Biomechanical Trade-offs Research Articles

Evolution and functional implications of stinger shape in ants.

Trait diversification is often driven by underlying performance tradeoffs in the context of different selective pressures. Evolutionary changes in task specialization may influence how species respond to tradeoffs and alter diversification. We conducted this study to investigate the functional morphology, evolutionary history, and tempo and mode of evolution of the Hymenoptera stinger using Ectatomminae ants as a model clade. We hypothesized that a performance tradeoff surface underlies the diversity of stinger morphology and that shifts between predatory and omnivorous diets mediate the diversification dynamics of the trait. Shape variation was characterized by X-ray microtomography, and the correlation between shape and average values of von Mises stress, as a measure of yield failure criteria under loading conditions typical of puncture scenarios, was determined using finite element analysis. We observed that stinger elongation underlies most of the shape variation but found no evidence of biomechanical tradeoffs in the performance characteristics measured. Additionally, omnivores have increased phenotypic shifts and accelerated evolution in performance metrics, suggesting the evolution of dietary flexibility releases selection pressure on a specific function, resulting in a greater phenotypic evolutionary rate. These results increase our understanding of the biomechanical basis of stinger shape, indicate that shape diversity is not the outcome of simple biomechanical optimization, and reveal connections between diet and trait diversification.

Biomineralization and biomechanical trade-offs under heterogeneous environments in the eastern oyster Crassostrea virginica

ABSTRACT Accurate biological models are critical to reliably predict vulnerability of marine organisms and ecosystems to rapid environmental changes. Current predictions on the biological impacts of climate change and human-caused disturbances primarily stem from controlled experiments but lack assessments of the mechanisms underlying biotic variations in natural systems, especially for habitat-forming, climate-sensitive species with key ecological roles. This study aimed to characterize and quantify spatial patterns of shell biomineralization and biomechanical properties in a key reef-building oyster, Crassostrea virginica, collected from restored reefs along natural estuarine gradients in the Hudson River Estuary (NY, USA). We characterized patterns of oyster shell deposition, structure, composition and mechanical performance at the macro- and microscale. Eastern oysters show a strong capacity for adjustments in shell biomineralization and biomechanics to maintain shell production and protective functions. We reveal salinity as a key predictor of oyster shell structure, mechanical integrity and resistance to dissolution, and describe the functional role of chalky calcite in shaping shell mechanical performance. Changes in shell production along salinity gradients indicate formation of shells with (1) high mechanical resistance but increased vulnerability to dissolution under marine conditions and (2) lower structural integrity but higher protection from dissolution under brackish conditions. Our work illustrates that biomineralization and biomechanical trade-offs may act as mechanisms in eastern oysters to maintain overall performance under heterogeneous estuarine environments and could represent a cornerstone for calcifying organisms to acclimate and maintain their ecological functions in a rapidly changing climate.

Biomechanics of biting in loggerhead shrikes: jaw-closing force, velocity and an argument for power.

Differences in the physical and behavioral attributes of prey are likely to impose disparate demands of force and speed on the jaws of a predator. Because of biomechanical trade-offs between force and speed, this presents an interesting conundrum for predators of diverse prey types. Loggerhead shrikes (Lanius ludovicianus) are medium-sized (∼50 g) passeriform birds that dispatch and feed on a variety of arthropod and vertebrate prey, primarily using their beaks. We used high-speed video of shrikes biting a force transducer in lateral view to obtain corresponding measurements of bite force, upper and lower bill linear and angular displacements, and velocities. Our results show that upper bill depression (about the craniofacial hinge) is more highly correlated with bite force, whereas lower bill elevation is more highly correlated with jaw-closing velocity. These results suggest that the upper and lower jaws might play different roles for generating force and speed (respectively) in these and perhaps other birds as well. We hypothesize that a division of labor between the jaws may allow shrikes to capitalize on elements of force and speed without compromising performance. As expected on theoretical grounds, bite force trades-off against jaw-closing velocity during the act of biting, although peak bite force and jaw-closing velocity across individual shrikes show no clear signs of a force-velocity trade-off. As a result, shrikes appear to bite with jaw-closing velocities and forces that maximize biting power, which may be selectively advantageous for predators of diverse prey that require both jaw-closing force and speed.

Ontogenetic transitions, biomechanical trade-offs and macroevolution of scyphozoan medusae swimming patterns

Ephyrae, the early stages of scyphozoan jellyfish, possess a conserved morphology among species. However, ontogenetic transitions lead to morphologically different shapes among scyphozoan lineages, with important consequences for swimming biomechanics, bioenergetics and ecology. We used high-speed imaging to analyse biomechanical and kinematic variables of swimming in 17 species of Scyphozoa (1 Coronatae, 8 “Semaeostomeae” and 8 Rhizostomeae) at different developmental stages. Swimming kinematics of early ephyrae were similar, in general, but differences related to major lineages emerged through development. Rhizostomeae medusae have more prolate bells, shorter pulse cycles and higher swimming performances. Medusae of “Semaeostomeae”, in turn, have more variable bell shapes and most species had lower swimming performances. Despite these differences, both groups travelled the same distance per pulse suggesting that each pulse is hydrodynamically similar. Therefore, higher swimming velocities are achieved in species with higher pulsation frequencies. Our results suggest that medusae of Rhizostomeae and “Semaeostomeae” have evolved bell kinematics with different optimized traits, rhizostomes optimize rapid fluid processing, through faster pulsations, while “semaeostomes” optimize swimming efficiency, through longer interpulse intervals that enhance mechanisms of passive energy recapture.

Biomechanical Tradeoffs in Foot Function from Variations in Shoe Design.

There is debate and confusion over how to evaluate the biomechanical effects of running shoe design. Here we use an evolutionary perspective to analyze how key design features of running shoes alter the evolved biomechanics of the foot, creating a range of tradeoffs in force production and transmission that may affect performance and vulnerability to injury.

Bill shape imposes biomechanical tradeoffs in cavity-excavating birds.

Organisms are subject to physical forces that influence morphological evolution. Birds use their bills as implements to perform various functions, each exerting unique physical demands. When excavating cavities, bird bills must resist a range of mechanical stresses to prevent fracture. However, the contribution of bill geometry and material composition to excavation stress resistance remains poorly understood. Here, we study the biomechanical consequences of bill diversification in the cavity-excavating palaeotropical barbets. Using finite-element models and beam theory, we compare excavation performance for two loading regimes experienced by barbet bills during cavity excavation: impact and torsion. We find that deeper and wider maxillae perform better for impact loads than for torsional loads, with the converse for narrower maxillae. This results in tradeoffs between impact and torsion resistance imposed by bill geometry. Analytical beam models validate this prediction, showing that this relationship holds even when maxillae are simplified to solid elliptical beams. Finally, we find that composite bill structures broadly exhibit lower stresses than homogeneous structures of the same geometry, indicating a functional synergy between the keratinous rhamphotheca and bony layers of the bill. Overall, our findings demonstrate the strong link between morphological evolution, behaviour and functional performance in organisms.

Functional Trade-Offs Asymmetrically Promote Phenotypic Evolution.

Trade-offs are thought to bias evolution and are core features of many anatomical systems. Therefore, trade-offs may have far-reaching macroevolutionary consequences, including patterns of morphological, functional, and ecological diversity. Jaws, like many complex anatomical systems, are comprised of elements involved in biomechanical trade-offs. We test the impact of a core mechanical trade-off, the transmission of velocity versus force (i.e., mechanical advantage), on rates of jaw evolution in Neotropical cichlids. Across 130 species representing a wide array of feeding ecologies, we find that the velocity-force trade-off impacts the evolution of the surrounding jaw system. Specifically, rates of jaw evolution are faster at functional extremes than in more functionally intermediate or unspecialized jaws. Yet, surprisingly, the effect on jaw evolution is uneven across the extremes of the velocity-force continuum. Rates of jaw evolution are 4- to 10-fold faster in velocity-modified jaws, whereas force-modified jaws are 7- to 18-fold faster, compared to unspecialized jaws, depending on the extent of specialization. Further, we find that a more extreme mechanical trade-off resulted in faster rates of jaw evolution. The velocity-force trade-off reflects a gradient from specialization on capture-intensive (e.g., evasive or buried) to processing-intensive prey (e.g., attached or shelled), respectively. The velocity extreme of the trade-off is characterized by large magnitudes of trait change leading to functionally divergent specialists and ecological stasis. By contrast, the force extreme of the trade-off is characterized by enhanced ecological lability made possible by phenotypes more readily co-opted for different feeding ecologies. This asymmetry of macroevolutionary outcomes along each extreme is likely the result of an enhanced utility of the pharyngeal jaw system as force-modified oral jaws are adapted for prey that requires intensive processing (e.g., algae, detritus, and mollusks). The velocity-force trade-off, a fundamental feature of many anatomical systems, promotes rapid phenotypic evolution of the surrounding jaw system in a canonical continental adaptive radiation. Considering that the velocity-force trade-off is an inherent feature of all jaw systems that involve a lower element that rotates at a joint, spanning the vast majority of vertebrates, our results may be widely applicable across the tree of life. [Adaptive radiation; constraint; decoupling; jaws; macroevolution; specialization.].

Identifying the best strategy for soccer penalty success: A predictive model for optimising behavioural and biomechanical trade-offs

Success in a soccer penalty can be the difference between winning and losing matches. The outcome is determined by a complex interaction between the shooter and goalkeeper, whose performances are constrained by biomechanical trade-offs. To overcome these performance constraints, each player has a range of available strategies. Shooters can kick at different speeds, affecting accuracy, while goalkeepers can move at various times (leave-times), affecting the time available to move and the probability they move in the correct direction. Previous models of penalty success ignore such trade-offs and how they interact to influence the outcome. Here, we present a model that accounts for shooting inaccuracy to predict the probability of success for all shooting strategies, defined as any combination of: shot speed, position where the shooter aims, shooter footedness, and kicking technique (side-foot or instep). To estimate the probability of success each shooting strategy is matched against all possible goalkeeper leave-times, considering the probability each leave-time is chosen. We test the model against an average goalkeeper and a goalkeeper who tends to move later. Against the average goalkeeper, aiming on the ground toward the centre of the goal is optimal; however, against a late moving goalkeeper, aiming on the ground to the extremities of the goal is effective, with the optimal target in the horizontal dimension dependent on shot speed, kick technique, and footedness. Coaches could use this model to identify their best penalty takers and each players’ optimal shooting strategy against either the average goalkeeper or a specific goalkeeper.

Differences in paretic lower limb loading and fluidity in sit-to-walk according to selection of the leading limb in individuals with stroke

BackgroundSit-to-walk is an asymmetric task that is challenging for individuals with stroke, and paretic limb loading at seat-off and movement fluidity may change according to whether the non-paretic or paretic leg is used as the leading limb. This study aimed to investigate differences in paretic limb loading and fluidity depending on whether the non-paretic limb or paretic limb was used as the leading limb. MethodsThirty-eight individuals with stroke performed sit-to-walk with each leg as the leading limb, and their movements were measured using a 3D motion analysis system. The paired t-test or Wilcoxon signed-rank test was used to assess differences according to limb selection in paretic limb loading ratio at seat-off and fluidity (Fluidity Index: ratio of the lowest to peak forward velocity before first initial contact). FindingsTwenty-two of 38 participants preferred to use the paretic limb as the leading limb. When leading with the paretic limb, the paretic limb loading ratio was significantly larger (p = 0.002), and the Fluidity Index was lower (p = 0.007). InterpretationSit-to-walk with the paretic leading limb seems to be an adaptive movement because many participants preferred leading with the paretic limb. However, selection of the leading limb in sit-to-walk involves a biomechanical tradeoff between paretic limb loading at seat-off and movement fluidity in individuals with stroke. Use of the paretic leading limb requires loading capacity of this limb, and the non-paretic leading limb must have high balance ability to merge sit-to-stand and gait initiation.

Development of a framework to assess the biomechanical impact of reverse shoulder arthroplasty placement modifications.

Reverse shoulder arthroplasty biomechanics can be improved by modifying the placement of prosthesis. Biomechanical studies have quantified the impact of placement modifications on the mobility and stability of the reverse shoulder. While these studies have provided detailed insights, direct comparisons between their finding are obfuscated by their use of differing methodologies. The aim of our study was to develop an assessment framework which used musculoskeletal simulations to consistently evaluate the biomechanics of various placement modifications. We conducted musculoskeletal simulations of humeral elevations and rotations using 15 reverse shoulder models. For each model, these simulations were conducted for a reference configuration of the prosthesis, established using surgical guidelines, and 34 modified configurations, which were based on commonplace adaptations to the placement of the glenosphere and humeral tray. The effect of each modified configuration on deltoid elongation, deltoid moment arm (DMA), joint stability, and impingement-free range of motion (IFROM) was determined relative to the reference configuration. We found that 16 of the 34 modified placements had an overall beneficial impact on reverse shoulder biomechanics. Within this subset, we identified two biomechanical trade-offs. First, there is an antagonistic relationship between IFROM and both the DMA and joint stability. Second, functional requirements differ between humeral elevations and rotations. Furthermore, we found that posteromedial translation of the humeral tray had the most beneficial impact on joint stability and inferior translation of the glenosphere had the most beneficial impact on IFROM and DMA.

Neural and biomechanical tradeoffs associated with human-exoskeleton interactions

Industrial passive low-back exoskeletons have gained recent attention as ergonomic interventions to manual handling tasks. This research utilized a two-armed experimental approach (single vs dual-task paradigms) to quantify neural and biomechanical tradeoffs associated with short-term human-exoskeleton interaction (HEI) during asymmetrical lifting in twelve healthy adults balanced by gender. A dynamic, electromyography-assisted spine model was employed that indicated statistical, but marginal, biomechanical benefits of the tested exoskeleton, which diminished with the introduction of the cognitive dual-task. Using Near Infrared Spectroscopy (fNIRS)-based brain connectivity analyses, we found that the tested exoskeleton imposed greater neurocognitive and motor adaptation efforts by engaging action monitoring and error processing brain networks. Collectively, these findings indicate that a wearer's biomechanical response to increased cognitive demands in the workplace may offset the mechanical advantages of exoskeletons. We also demonstrate the utility of ambulatory fNIRS to capture the neural cost of HEI without the need for elaborate dual-task manipulations.

Locomotion and Energetics of Divergent Foraging Strategies in Hummingbirds: A Review.

Hummingbirds have two main foraging strategies: territoriality (defending a patch of flowers) and traplining (foraging over routine circuits of isolated patches). Species are often classified as employing one or the other. Not only have these strategies been inconsistently defined within the behavioral literature, but this simple framework also neglects the substantial evidence for flexible foraging behavior displayed by hummingbirds. Despite these limitations, research on hummingbird foraging has explored the distinct avenues of selection that proponents of either strategy presumably face: trapliners maximizing foraging efficiency, and territorialists favoring speed and maneuverability for resource defense. In earlier studies, these functions were primarily examined through wing disc loading (ratio of body weight to the circular area swept out by the wings, WDL) and predicted hovering costs, with trapliners expected to exhibit lower WDL than territorialists and thus lower hovering costs. While these pioneering models continue to play a role in current research, early studies were constrained by modest technology, and the original expectations regarding WDL have not held up when applied across complex hummingbird assemblages. Current technological advances have allowed for innovative research on the biomechanics/energetics of hummingbird flight, such as allometric scaling relationships (e.g., wing area-flight performance) and the link between high burst lifting performance and territoriality. Providing a predictive framework based on these relationships will allow us to reexamine previous hypotheses, and explore the biomechanical trade-offs to different foraging strategies, which may yield divergent routes of selection for quintessential territoriality and traplining. With a biomechanical and morphofunctional lens, here we examine the locomotor and energetic facets that dictate hummingbird foraging, and provide (a) predictions regarding the behavioral, biomechanical, and morphofunctional associations with territoriality and traplining; and (b) proposed methods of testing them. By pursuing these knowledge gaps, future research could use a variety of traits to help clarify the operational definitions of territoriality and traplining, to better apply them in the field.

Biomechanical trade-offs in the pelvic floor constrain the evolution of the human birth canal

Biomechanical trade-offs in the pelvic floor constrain the evolution of the human birth canal

Biomechanical trade-offs in the pelvic floor constrain the evolution of the human birth canal

Biomechanical trade-offs in the pelvic floor constrain the evolution of the human birth canal

Biomechanical trade-offs in the pelvic floor constrain the evolution of the human birth canal

Compared with most other primates, humans are characterized by a tight fit between the maternal birth canal and the fetal head, leading to a relatively high risk of neonatal and maternal mortality and morbidities. Obstetric selection is thought to favor a spacious birth canal, whereas the source for opposing selection is frequently assumed to relate to bipedal locomotion. Another, yet underinvestigated, hypothesis is that a more expansive birth canal suspends the soft tissue of the pelvic floor across a larger area, which is disadvantageous for continence and support of the weight of the inner organs and fetus. To test this "pelvic floor hypothesis," we generated a finite element model of the human female pelvic floor and varied its radial size and thickness while keeping all else constant. This allowed us to study the effect of pelvic geometry on pelvic floor deflection (i.e., the amount of bending from the original position) and tissue stresses and stretches. Deflection grew disproportionately fast with increasing radial size, and stresses and stretches also increased. By contrast, an increase in thickness increased pelvic floor stiffness (i.e., the resistance to deformation), which reduced deflection but was unable to fully compensate for the effect of increasing radial size. Moreover, larger thicknesses increase the intra-abdominal pressure necessary for childbirth. Our results support the pelvic floor hypothesis and evince functional trade-offs affecting not only the size of the birth canal but also the thickness and stiffness of the pelvic floor.

Anticipating the Direction of Soccer Penalty Shots Depends on the Speed and Technique of the Kick.

To succeed at a sport, athletes must manage the biomechanical trade-offs that constrain their performance. Here, we investigate a previously unknown trade-off in soccer: how the speed of a kick makes the outcome more predictable to an opponent. For this analysis, we focused on penalty kicks to build on previous models of factors that influence scoring. More than 700 participants completed an online survey, watching videos of penalty shots from the perspective of a goalkeeper. Participants (ranging in soccer playing experience from never played to professional) watched 60 penalty kicks, each of which was occluded at a particular moment (−0.4 s to 0.0 s) before the kicker contacted the ball. For each kick, participants had to predict shot direction toward the goal (left or right). As expected, predictions became more accurate as time of occlusion approached ball contact. However, the effect of occlusion was more pronounced when players kicked with the side of the foot than when they kicked with the top of the foot (instep). For side-foot kicks, the direction of shots was predicted more accurately for faster kicks, especially when a large portion of the kicker’s approach was presented. Given the trade-off between kicking speed and directional predictability, a penalty kicker might benefit from kicking below their maximal speed.

Modeling the two-dimensional accuracy of soccer kicks

In many sports, athletes perform motor tasks that simultaneously require both speed and accuracy for success, such as kicking a ball. Because of the biomechanical trade-off between speed and accuracy, athletes must balance these competing demands. Modelling the optimal compromise between speed and accuracy requires one to quantifyhow task speed affects the dispersion around a target, a level of experimental detail not previously addressed. Using soccer penalties as a system, we measured two-dimensional kicking error over a range of speeds, target heights, and kicking techniques. Twenty experienced soccer players executed a total of 8466 kicks at two targets (high and low). Players kicked with the side of their foot or the instep at ball speeds ranging from 40% to 100% of their maximum. The inaccuracy of kicks was measured in horizontal and vertical dimensions. For both horizontal and vertical inaccuracy, variance increased as a power function of speed, whose parameter values depended on the combination of kicking technique and target height. Kicking precision was greater when aiming at a low target compared to a high target. Side-foot kicks were more accurate than instep kicks. The centre of the dispersion of shots shifted as a function of speed. An analysis of the covariance between horizontal and vertical error revealed right-footed kickers tended to miss below and to the left of the target or above and to the right, while left-footed kickers tended along the reflected axis. Our analysis provides relationships needed to model the optimal strategy for penalty kickers.

Intrinsic Constraints on the Diversification of Neotropical Cichlid Adductor Mandibulae Size.

The diversification of functional traits may be constrained by intrinsic factors, such as structural, mechanical, developmental, or physiological limitations. We explored the biomechanical and constructional constraints on the size of the major jaw closing muscles, the adductor mandibulae complex (AM), in a diverse clade of freshwater fish - the Neotropical cichlids. Using phylogenetic comparative methods, we contrasted patterns of size variation and diversification rates of three AM divisions with variables describing head size and biomechanical coefficients describing force and velocity transmission. We found that all three AM muscles examined were impacted by constructional constraints, namely, (1) the space available in the head (head length and width-all AMs), (2) competition with the eye (AM1 and AM2), (3) competition for space among the three major AM divisions (e.g., AM1 vs. AM3), and (4) potentially the shape of the lower jaw (AM2). Only AM2 size was significantly associated with lower jaw biomechanical coefficients, but opposite predictions based on force transmission (i.e., no compensation for low mechanical advantage). Diversification rates of the mass of the divisions of the AM were also not connected to the diversification rates of their biomechanical coefficients. Previously suggested compensation in AM mass for reduced force transmission among ram-feeding predators appears to be driven by overall body plan changes (lengthening of the head in elongate bodies) and only indirectly to biomechanical trade-offs. Strong constructional constraints on AM size likely limit potentially functional morphospace occupation, and highlight the highly integrated nature of ram-suction feeding functional adaptations in Neotropical cichlids. Anat Rec, 301:216-226, 2018. © 2018 Wiley Periodicals, Inc.

Surface friction alters the agility of a small Australian marsupial.

Movement speed can underpin an animal's probability of success in ecological tasks. Prey often use agility to outmanoeuvre predators; however, faster speeds increase inertia and reduce agility. Agility is also constrained by grip, as the foot must have sufficient friction with the ground to apply the forces required for turning. Consequently, ground surface should affect optimum turning speed. We tested the speed-agility trade-off in buff-footed antechinus (Antechinus mysticus) on two different surfaces. Antechinus used slower turning speeds over smaller turning radii on both surfaces, as predicted by the speed-agility trade-off. Slipping was 64% more likely on the low-friction surface, and had a higher probability of occurring the faster the antechinus were running before the turn. However, antechinus compensated for differences in surface friction by using slower pre-turn speeds as their amount of experience on the low-friction surface increased, which consequently reduced their probability of slipping. Conversely, on the high-friction surface, antechinus used faster pre-turn speeds in later trials, which had no effect on their probability of slipping. Overall, antechinus used larger turning radii (0.733±0.062 versus 0.576±0.051 m) and slower pre-turn (1.595±0.058 versus 2.174±0.050 m s-1) and turning speeds (1.649±0.061 versus 2.01±0.054 m s-1) on the low-friction surface. Our results demonstrate the interactive effect of surface friction and the speed-agility trade-off on speed choice. To predict wild animals' movement speeds, future studies should examine the interactions between biomechanical trade-offs and terrain, and quantify the costs of motor mistakes in different ecological activities.

Manakins’ superfast wings are on steroids

![Graphic][1] Your monster truck will never win a Grand Prix and my Lamborghini will never crush a row of pick-ups. In much the same way, the muscles that make animals move are rarely both fast and furious because there is a biomechanical trade-off between speed and strength. An exception to