Shell biomechanics in the bivalve Laternula

The ligament in Laternula is modified into a complex mechanical structure composed of rigid and flexible parts with multiple connection points. Rather than allowing the valves to move with respect to each other, this ligament tends to keep them immobile. Providing space inside the shell for the modified ligament requires a secondary increase in shell curvature of the umbones. This is achieved by the shell flexing by muscular contraction and stiffening while flexed through the construction of a rigid buttress. A vertical slit in each valve, produced by secondary resorption and breakage of shell material, confines mechanical stress to the posterior slope of the shell. □ Mollusca, Bivalvia, Laternulidae, Holocene, Indo-Pacific, functional morphology.     

Modelling approaches in biomechanics

Conceptual, physical and mathematical models have all proved useful in biomechanics. Conceptual models, which have been used only occasionally, clarify a point without having to be constructed physically or analysed mathematically. Some physical models are designed to demonstrate a proposed mechanism, for example the folding mechanisms of insect wings. Others have been used to check the conclusions of mathematical modelling. However, others facilitate observations that would be difficult to make on real organisms, for example on the flow of air around the wings of small insects. Mathematical models have been used more often than physical ones. Some of them are predictive, designed for example to calculate the effects of anatomical changes on jumping performance, or the pattern of flow in a 3D assembly of semicircular canals. Others seek an optimum, for example the best possible technique for a high jump. A few have been used in inverse optimization studies, which search for variables that are optimized by observed patterns of behaviour. Mathematical models range from the extreme simplicity of some models of walking and running, to the complexity of models that represent numerous body segments and muscles, or elaborate bone shapes. The simpler the model, the clearer it is which of its features is essential to the calculated effect.     

Physical modelling in biomechanics

Physical models, like mathematical models, are useful tools in biomechanical research. Physical models enable investigators to explore parameter space in a way that is not possible using a comparative approach with living organisms: parameters can be varied one at a time to measure the performance consequences of each, while values and combinations not found in nature can be tested. Experiments using physical models in the laboratory or field can circumvent problems posed by uncooperative or endangered organisms. Physical models also permit some aspects of the biomechanical performance of extinct organisms to be measured. Use of properly scaled physical models allows detailed physical measurements to be made for organisms that are too small or fast to be easily studied directly. The process of physical modelling and the advantages and limitations of this approach are illustrated using examples from our research on hydrodynamic forces on sessile organisms, mechanics of hydraulic skeletons, food capture by zooplankton and odour interception by olfactory antennules.     

Dinosaur biomechanics

Biomechanics has made large contributions to dinosaur biology. It has enabled us to estimate both the speeds at which dinosaurs generally moved and the maximum speeds of which they may have been capable. It has told us about the range of postures they could have adopted, for locomotion and for feeding, and about the problems of blood circulation in sauropods with very long necks. It has made it possible to calculate the bite forces of predators such as Tyrannosaurus, and the stresses they imposed on its skull; and to work out the remarkable chewing mechanism of hadrosaurs. It has shown us how some dinosaurs may have produced sounds. It has enabled us to estimate the effectiveness of weapons such as the tail spines of Stegosaurus. In recent years, techniques such as computational tomography and finite element analysis, and advances in computer modelling, have brought new opportunities. Biomechanists should, however, be especially cautious in their work on animals known only as fossils. The lack of living specimens and even soft tissues oblige us to make many assumptions. It is important to be aware of the often wide ranges of uncertainty that result.     

Spine biomechanics

Current trends in spine research are reviewed in order to suggest future opportunities for biomechanics. Recent studies show that psychosocial factors influence back pain behaviour but are not important causes of pain itself. Severe back pain most often arises from intervertebral discs, apophyseal joints and sacroiliac joints, and physical disruption of these structures is strongly but variably linked to pain. Typical forms of structural disruption can be reproduced by severe mechanical loading in-vitro, with genetic and age-related weakening sometimes leading to injury under moderate loading. Biomechanics can be used to quantify spinal loading and movements, to analyse load distributions and injury mechanisms, and to develop therapeutic interventions. The authors suggest that techniques for quantifying spinal loading should be capable of measurement "in the field" so that they can be used in epidemiological surveys and ergonomic interventions. Great accuracy is not required for this task, because injury risk depends on tissue weakness as much as peak loading. Biomechanical tissue testing and finite-element modelling should complement each other, with experiments establishing proof of concept, and models supplying detail and optimising designs. Suggested priority areas for future research include: understanding interactions between intervertebral discs and adjacent vertebrae; developing prosthetic and tissue-engineered discs; and quantifying spinal function during rehabilitation. "Mechanobiology" has perhaps the greatest future potential, because spinal degeneration and healing are both mediated by the activity of cells which are acutely sensitive to their local mechanical environment. Precise characterisation and manipulation of this environment will be a major challenge for spine biomechanics.     

Cellular disposition of transported polyamines in hypoxic rat lung and pulmonary arteries

The polyamines putrescine, spermidine (SPD), and spermine are a family of low-molecular-weight organic cations essential for cell growth and differentiation and other aspects of signal transduction. Hypoxic pulmonary vascular remodeling is accompanied by depressed lung polyamine synthesis and markedly augmented polyamine uptake. Cell types in which hypoxia induces polyamine transport in intact lung have not been delineated. Accordingly, rat lung and rat main pulmonary arterial explants were incubated with [(14)C]SPD in either normoxic (21% O(2)) or hypoxic (2% O(2)) environments for 24 h. Autoradiographic evaluation confirmed previous studies showing that, in normoxia, alveolar epithelial cells are dominant sites of polyamine uptake. In contrast, hypoxia was accompanied by prominent localization of [(14)C]SPD in conduit, muscularized, and partially muscularized pulmonary arteries, which was not evident in normoxic lung tissue. Hypoxic main pulmonary arterial explants also exhibited substantial increases in [(14)C]SPD uptake relative to control explants, and autoradiography revealed that enhanced uptake was most evident in the medial layer. Main pulmonary arterial explants denuded of endothelium failed to increase polyamine transport in hypoxia. Conversely, medium conditioned by endothelial cells cultured in hypoxic, but not in normoxic, environments enabled hypoxic transport induction in denuded arterial explants. These findings in arterial explants were recapitulated in rat cultured main pulmonary artery cells, including the enhancing effect of a soluble endothelium-derived factor(s) on hypoxic induction of [(14)C]SPD uptake in smooth muscle cells. Viewed collectively, these results show in intact lung tissue that hypoxia enhances polyamine transport in pulmonary artery smooth muscle by a mechanism requiring elaboration of an unknown factor(s) from endothelial cells.     

A minimal principle in biomechanics

Expenditure of energy under several simultaneous forms (mechanical, chemical, etc.) is associated with all muscular activity. The energy is directly related to what is commonly called exertion or effort. This paper defines “muscular effort” quantitatively in terms of some of the elements of the dynamics of the human (and animal) body. It postulates that in all likelihood the individual will, consciously or otherwise, determine his motion (or his posture, if at rest) in such a manner as to reduce his total muscular effort to a minimum consistent with imposed conditions, or “constraints”. The principle, formulated in mathematical terms, is sufficient to ascribe to the moments at all body joints—a matter generally of free choice on the part of the individual—their most likely magnitudes. It therefore renders the equations of human (and animal) motion determinate within this context. The paper also describes briefly an iteration method for the solution of these equations, once they have been made determinate. A simple illustrative application of the principle is included.     

On the biomechanics of cytokinesis in animal cells

The material properties of the cell membrane are discussed. Various theories concerning the mechanism of cytokinesis in animal cells are presented. The currently accepted mechanism is that of active muscle-like contraction of the furrow base itself. A mathematical model is developed based on this theory. The cell membrane is modelled as a spherical membrane of nonlinear, elastic material. The membrane undergoes large deformations under the action of a contractile ring force in its equatorial plane. The numerical procedure employed in the solution of the governing equations is explained. The numerical results are compared with the experimental observations available in the literature. It is concluded that the cell membrane stiffness increases during the early stages of cleavage and it, later, decreases. The cell membrane division is a biomechanical instability problem. The factors that may facilitate or block cleavage are discussed. The experimental evidences that support the conjectures of the model are pointed out.     

Modeling the biomechanics of fetal movements

Fetal movements in the uterus are a natural part of development and are known to play an important role in normal musculoskeletal development. However, very little is known about the biomechanical stimuli that arise during movements in utero, despite these stimuli being crucial to normal bone and joint formation. Therefore, the objective of this study was to create a series of computational steps by which the forces generated during a kick in utero could be predicted from clinically observed fetal movements using novel cine-MRI data of three fetuses, aged 20–22 weeks. A custom tracking software was designed to characterize the movements of joints in utero, and average uterus deflection of \(6.95 \pm 0.41\) mm due to kicking was calculated. These observed displacements provided boundary conditions for a finite element model of the uterine environment, predicting an average reaction force of \(0.52 \pm 0.15\) N generated by a kick against the uterine wall. Finally, these data were applied as inputs for a musculoskeletal model of a fetal kick, resulting in predicted maximum forces in the muscles surrounding the hip joint of approximately 8 N, while higher maximum forces of approximately 21 N were predicted for the muscles surrounding the knee joint. This study provides a novel insight into the closed mechanical environment of the uterus, with an innovative method allowing elucidation of the biomechanical interaction of the developing fetus with its surroundings.     

Cellular events associated with lung branching morphogenesis including the deposition of collagen type IV

In this study mouse lung development was examined using an in vitro model system. The culture system permitted examination of a morphogenic process that eventually led to the formation of presumptive alveoli (terminal sacs). The observations included changes in epithelial cell morphology (transition from a columnar to a spindle shape), and evidence for motile activity on the part of primitive airway epithelial cells. The importance of Type IV collagen to the cellular events associated with branching morphogenesis was investigated by immunolocalization. In addition, we assessed the similarity of normal lung development to in vitro development by comparing cultured lungs with equivalent stages of embryonic and fetal mouse lungs. The results show that cultured embryonic lung explants proceed along a morphogenic pathway that parallels normal lung development; that primitive pulmonary epithelial cells engage in motile activity and transiently acquire an extended cell shape both in vitro and in vivo; that, as suggested by others, the pattern of late branching morphogenesis is not dichotomous, but irregular; and that short wisplike fibers of Type IV collagen are present in developing embryonic and fetal lung mesenchyme. Taken together, the results show that early and late lung branching patterns differ significantly, and suggest that later stages of lung branching involve distinct epithelial cell shape transitions. The immunofluorescence data suggest that fibrous Type IV collagen may be the extracellular matrix scaffold within which early epithelial cells accomplish lung branching morphogenesis.     

Plant biomechanics in an ecological context

Fundamental plant traits such as support, anchorage, and protection against environmental stress depend substantially on biomechanical design. The costs, subsequent trade-offs, and effects on plant performance of mechanical traits are not well understood, but it appears that many of these traits have evolved in response to abiotic and biotic mechanical forces and resource deficits. The relationships between environmental stresses and mechanical traits can be specific and direct, as in responses to strong winds, with structural reinforcement related to plant survival. Some traits such as leaf toughness might provide protection from multiple forms of stress. In both cases, the adaptive value of mechanical traits may vary between habitats, so is best considered in the context of the broader growth environment, not just of the proximate stress. Plants can also show considerable phenotypic plasticity in mechanical traits, allowing adjustment to changing environments across a range of spatial and temporal scales. However, it is not always clear whether a mechanical property is adaptive or a consequence of the physiology associated with stress. Mechanical traits do not only affect plant survival; evidence suggests they have downstream effects on ecosystem organization and functioning (e.g., diversity, trophic relationships, and productivity), but these remain poorly explored.     

Discrete element analysis in musculoskeletal biomechanics

This paper is written to honor Professor Y. C. Fung, the applied mechanician who has made seminal contributions in biomechanics. His work has generated great spin-off utility in the field of musculoskeletal biomechanics. Following the concept of the Rigid Body-Spring Model theory by T. Kawai (1978) for non-linear analysis of beam, plate, and shell structures and the soil-gravel mixture foundation, we have derived a generalized Discrete Element Analysis (DEA) method to determine human articular joint contact pressure, constraining ligament tension and bone-implant interface stresses. The basic formulation of DEA to solve linear problems is reviewed. The derivation of non-linear springs for the cartilage in normal diarthrodial joint contact problem was briefly summarized. Numerical implementation of the DEA method for both linear and non-linear springs is presented. This method was able to generate comparable results to the classic contact stress problem (the Hertzian solution) and the use of Finite Element Modeling (FEM) technique on selected models. Selected applications in human knee and hip joints are demonstrated. In addition, the femoral joint prosthesis stem/bone interface stresses in a non-cemented fixation were analyzed using a 2D plane-strain approach. The DEA method has the advantages of ease in creating the model and reducing computational time for joints of irregular geometry. However, for the analysis of joint tissue stresses, the FEA technique remains the method of choice.     

Knee joint biomechanics in closed-kinetic-chain exercises

Effective management of knee joint disorders demands appropriate rehabilitation programs to restore function while strengthening muscles. Excessive stresses in cartilage/menisci and forces in ligaments should be avoided to not exacerbate joint condition after an injury or reconstruction. Using a validated 3D nonlinear finite element model, detailed biomechanics of the entire joint in closed-kinetic-chain squat exercises are investigated at different flexion angles, weights in hands, femur-tibia orientations and coactivity in hamstrings. Predictions are in agreement with results of earlier studies. Estimation of small forces in cruciate ligaments advocates the use of squat exercises at all joint angles and external loads. In contrast, large contact stresses, especially at the patellofemoral joint, that approach cartilage failure threshold in compression suggest avoiding squatting at greater flexion angles, joint moments and weights in hands. Current results are helpful in comprehensive evaluation and design of effective exercise therapies and trainings with minimal risk to various components.     

Age-related changes in the biomechanics of healing patellar tendon

By 2030, there will be 70 million people in the United States over the age of 65, and by 2050, 22% of the US population will be considered elderly. It is generally believed that injuries in the elderly heal slower and less completely than in adolescents or young adults. To evaluate aging effects on tissue repair a surgical injury was created in the middle third of one patellar tendon in 1- and 4-5-year-old New Zealand White rabbits. The biomechanical properties of the isolated repair tissues and contralateral normal tendon tissues were compared at 6, 12 and 26 weeks post-injury. We hypothesized that repair tissues would exhibit age-related reductions in biomechanical properties at all time intervals of healing, both based on raw data and when normalized to values from contralateral tendons. Repairs from both age groups were similar, with no significant increase in maximum stress, strain at maximum stress, or modulus between 6 and 12 weeks. At 26 weeks, the repairs in the 4-year-old rabbits had higher maximum stress values than repairs in the 1-year-old rabbits (p=0.03). There were no significant differences in the strain at maximum stress or modulus. When repair tissue properties were normalized to values in the contralateral normal tendon, the maximum stress of the patellar tendon repair tissue from the 4 year old was significantly greater than the corresponding value from the 1 year old at the 26 week time point (p=0.04). In conclusion, these findings do not support the presence of age-related declines in the biomechanics of healing tendon.