments are minimized.

Functional performance of turtle humerus shape across an ecological adaptive landscape

The concept of the adaptive landscape has been invaluable to evolutionary biologists for visualizing the dynamics of selection and adaptation, and is increasingly being used to study morpho-functional data. Here, we construct adaptive landscapes to explore functional trade-offs associated with variation in humerus morphology among turtles adapted to three different locomotor environments: marine, semiaquatic, and terrestrial. Humerus shape from 40 species of cryptodire turtles was quantified using a pseudolandmark approach. Hypothetical shapes were extracted in a grid across morphospace and four functional traits (strength, stride length, mechanical advantage, and hydrodynamics) measured on those shapes. Quantitative trait modeling was used to construct adaptive landscapes that optimize the functional traits for each of the three locomotor ecologies. Our data show that turtles living in different environments have statistically different humeral shapes. The optimum adaptive landscape for each ecology is defined by a different combination of performance trade-offs, with turtle species clustering around their respective adaptive peak. Further, species adhere to pareto fronts between marine-semiaquatic and semiaquatic-terrestrial optima, but not between marine-terrestrial. Our study demonstrates the utility of adaptive landscapes in informing the link between form, function, and ecological adaptation, and establishes a framework for reconstructing turtle ecological evolution using isolated humeri from the fossil record.

Comparative limb bone scaling in turtles: Phylogenetic transitions with changes in functional demands?

Several terrestrial vertebrate clades include lineages that have evolved nearly exclusive use of aquatic habitats. In many cases, such transitions are associated with the evolution of flattened limbs that are used to swim via dorsoventral flapping. Such changes in shape may have been facilitated by changes in limb bone loading in novel aquatic environments. Studies on limb bone loading in turtles found that torsion is high relative to bending loads on land, but reduced compared to bending during aquatic rowing. Release from torsion among rowers could have facilitated the evolution of hydrodynamically advantageous flattened limbs among aquatic species. Because rowing is regarded as an intermediate locomotor stage between walking and flapping, rowing species might show limb bone flattening intermediate between the tubular shapes of walkers and the flattened shapes of flappers. We collected measurements of humeri and femora from specimens representing four functionally divergent turtle clades: sea turtles (marine flappers), softshells (specialized freshwater rowers), emydids (generalist semiaquatic rowers), and tortoises (terrestrial walkers). Patterns of limb bone scaling with size were compared across lineages using phylogenetic comparative methods. Although rowing taxa did not show the intermediate scaling patterns we predicted, our data provide other functional insights. For example, flattening of sea turtle humeri was associated with positive allometry (relative to body mass) for the limb bone diameter perpendicular to the flexion-extension plane of the elbow. Moreover, softshell limb bones exhibit positive allometry of femoral diameters relative to body mass, potentially helping them maintain their typical benthic position in water by providing additional weight to compensate for shell reduction. Tortoise limb bones showed positive allometry of diameters, as well as long humeri, relative to body mass, potentially reflecting specializations for resisting loads associated with digging. Overall, scaling patterns of many turtle lineages appear to correlate with distinctive behaviors or locomotor habits.

The application of nanogenerators and piezoelectricity in osteogenesis

Bone is a complex organ possessing both physicomechanical and bioelectrochemical properties. In the view of Wolff's Law, bone can respond to mechanical loading and is subsequently reinforced in the areas of stress. Piezoelectricity is one of several mechanical responses of the bone matrix that allows osteocytes, osteoblasts, osteoclasts, and osteoprogenitors to react to changes in their environment. The present review details how osteocytes convert external mechanical stimuli into internal bioelectrical signals and the induction of intercellular cytokines from the standpoint of piezoelectricity. In addition, this review introduces piezoelectric and triboelectric materials used as self-powered electrical generators to promote osteogenic proliferation and differentiation due to their electromechanical properties, which could promote the development of promising applications in tissue engineering and bone regeneration.

Cancellous bone and theropod dinosaur locomotion. Part I-an examination of cancellous bone architecture in the hindlimb bones of theropods

This paper is the first of a three-part series that investigates the architecture of cancellous ('spongy') bone in the main hindlimb bones of theropod dinosaurs, and uses cancellous bone architectural patterns to infer locomotor biomechanics in extinct non-avian species. Cancellous bone is widely known to be highly sensitive to its mechanical environment, and has previously been used to infer locomotor biomechanics in extinct tetrapod vertebrates, especially primates. Despite great promise, cancellous bone architecture has remained little utilized for investigating locomotion in many other extinct vertebrate groups, such as dinosaurs. Documentation and quantification of architectural patterns across a whole bone, and across multiple bones, can provide much information on cancellous bone architectural patterns and variation across species. Additionally, this also lends itself to analysis of the musculoskeletal biomechanical factors involved in a direct, mechanistic fashion. On this premise, computed tomographic and image analysis techniques were used to describe and analyse the three-dimensional architecture of cancellous bone in the main hindlimb bones of theropod dinosaurs for the first time. A comprehensive survey across many extant and extinct species is produced, identifying several patterns of similarity and contrast between groups. For instance, more stemward non-avian theropods (e.g. ceratosaurs and tyrannosaurids) exhibit cancellous bone architectures more comparable to that present in humans, whereas species more closely related to birds (e.g. paravians) exhibit architectural patterns bearing greater similarity to those of extant birds. Many of the observed patterns may be linked to particular aspects of locomotor biomechanics, such as the degree of hip or knee flexion during stance and gait. A further important observation is the abundance of markedly oblique trabeculae in the diaphyses of the femur and tibia of birds, which in large species produces spiralling patterns along the endosteal surface. Not only do these observations provide new insight into theropod anatomy and behaviour, they also provide the foundation for mechanistic testing of locomotor hypotheses via musculoskeletal biomechanical modelling.

Humeral loads during swimming and walking in turtles: implications for morphological change during aquatic reinvasions

During evolutionary reinvasions of water by terrestrial vertebrates, ancestrally tubular limb bones often flatten to form flippers. Differences in skeletal loading between land and water might have facilitated such changes. In turtles, femoral shear strains are significantly lower during swimming than during walking, potentially allowing a release from loads favoring tubular shafts. However, flipper-like morphology in specialized tetrapod swimmers is most accentuated in the forelimbs. To test whether the forelimbs of turtles also experience reduced torsional loading in water, we compared strains on the humerus of river cooters (Pseudemys concinna) between swimming and terrestrial walking. We found that humeral shear strains are also lower during swimming than during terrestrial walking; however, this appears to relate to a reduction in overall strain magnitude, rather than a specific reduction in twisting. These results indicate that shear strains show similar reductions between swimming and walking for forelimb and hindlimb, but these reductions are produced through different mechanisms.

"On the Fence" versus "All in": Insights from Turtles for the Evolution of Aquatic Locomotor Specializations and Habitat Transitions in Tetrapod Vertebrates

Though ultimately descended from terrestrial amniotes, turtles have deep roots as an aquatic lineage and are quite diverse in the extent of their aquatic specializations. Many taxa can be viewed as "on the fence" between aquatic and terrestrial realms, whereas others have independently hyperspecialized and moved "all in" to aquatic habitats. Such differences in specialization are reflected strongly in the locomotor system. We have conducted several studies to evaluate the performance consequences of such variation in design, as well as the mechanisms through which specialization for aquatic locomotion is facilitated in turtles. One path to aquatic hyperspecialization has involved the evolutionary transformation of the forelimbs from rowing, tubular limbs with distal paddles into flapping, flattened flippers, as in sea turtles. Prior to the advent of any hydrodynamic advantages, the evolution of such flippers may have been enabled by a reduction in twisting loads on proximal limb bones that accompanied swimming in rowing ancestors, facilitating a shift from tubular to flattened limbs. Moreover, the control of flapping movements appears related primarily to shifts in the activity of a single forelimb muscle, the deltoid. Despite some performance advantages, flapping may entail a locomotor cost in terms of decreased locomotor stability. However, other morphological specializations among rowing species may enhance swimming stability. For example, among highly aquatic pleurodiran turtles, fusion of the pelvis to the shell appears to dramatically reduce motions of the pelvis compared to freshwater cryptodiran species. This could contribute to advantageous increases in aquatic stability among predominantly aquatic pleurodires. Thus, even within the potential constraints of a body plan in which the body is encased by a shell, turtles exhibit diverse locomotor capacities that have enabled diversification into a wide range of aquatic habitats.

Pelvic girdle mobility of cryptodire and pleurodire turtles during walking and swimming

Movements of the pelvic girdle facilitate terrestrial locomotor performance in a wide range of vertebrates by increasing hind limb excursion and stride length. The extent to which pelvic movements contribute to limb excursion in turtles is unclear because the bony shell surrounding the body presents a major obstacle to their visualization. In the Cryptodira, which are one of the two major lineages of turtles, pelvic anatomy indicates the potential for rotation inside the shell. However, in the Pleurodira, the other major suborder, the pelvis shows a derived fusion to the shell, preventing pelvic motion. In addition, most turtles use their hind limbs for propulsion during swimming as well as walking, and the different locomotor demands between water and land could lead to differences in the contributions of pelvic rotation to limb excursion in each habitat. To test these possibilities, we used X-ray reconstruction of moving morphology (XROMM) to compare pelvic mobility and femoral motion during walking and swimming between representative species of cryptodire (Pseudemys concinna) and pleurodire (Emydura subglobosa) turtles. We found that the pelvis yawed substantially in cryptodires during walking and, to a lesser extent, during swimming. These movements contributed to greater femoral protraction during both walking and swimming in cryptodires when compared with pleurodires. Although factors related to the origin of pelvic-shell fusion in pleurodires are debated, its implications for their locomotor function may contribute to the restriction of this group to primarily aquatic habits.

Limb bone loading in swimming turtles: changes in loading facilitate transitions from tubular to flipper-shaped limbs during aquatic invasions

Members of several terrestrial vertebrate lineages have returned to nearly exclusive use of aquatic habitats. These transitions were often accompanied by changes in skeletal morphology, such as flattening of limb bone shafts. Such morphological changes might be correlated with the exposure of limb bones to altered loading. Though the environmental forces acting on the skeleton differ substantially between water and land, no empirical data exist to quantify the impact of such differences on the skeleton, either in terms of load magnitude or regime. To test how locomotor loads change between water and land, we compared in vivo strains from femora of turtles (Trachemys scripta) during swimming and terrestrial walking. As expected, strain magnitudes were much lower (by 67.9%) during swimming than during walking. However, the loading regime of the femur also changed between environments: torsional strains are high during walking, but torsion is largely eliminated during swimming. Changes in loading regime between environments may have enabled evolutionary shifts to hydrodynamically advantageous flattened limb bones in highly aquatic species. Although circular cross sections are optimal for resisting torsional loads, the removal of torsion would reduce the advantage of tubular shapes, facilitating the evolution of flattened limbs.

Experimental tests of planar strain theory for predicting bone cross-sectional longitudinal and shear strains

Understanding the diversity of skeletal loading regimes in vertebrate long bones during locomotion has been significantly enhanced by the application of planar strain theory (PST) to in vivo bone strain data. PST is used to model the distribution of longitudinal strains normal to the bone's transverse cross-section and the location of the neutral axis of bending. To our knowledge, the application of this theory to skeletal biomechanics has not been experimentally validated. We evaluated the accuracy of PST using strain measurements from emu tibiotarsi instrumented with four strain gauges and loaded in ex vivo four-point bending. Using measured strains from three-gauge combinations, PST was applied to predict strain values at a fourth gauge's location. Experimentally measured and predicted strain values correlated linearly with a slope near 1.0, suggesting that PST accurately predicts longitudinal strains. Additionally, we assessed the use of PST to extrapolate shear strains to locations on a bone not instrumented with rosette strain gauges. Guineafowl TBTs were instrumented with rosette strain gauges and in vivo longitudinal and shear strains were measured during treadmill running. Individual-specific and sample-mean ratios between measured longitudinal strains from the medial and posterior bone surfaces were used to extrapolate posterior-site shear strain from shear strains measured on the medial surface. Measured and predicted shear strains at the posterior gauge site using either ratio showed trends for a positive correlation between measured and predicted strains, but the correlation did not equal 1.0 and had a non-zero intercept, suggesting that the use of PST should be carefully considered in the context of the goals of the study and the desired precision for the predicted shear strains.

HOW DO BONE CELLS SENSE MECHANICAL LOADING?

Influenced by gravidity, bone tissue experiences stronger or lighter deformation according to the strength of the activities of daily life. Activities resulting in impact are particularly known to stimulate osteogenesis, thus reducing bone mass loss. Knowing how bone cells recognize the mechanical deformation imposed to the bone and trigger a series of biochemical chain reactions is of crucial importance for the development of therapeutic and preventive practices in orthopaedic activity. There is still a long way to run until we can understand the whole process, but current knowledge has shown a strong progression, with researches being conducted focused on therapies. For a mechanical sign to be transformed into a biological one (mechanotransduction), it must be amplified at cell level by the histological structure of bone tissue, producing tensions in cell membrane proteins (integrins) and changing their spatial structure. Such change activates bindings between these and the cytoskeleton, producing focal adhesions, where cytoplasmatic proteins are recruited to enable easier biochemical reactions. Focal adhesion kinase (FAK) is the most important one being self-activated when its structure is changed by integrins. Activated FAK triggers a cascade of reactions, resulting in the activation of ERK-1/2 and Akt, which are proteins that, together with FAK, regulate the production of bone mass. Osteocytes are believed to be the mechanosensor cells of the bone and to transmit the mechanical deformation to osteoblasts and osteoclasts. Ionic channels and gap junctions are considered as intercellular communication means for biochemical transmission of a mechanical stimulus. These events occur continuously on bone tissue and regulate bone remodeling.

Bone strain magnitude is correlated with bone strain rate in tetrapods: implications for models of mechanotransduction

Hypotheses suggest that structural integrity of vertebrate bones is maintained by controlling bone strain magnitude via adaptive modelling in response to mechanical stimuli. Increased tissue-level strain magnitude and rate have both been identified as potent stimuli leading to increased bone formation. Mechanotransduction models hypothesize that osteocytes sense bone deformation by detecting fluid flow-induced drag in the bone's lacunar-canalicular porosity. This model suggests that the osteocyte's intracellular response depends on fluid-flow rate, a product of bone strain rate and gradient, but does not provide a mechanism for detection of strain magnitude. Such a mechanism is necessary for bone modelling to adapt to loads, because strain magnitude is an important determinant of skeletal fracture. Using strain gauge data from the limb bones of amphibians, reptiles, birds and mammals, we identified strong correlations between strain rate and magnitude across clades employing diverse locomotor styles and degrees of rhythmicity. The breadth of our sample suggests that this pattern is likely to be a common feature of tetrapod bone loading. Moreover, finding that bone strain magnitude is encoded in strain rate at the tissue level is consistent with the hypothesis that it might be encoded in fluid-flow rate at the cellular level, facilitating bone adaptation via mechanotransduction.

In vivo strains in the femur of the nine-banded armadillo (Dasypus novemcinctus)

The capacity of limb bones to resist the locomotor loads they encounter depends on both the pattern of those loads and the material properties of the skeletal elements. Among mammals, understanding of the interplay between these two factors has been based primarily on evidence from locomotor behaviors in upright placentals, which show limb bones that are loaded predominantly in anteroposterior bending with minimal amounts of torsion. However, loading patterns from the femora of opossums, marsupials using crouched limb posture, show appreciable torsion while the bone experiences mediolateral (ML) bending. These data indicated greater loading diversity in mammals than was previously recognized, and suggested the possibility that ancestral loading patterns found in sprawling lineages (e.g., reptilian sauropsids) might have been retained among basal mammals. To further test this hypothesis, we recorded in vivo locomotor strains from the femur of the nine-banded armadillo (Dasypus novemcinctus), a member of the basal xenarthran clade of placental mammals that also uses crouched limb posture. Orientations of principal strains and magnitudes of shear strains indicate that armadillo femora are exposed to only limited torsion; however, bending is essentially ML, placing the medial aspect of the femur in compression and the lateral aspect in tension. This orientation of bending is similar to that found in opossums, but planar strain analyses indicate much more of the armadillo femur experiences tension during bending, potentially due to muscles pulling on the large, laterally positioned third trochanter. Limb bone safety factors were estimated between 3.3 and 4.3 in bending, similar to other placental mammals, but lower than opossums and most sprawling taxa. Thus, femoral loading patterns in armadillos show a mixture of similarities to both opossums (ML bending) and other placentals (limited torsion and low safety factors), along with unique features (high axial tension) that likely relate to their distinctive hindlimb anatomy.

Performance and scaling of a novel locomotor structure: adhesive capacity of climbing gobiid fishes

Many species of gobiid fishes adhere to surfaces using a sucker formed from fusion of the pelvic fins. Juveniles of many amphidromous species use this pelvic sucker to scale waterfalls during migrations to upstream habitats after an oceanic larval phase. However, adults may still use suckers to re-scale waterfalls if displaced. If attachment force is proportional to sucker area and if growth of the sucker is isometric, then increases in the forces that climbing fish must resist might outpace adhesive capacity, causing climbing performance to decline through ontogeny. To test for such trends, we measured pressure differentials and adhesive suction forces generated by the pelvic sucker across wide size ranges in six goby species, including climbing and non-climbing taxa. Suction was achieved via two distinct growth strategies: (1) small suckers with isometric (or negatively allometric) scaling among climbing gobies and (2) large suckers with positively allometric growth in non-climbing gobies. Species using the first strategy show a high baseline of adhesive capacity that may aid climbing performance throughout ontogeny, with pressure differentials and suction forces much greater than expected if adhesion were a passive function of sucker area. In contrast, large suckers possessed by non-climbing species may help compensate for reduced pressure differentials, thereby producing suction sufficient to support body weight. Climbing Sicyopterus species also use oral suckers during climbing waterfalls, and these exhibited scaling patterns similar to those for pelvic suckers. However, oral suction force was considerably lower than that for pelvic suckers, reducing the ability for these fish to attach to substrates by the oral sucker alone.

A universal scaling relationship between body mass and proximal limb bone dimensions in quadrupedal terrestrial tetrapods

BACKGROUND

Body size is intimately related to the physiology and ecology of an organism. Therefore, accurate and consistent body mass estimates are essential for inferring numerous aspects of paleobiology in extinct taxa, and investigating large-scale evolutionary and ecological patterns in the history of life. Scaling relationships between skeletal measurements and body mass in birds and mammals are commonly used to predict body mass in extinct members of these crown clades, but the applicability of these models for predicting mass in more distantly related stem taxa, such as non-avian dinosaurs and non-mammalian synapsids, has been criticized on biomechanical grounds. Here we test the major criticisms of scaling methods for estimating body mass using an extensive dataset of mammalian and non-avian reptilian species derived from individual skeletons with live weights.

RESULTS

Significant differences in the limb scaling of mammals and reptiles are noted in comparisons of limb proportions and limb length to body mass. Remarkably, however, the relationship between proximal (stylopodial) limb bone circumference and body mass is highly conserved in extant terrestrial mammals and reptiles, in spite of their disparate limb postures, gaits, and phylogenetic histories. As a result, we are able to conclusively reject the main criticisms of scaling methods that question the applicability of a universal scaling equation for estimating body mass in distantly related taxa.

CONCLUSIONS

The conserved nature of the relationship between stylopodial circumference and body mass suggests that the minimum diaphyseal circumference of the major weight-bearing bones is only weakly influenced by the varied forces exerted on the limbs (that is, compression or torsion) and most strongly related to the mass of the animal. Our results, therefore, provide a much-needed, robust, phylogenetically corrected framework for accurate and consistent estimation of body mass in extinct terrestrial quadrupeds, which is important for a wide range of paleobiological studies (including growth rates, metabolism, and energetics) and meta-analyses of body size evolution.

Evolution of limb bone loading and body size in varanid lizards

Geometric scaling predicts that stresses on limb bones and muscles should increase with body size. Mammals counter this size-related increase in stress partially through changes in bone geometry, but largely through changes in posture, with larger species having a more erect stance. However, the ability to counter size-related stresses in this fashion may be limited to those taxa that have a parasagittal gait (such as mammals), where legs are swung underneath the body. We examined locomotor kinematics for 11 species of varanid lizards (from 0.04 to 8 kg body mass) that have a sprawling gait, to determine how they moderate size-related stresses. Posture, as indicated by femur adduction and hip heights, did not change significantly with body size, beyond that expected from geometrical scaling. Instead, lizards mitigated size-related increases in stress by increasing duty factor and possibly reducing femur rotation. Incorporating these factors in biomechanical models predicted that both bending (∝M(0.016), where M is mass) and torsional (∝M(-0.049)) stresses should be nearly independent of body size over the size range examined. However, increasing duty factor and reducing femur rotation probably have deleterious effects on speed, and this difference in kinematics with size may explain why speed scales lower for sprawling lizards than for parasagittal mammals (∝M(0.17) and ∝M(0.24), respectively). Further, paralleling conclusions for the synapsid lineage, these findings suggest that evolution from sprawling to upright posture did not occur in archosaurs as a response to larger size; rather, these archosaurs likely became upright first and larger later.

Locomotor loading mechanics in the hindlimbs of tegu lizards (Tupinambis merianae): comparative and evolutionary implications

Skeletal elements are usually able to withstand several times their usual load before they yield, and this ratio is known as the bone's safety factor. Limited studies on amphibians and non-avian reptiles have shown that they have much higher limb bone safety factors than birds and mammals. It has been hypothesized that this difference is related to the difference in posture between upright birds and mammals and sprawling ectotherms; however, limb bone loading data from a wider range of sprawling species are needed in order to determine whether the higher safety factors seen in amphibians and non-avian reptiles are ancestral or derived conditions. Tegus (family Teiidae) are an ideal lineage with which to expand sampling of limb bone loading mechanics for sprawling taxa, particularly for lizards, because they are from a different clade than previously sampled iguanas and exhibit different foraging and locomotor habits (actively foraging carnivore versus burst-activity herbivore). We evaluated the mechanics of locomotor loading for the femur of the Argentine black and white tegu (Tupinambus merianae) using three-dimensional measurements of the ground reaction force and hindlimb kinematics, in vivo bone strains and femoral mechanical properties. Peak bending stresses experienced by the femur were low (tensile: 10.4 ± 1.1 MPa; compressive: -17.4 ± 0.9 MPa) and comparable to those in other reptiles, with moderate shear stresses and strains also present. Analyses of peak femoral stresses and strains led to estimated safety factor ranges of 8.8-18.6 in bending and 7.8-17.5 in torsion, both substantially higher than typical for birds and mammals but similar to other sprawling tetrapods. These results broaden the range of reptilian and amphibian taxa in which high femoral safety factors have been evaluated and further indicate a trend for the independent evolution of lower limb bone safety factors in endothermic taxa.

Loading mechanics of the femur in tiger salamanders (Ambystoma tigrinum) during terrestrial locomotion

Salamanders are often used as representatives of the basal tetrapod body plan in functional studies, but little is known about the loads experienced by their limb bones during locomotion. Although salamanders' slow walking speeds might lead to low locomotor forces and limb bone stresses similar to those of non-avian reptiles, their highly sprawled posture combined with relatively small limb bones could produce elevated limb bone stresses closer to those of avian and mammalian species. This study evaluates the loads on the femur of the tiger salamander (Ambystoma tigrinum) during terrestrial locomotion using three-dimensional measurements of the ground reaction force (GRF) and hindlimb kinematics, as well as anatomical measurements of the femur and hindlimb muscles. At peak stress (29.8 ± 2.0% stance), the net GRF magnitude averaged 0.42 body weights and was directed nearly vertically for the middle 20-40% of the contact interval, essentially perpendicular to the femur. Although torsional shear stresses were significant (4.1 ± 0.3 MPa), bending stresses experienced by the femur were low compared with other vertebrate lineages (tensile: 14.9 ± 0.8 MPa; compressive: -18.9 ± 1.0 MPa), and mechanical property tests indicated yield strengths that were fairly standard for tetrapods (157.1 ± 3.7 MPa). Femoral bending safety factors (10.5) were considerably higher than values typical for birds and mammals, and closer to the elevated values calculated for reptilian species. These results suggest that high limb bone safety factors may have an ancient evolutionary history, though the underlying cause of high safety factors (e.g. low limb bone loads, high bone strength or a combination of the two) may vary among lineages.

In vivo strains in the femur of the Virginia opossum (Didelphis virginiana) during terrestrial locomotion: testing hypotheses of evolutionary shifts in mammalian bone loading and design

Terrestrial locomotion can impose substantial loads on vertebrate limbs. Previous studies have shown that limb bones from cursorial species of eutherian mammals experience high bending loads with minimal torsion, whereas the limb bones of non-avian reptiles (and amphibians) exhibit considerable torsion in addition to bending. It has been hypothesized that these differences in loading regime are related to the difference in limb posture between upright mammals and sprawling reptiles, and that the loading patterns observed in non-avian reptiles may be ancestral for tetrapod vertebrates. To evaluate whether non-cursorial mammals show loading patterns more similar to those of sprawling lineages, we measured in vivo strains in the femur during terrestrial locomotion of the Virginia opossum (Didelphis virginiana), a marsupial that uses more crouched limb posture than most mammals from which bone strains have been recorded, and which belongs to a clade phylogenetically between reptiles and the eutherian mammals studied previously. The presence of substantial torsion in the femur of opossums, similar to non-avian reptiles, would suggest that this loading regime likely reflects an ancestral condition for tetrapod limb bone design. Strain recordings indicate the presence of both bending and appreciable torsion (shear strain: 419.1 ± 212.8 με) in the opossum femur, with planar strain analyses showing neutral axis orientations that placed the lateral aspect of the femur in tension at the time of peak strains. Such mediolateral bending was unexpected for a mammal running with near-parasagittal limb kinematics. Shear strains were similar in magnitude to peak compressive axial strains, with opossum femora experiencing similar bending loads but higher levels of torsion compared with most previously studied mammals. Analyses of peak femoral strains led to estimated safety factor ranges of 5.1-7.2 in bending and 5.5-7.3 in torsion, somewhat higher than typical mammalian values for bending, but approaching typical reptilian values for shear. Loading patterns of opossum limb bones therefore appear intermediate in some respects between those of eutherian mammals and non-avian reptiles, providing further support for hypotheses that high torsion and elevated limb bone safety factors may represent persistent ancestral conditions in the evolution of tetrapod limb bone loading and design.

Femoral loading mechanics in the Virginia opossum, Didelphis virginiana: torsion and mediolateral bending in mammalian locomotion

Studies of limb bone loading in terrestrial mammals have typically found anteroposterior bending to be the primary loading regime, with torsion contributing minimally. However, previous studies have focused on large, cursorial eutherian species in which the limbs are held essentially upright. Recent in vivo strain data from the Virginia opossum (Didelphis virginiana), a marsupial that uses a crouched rather than an upright limb posture, have indicated that its femur experiences appreciable torsion during locomotion as well as strong mediolateral bending. The elevated femoral torsion and strong mediolateral bending observed in D. virginiana might result from external forces such as a medial inclination of the ground reaction force (GRF), internal forces deriving from a crouched limb posture, or a combination of these factors. To evaluate the mechanism underlying the loading regime of opossum femora, we filmed D. virginiana running over a force platform, allowing us to measure the magnitude of the GRF and its three-dimensional orientation relative to the limb, facilitating estimates of limb bone stresses. This three-dimensional analysis also allows evaluations of muscular forces, particularly those of hip adductor muscles, in the appropriate anatomical plane to a greater degree than previous two-dimensional analyses. At peak GRF and stress magnitudes, the GRF is oriented nearly vertically, inducing a strong abductor moment at the hip that is countered by adductor muscles on the medial aspect of the femur that place this surface in compression and induce mediolateral bending, corroborating and explaining loading patterns that were identified in strain analyses. The crouched orientation of the femur during stance in opossums also contributes to levels of femoral torsion as high as those seen in many reptilian taxa. Femoral safety factors were as high as those of non-avian reptiles and greater than those of upright, cursorial mammals, primarily because the load magnitudes experienced by opossums are lower than those of most mammals. Thus, the evolutionary transition from crouched to upright posture in mammalian ancestors may have been accompanied by an increase in limb bone load magnitudes.

Forelimb kinematics and motor patterns of the slider turtle (Trachemys scripta) during swimming and walking: shared and novel strategies for meeting locomotor demands of water and land

Turtles use their limbs during both aquatic and terrestrial locomotion, but water and land impose dramatically different physical requirements. How must musculoskeletal function be adjusted to produce locomotion through such physically disparate habitats? We addressed this question by quantifying forelimb kinematics and muscle activity during aquatic and terrestrial locomotion in a generalized freshwater turtle, the red-eared slider (Trachemys scripta), using digital high-speed video and electromyography (EMG). Comparisons of our forelimb data to previously collected data from the slider hindlimb allow us to test whether limb muscles with similar functional roles show qualitatively similar modulations of activity across habitats. The different functional demands of water and air lead to a prediction that muscle activity for limb protractors (e.g. latissimus dorsi and deltoid for the forelimb) should be greater during swimming than during walking, and activity in retractors (e.g. coracobrachialis and pectoralis for the forelimb) should be greater during walking than during swimming. Differences between aquatic and terrestrial forelimb movements are reflected in temporal modulation of muscle activity bursts between environments, and in some cases the number of EMG bursts as well. Although patterns of modulation between water and land are similar between the fore- and hindlimb in T. scripta for propulsive phase muscles (retractors), we did not find support for the predicted pattern of intensity modulation, suggesting that the functional demands of the locomotor medium alone do not dictate differences in intensity of muscle activity across habitats.

Limb-bone histology of temnospondyls: implications for understanding the diversification of palaeoecologies and patterns of locomotion of Permo-Triassic tetrapods

The locomotion of early tetrapods has long been a subject of great interest in the evolutionary history of vertebrates. However, we still do not have a precise understanding of the evolutionary radiation of their locomotory strategies. We present here the first palaeohistological study based on theoretical biomechanical considerations among a highly diversified group of early tetrapods, the temnospondyls. Based on the quantification of microanatomical and histological parameters in the humerus and femur of nine genera, this multivariate analysis provides new insights concerning the adaptations of temnospondyls to their palaeoenvironments during the Early Permian, and clearly after the Permo-Triassic crisis. This study therefore presents a methodology that, if based on a bigger sample, could contribute towards a characterization of the behaviour of species during great evolutionary events.

In vivo tibial stiffness is maintained by whole bone morphology and cross-sectional geometry in growing female mice

Whole bone morphology, cortical geometry, and tissue material properties modulate skeletal stresses and strains that in turn influence skeletal physiology and remodeling. Understanding how bone stiffness, the relationship between applied load and tissue strain, is regulated by developmental changes in bone structure and tissue material properties is important in implementing biophysical strategies for promoting healthy bone growth and preventing bone loss. The goal of this study was to relate developmental patterns of in vivo whole bone stiffness to whole bone morphology, cross-sectional geometry, and tissue properties using a mouse axial loading model. We measured in vivo tibial stiffness in three age groups (6, 10, 16 wk old) of female C57Bl/6 mice during cyclic tibial compression. Tibial stiffness was then related to cortical geometry, longitudinal bone curvature, and tissue mineral density using microcomputed tomography (microCT). Tibial stiffness and the stresses induced by axial compression were generally maintained from 6 to 16 wks of age. Growth-related increases in cortical cross-sectional geometry and longitudinal bone curvature had counteracting effects on induced bone stresses and, therefore, maintained tibial stiffness similarly with growth. Tissue mineral density increased slightly from 6 to 16 wks of age, and although the effects of this increase on tibial stiffness were not directly measured, its role in the modulation of whole bone stiffness was likely minor over the age range examined. Thus, whole bone morphology, as characterized by longitudinal curvature, along with cortical geometry, plays an important role in modulating bone stiffness during development and should be considered when evaluating and designing in vivo loading studies and biophysical skeletal therapies.

Allometry of masticatory loading parameters in mammals

Considerable research on the scaling of loading patterns in mammalian locomotor systems has not been accompanied by a similarly comprehensive analysis of the interspecific scaling of loading regimes in the mammalian masticatory complex. To address this deficiency, we analyzed mandibular corpus bone strain in 11 mammalian taxa varying in body size by over 2.5 orders of magnitude, including goats, horses, alpacas, pigs, and seven primate taxa. During alert chewing and biting of hard/tough foods, bone-strain data were collected with rosette gauges placed along the lateral aspect of the mandibular corpus below the molars or premolars. Bone-strain data were used to characterize relevant masticatory loading parameters: peak loading magnitudes, chewing cycle duration, chewing frequency, occlusal duty factor, loading rate, and loading time. Interspecific analyses indicate that much as observed in limb elements, corpus peak-strain magnitudes are similar across mammals of disparate body sizes. Chewing frequency is inversely correlated with body size, much as with locomotor stride frequency. Some of this allometric variation in chewing frequency appears to be due to a negative correlation with loading time, which increases with body size. Similar to the locomotor apparatus, occlusal duty factor, or the duration of the chewing cycle during which the corpus is loaded, does not vary with body size. Peak principal-strain magnitudes are most strongly positively correlated with loading rate and only secondarily with loading, with this complex relationship best described by a multiple regression equation with an interaction term between loading rate and loading time. In addition to informing interpretations of craniomandibular growth, form, function, and allometry, these comparisons provide a skeleton-wide perspective on the patterning of osteogenic stimuli across body sizes.

Mechanical properties of the hindlimb bones of bullfrogs and cane toads in bending and torsion

When compared with most vertebrates, frogs use a novel style of jumping locomotion powered by the hindlimbs. Hindlimb bones of frogs must withstand the potentially erratic loads associated with such saltatory locomotion. To evaluate the load bearing capacity of anuran limb bones, we used three-point bending, torsion, and hardness tests to measure the mechanical properties of the femur and tibiofibula from adults of two species that use different jumping styles: explosively jumping bullfrogs (Rana (Lithobates) catesbeiana) and cyclically hopping cane toads (Bufo (Chaunus) marinus). Yield stress and strain values for R. catesbeiana and B. marinus hindlimb bones are within the range of values previously reported for other vertebrates. However, anuran hindlimb bones generally stand out as having higher yield stresses in bending than those of closely related, nonsaltatory salamanders, highlighting the importance of considering phylogenetic context in comparisons of bone functional capacity and adaptation. Stiffness values for both frog species tested were also high, which may facilitate efficient transmission of muscular forces while jumping. Elevated stiffness may also contribute to some discrepancies between determinations of bone properties via hardness versus bending tests. In comparisons between species, B. marinus bones showed significantly higher bending yield stresses than R. catesbeiana, whereas R. catesbeiana bones showed significantly higher torsional yield stresses than B. marinus. These differences may correlate with differences in jumping style and limb anatomy between ranid and bufonid frogs, suggesting that evolutionary changes in bone mechanical properties may help to accommodate new functional demands that emerge in lineages.