Lee Margetts

Tenocyte contraction induces crimp formation in tendon-like tissue

Tendons are composed of longitudinally aligned collagen fibrils arranged in bundles with an undulating pattern, called crimp. The crimp structure is established during embryonic development and plays a vital role in the mechanical behaviour of tendon, acting as a shock-absorber during loading. However, the mechanism of crimp formation is unknown, partly because of the difficulties of studying tendon development in vivo. Here, we used a 3D cell culture system in which embryonic tendon fibroblasts synthesise a tendon-like construct comprised of collagen fibrils arranged in parallel bundles. Investigations using polarised light microscopy, scanning electron microscopy and fluorescence microscopy showed that tendon constructs contained a regular pattern of wavy collagen fibrils. Tensile testing indicated that this superstructure was a form of embryonic crimp producing a characteristic toe region in the stress–strain curves. Furthermore, contraction of tendon fibroblasts was the critical factor in the buckling of collagen fibrils during the formation of the crimp structure. Using these biological data, a finite element model was built that mimics the contraction of the tendon fibroblasts and monitors the response of the Extracellular matrix. The results show that the contraction of the fibroblasts is a sufficient mechanical impulse to build a planar wavy pattern. Furthermore, the value of crimp wavelength was determined by the mechanical properties of the collagen fibrils and inter-fibrillar matrix. Increasing fibril stiffness combined with constant matrix stiffness led to an increase in crimp wavelength. The data suggest a novel mechanism of crimp formation, and the finite element model indicates the minimum requirements to generate a crimp structure in embryonic tendon.

Sensitivity analysis in evolutionary robotic simulations of bipedal dinosaur running

ABSTRACT Constructing musculoskeletal models of extinct vertebrates requires subjective assumptions about soft tissue parameters rarely preserved in the fossil record. Despite these necessary assumptions about fundamental input values, paleobiologists rarely perform objective tests of best-estimate models before reaching conclusions based on predicted results. The extent to which lack of knowledge on soft tissue anatomy limits the accuracy of running speed estimates of extinct dinosaurs is therefore poorly understood. In this study, a sensitivity analysis is performed on an evolutionary robotics model of the non-avian theropod dinosaur Allosaurus, used previously to estimate maximum running speed in this extinct animal. A range of muscle parameters were varied over the range observed in extant vertebrates, whereas mass-related parameters were altered across the range of published estimates for Allosaurus. Muscle parameters have a linear relationship with maximum running speed, whereas surprisingly total body mass and torso center of mass have little effect. Muscle force values produced the greatest range in predicted running speeds (4.5–10.7 m/s) and stride lengths (4–5.8 m) in the sensitivity analysis, equating to 65.9% and 30.7% variation about the original ‘best-estimate’ prediction, a relatively high potential margin of error. These results highlight the importance of sensitivity analyses in biomechanical modeling of extinct taxa, particularly where values for soft tissues parameters are not tightly constrained. The current range in plausible values for soft tissue properties makes a robust quantitative assessment of behavioral ecology and species interactions in dinosaurian communities extremely difficult.

Simulating sauropod manus-only trackway formation using finite-element analysis

The occurrence of sauropod manus-only trackways in the fossil record is poorly understood, limiting their potential for understanding locomotor mechanics and behaviour. To elucidate possible causative mechanisms for these traces, finite-element analyses were conducted to model the indentation of substrate by the feet of Diplodocus and Brachiosaurus. Loading was accomplished by applying mass, centre of mass and foot surface area predictions to a range of substrates to model track formation. Experimental results show that when pressure differs between manus and pes, as determined by the distribution of weight and size of respective autopodia, there is a range of substrate shear strengths for which only the manus (or pes) produce enough pressure to deform the substrate, generating a track. If existing reconstructions of sauropod feet and mass distributions are correct, then different taxa will produce either manus- or pes-only trackways in specific substrates. As a result of this work, it is predicted that the occurrence of manus- or pes-only trackways may show geo-temporal correlation with the occurrence of body fossils of specific taxa.

The convergence variability of parallel iterative solvers

Purpose – To investigate the cause of a well‐known phenomenon associated with a range of parallel iterative solvers – the variability in the number of iterations required to achieve convergence.Design/methodology/approach – The conclusions are based on extensive experiments undertaken using parallel computers. Recently published works are also used to provide additional examples of variability in iteration count.Findings – The variability of iteration counts experienced by parallelised, element‐by‐element iterative solvers is caused by numerical precision and roundoff.Research limitations/implications – A theoretical examination of the phenomenon may bring to light a methodology in which the iteration count could be limited to the lower end of the variable range – thus reducing solution times.Practical implications – The authors believe that the variability in iteration count described for element‐by‐element methods presents no real difficulty to the engineering analyst.Originality/value – The paper gives...

Parallel 3D Finite Element Analysis of Coupled Problems

Steady advances in computer power have enabled researchers to consider tackling increasingly complex problems. In the academic community, current focus is on multiscale modelling and multiphysics. The aim is for simulation to be more realistically representative of real world processes. This paper considers the simulation of coupled problems involving more than one physical process, multiphysics. In particular, the authors present some ideas and experiences regarding the use of the finite element method and parallel computers to solve 3D coupled problems. In the literature, two main approaches have been used to solve coupled problems. These are sometimes referred to as (i) fully coupled modelling and (ii) un-coupled multi-physics. Both methods have their advantages and disadvantages. In the paper, the authors discuss some of the issues that should be considered when selecting a particular strategy, to ensure computational efficiency. Particular attention is given to an example from the field of magnetohydrodynamics: three dimensional steady state flow in a perfectly insulated rectangular duct. The magnetohydrodynamics example involves solving a system in which both magnetic and hydrodynamic forces influence the behaviour of the fluid. Visualisation of the problem, using streamlines to represent fluid flow (Figure 1) shows that a three dimensional representation is essential to capture the full complexity of the flow. A fully coupled solution strategy is presented in which the full system is represented by a single “stiffness” matrix and solved by a single computer program. A parallel implementation of an element-by-element variant of BiCGStab(l) is used to solve the equations, demonstrating the efficient use of up to 128 processors. nOpen image in new window n nFigure 1 nVelocity Streamlines from Three Different Viewpoints Along a Rectangular Duct