Muscle power attenuation by tendon during energy dissipation

An important function of skeletal muscle is deceleration via active muscle fascicle lengthening, which dissipates movement energy. The mechanical interplay between muscle contraction and tendon elasticity is critical when muscles produce energy. However, the role of tendon elasticity during muscular energy dissipation remains unknown. We tested the hypothesis that tendon elasticity functions as a mechanical buffer, preventing high (and probably damaging) velocities and powers during active muscle fascicle lengthening. We directly measured lateral gastrocnemius muscle force and length in wild turkeys during controlled landings requiring rapid energy dissipation. Muscle-tendon unit (MTU) strain was measured via video kinematics, independent of muscle fascicle strain (measured via sonomicrometry). We found that rapid MTU lengthening immediately following impact involved little or no muscle fascicle lengthening. Therefore, joint flexion had to be accommodated by tendon stretch. After the early contact period, muscle fascicles lengthened and absorbed energy. This late lengthening occurred after most of the joint flexion, and was thus mainly driven by tendon recoil. Temporary tendon energy storage led to a significant reduction in muscle fascicle lengthening velocity and the rate of energy absorption. We conclude that tendons function as power attenuators that probably protect muscles against damage from rapid and forceful lengthening during energy dissipation.     

Energetic and biomechanical constraints on animal migration distance

Animal migration is one of the great wonders of nature, but the factors that determine how far migrants travel remain poorly understood. We present a new quantitative model of animal migration and use it to describe the maximum migration distance of walking, swimming and flying migrants. The model combines biomechanics and metabolic scaling to show how maximum migration distance is constrained by body size for each mode of travel. The model also indicates that the number of body lengths travelled by walking and swimming migrants should be approximately invariant of body size. Data from over 200 species of migratory birds, mammals, fish, and invertebrates support the central conclusion of the model - that body size drives variation in maximum migration distance among species through its effects on metabolism and the cost of locomotion. The model provides a new tool to enhance general understanding of the ecology and evolution of migration.     

The evolution of cranial form and function in theropod dinosaurs: insights from geometric morphometrics

Theropod dinosaurs, an iconic clade of fossil species including Tyrannosaurus and Velociraptor, developed a great diversity of body size, skull form and feeding habits over their 160+ million year evolutionary history. Here, we utilize geometric morphometrics to study broad patterns in theropod skull shape variation and compare the distribution of taxa in cranial morphospace (form) to both phylogeny and quantitative metrics of biting behaviour (function). We find that theropod skulls primarily differ in relative anteroposterior length and snout depth and to a lesser extent in orbit size and depth of the cheek region, and oviraptorosaurs deviate most strongly from the "typical" and ancestral theropod morphologies. Noncarnivorous taxa generally fall out in distinct regions of morphospace and exhibit greater overall disparity than carnivorous taxa, whereas large-bodied carnivores independently converge on the same region of morphospace. The distribution of taxa in morphospace is strongly correlated with phylogeny but only weakly correlated with functional biting behaviour. These results imply that phylogeny, not biting function, was the major determinant of theropod skull shape.     

The rising cost of warming waters: effects of temperature on the cost of swimming in fishes

Understanding the effects of water temperature on the swimming performance of fishes is central in understanding how fish species will respond to global climate change. Metabolic cost of transport (COT)-a measure of the energy required to swim a given distance-is a key performance parameter linked to many aspects of fish life history. We develop a quantitative model to predict the effect of water temperature on COT. The model facilitates comparisons among species that differ in body size by incorporating the body mass-dependence of COT. Data from 22 fish species support the temperature and mass dependencies of COT predicted by our model, and demonstrate that modest differences in water temperature can result in substantial differences in the energetic cost of swimming.     

The penguin waddling gait pattern has a more consistent step width than step length

Previous research has indicated that the sagittal plane gait dynamics of humans are more stable and less dependent on active neural control, while the frontal plane dynamics are less stable and require greater neural control. The higher neural demands of the frontal plane dynamics are reflected in a more variable step width than step length. Greater variability in the step width occurs because humans modulate their foot placement for each step to ensure stability and prevent falls. Compared to other terrestrial animals, penguins appear to have excessive amount of frontal plane motion in their gait that is characterized as waddling. If excessive frontal plane motion requires additional neural control and is associated with falls, it would seem that evolutionary pressures would have eliminated such locomotive strategies. Here we measured the step length and width variability to determine if waddling results in a less stable gait. Remarkably, the variability of the step width was less than the variability of the step length. These results are directly opposite of what has been reported for humans. Hence, our data indicate that waddling may be an effective strategy for ensuring stability in the frontal plane dynamics.     

The effects of locomotion on the structural characteristics of avian limb bones

Despite the wide range of locomotor adaptations in birds, little detailed attention has been given to the relationships between the quantitative structural characteristics of avian limb bones and bird behaviour. Possible differences in forelimb relative to hindlimb strength across species have been especially neglected. We generated cross‐sectional, geometric data from peripheral quantitative computed tomography scans of the humerus and femur of 127 avian skeletons, representing 15 species of extant birds in 13 families. The sample includes terrestrial runners, arboreal perchers, hindlimb‐propelled divers, forelimb‐propelled divers and dynamic soarers. The hindlimb‐propelled diving class includes a recently flightless island form. Our results demonstrate that locomotor dynamics can be differentiated in most cases based on cross‐sectional properties, and that structural proportions are often more informative than bone length proportions for determining behaviour and locomotion. Recently flightless forms, for example, are more easily distinguished using structural ratios than using length ratios. A proper phylogenetic context is important for correctly interpreting structural characteristics, especially for recently flightless forms. Some of the most extreme adaptations to mechanical loading are seen in aquatic forms. Penguins have forelimbs adapted to very high loads. Aquatic species differ from non‐aquatic species on the basis of relative cortical thickness. The combination of bone structural strength and relative cortical area of the humerus successfully differentiates all of our locomotor groups. The methods used in this study are highly applicable to fossil taxa, for which morphology is known but behaviour is not. The use of bone structural characteristics is particularly useful in palaeontology not only because it generates strong signals for many locomotor guilds, but also because analysing such traits does not require knowledge of body mass, which can be difficult to estimate reliably for fossil taxa. © 2008 The Linnean Society of London, Zoological Journal of the Linnean Society, 2008, 153 , 601–624.     

Systemic induced resistance: a risk-spreading strategy in clonal plant networks?

Clonal plant networks consist of interconnected individuals (ramets) of different sizes and ages. They represent heterogeneous ramet assemblages with marked differences in quality and attractiveness for herbivores. Here, feeding preferences of a generalist herbivore (Spodoptera exigua) for differently-aged ramets of Trifolium repens were studied, and changes in herbivore preference in response to systemic defense induction were investigated. Dual-choice tests were used to assess the preference of herbivores for young versus mature ramets of induced and uninduced plants, respectively. Additionally, leaf traits related to nutrition, biomechanics and chemical defense were measured to explain variation in tissue quality and herbivore preference. Young ramets were heavily damaged in control plants. After systemic defense induction, damage on young ramets was greatly reduced, while damage on mature ramets increased slightly. Defense induction increased leaf strength and thickness, decreased leaf soluble carbohydrates and substantially changed phenolic composition of undamaged ramets connected to attacked individuals. Systemic induced resistance led to a more dispersed feeding pattern among ramets of different ages. It is proposed that inducible defense acts as a risk-spreading strategy in clonal plants by equalizing herbivore preference within the clone, thereby avoiding extended selective feeding on valuable plant tissues.     

Patterns of clavicular bilateral asymmetry in relation to the humerus: variation among humans

Studies of directional asymmetry in the human upper limb have extensively examined bones of the arm, forearm, and hand, but have rarely considered the clavicle. Physiologically, the clavicle is an integrated element of the upper limb, transmitting loads to the axial skeleton and supporting the distal bones. However, clavicles develop in a manner that is unique among the bones of the upper limb. Previous studies have indicated that the clavicle has a right-biased asymmetry in diaphyseal breadth, as in humeri, radii, ulnae, and metacarpals, but unlike these other elements, a left-biased length asymmetry. Few studies have assessed how clavicular asymmetry relates to these other bones of the upper limb. Bilateral directional asymmetry of the clavicle is examined in relation to the humerus in a large, geographically diverse human sample, comparing lengths and diaphyseal breadths. Dimensions were converted into percentage directional (%DA) and absolute (%AA) asymmetries. Results indicate that humans have same-side %DA bias in the clavicles and humeri, and contralateral length %DA between these elements. Diaphyseal breadths in both clavicles and humeri are more asymmetric-both in direction and amount-than lengths. Differences in diaphyseal asymmetry are shown to relate to variation in physical activities among groups, but a relationship between activity and length asymmetry is not supported. This further supports previous research, which suggests different degrees of sensitivity to loading between diaphyseal breadths and maximum lengths of long bones. Differences in lateralized behavior and the potential effects of different bone development are examined as possible influences on the patterns observed among human groups.     

Muscle work is biased toward energy generation over dissipation in non-level running

This study tested the hypothesis that skeletal muscles generate more mechanical energy in gait tasks that raise the center of mass compared to the mechanical energy they dissipate in gait tasks that lower the center of mass despite equivalent changes in total mechanical energy. Thirteen adults ran on a 10° decline and incline surface at a constant average velocity. Three-dimensional (3D) joint powers were calculated from ground force and 3D kinematic data using inverse dynamics. Joint work was calculated from the power curves and assumed to be due to skeletal muscle–tendon actuators. External work was calculated from the kinematics of the pelvis through the gait cycle. Incline vs. decline running was characterized with smaller ground forces that operated over longer lever arms causing larger joint torques and work from these torques. Total lower extremity joint work was 28% greater in incline vs. decline running (1.32 vs. −1.03 J/kg m, p<0.001). Total lower extremity joint work comprised 86% and 71% of the total external work in incline (1.53 J/kg m) and decline running (−1.45 J/kg m), which themselves were not significantly different (p<0.180). We conjectured that the larger ground forces in decline vs. incline running caused larger accelerations of all body tissues and initiated a greater energy-dissipating response in these tissues compared to their response in incline running. The runners actively lowered themselves less during decline stance and descended farther as projectiles than they lifted themselves during incline stance and ascended as projectiles. These data indicated that despite larger ground forces in decline running, the reduced displacement during downhill stance phases limited the work done by muscle contraction in decline compared to incline running.