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Latest Articles Include:

• Editorial Board and Publication Information
  - J Biomech 44(6):IFC (2011)
  
• In vivo kinematics of two-component total ankle arthroplasty during non-weightbearing and weightbearing dorsiflexion/plantarflexion
  - J Biomech 44(6):995-1000 (2011)
  Relatively high rates of loosening and implant failure have been reported after total ankle arthroplasty, especially in first and second generation implants. Abnormal kinematics and incongruency of the articular surface may cause increased loads applied to the implant with concomitant polyethylene wear, resulting in loosening and implant failure. The purpose of this study was to measure three-dimensional kinematics of two-component total ankle arthroplasty during non-weightbearing and weightbearing activities, and to investigate incongruency of the articular surfaces during these activities. Forty-seven patients with a mean age of 71 years were enrolled. Radiographs were taken at non-weightbearing maximal dorsiflexion and plantarflexion, and weightbearing maximal dorsiflexion, plantarflexion, and neutral position. 3D–2D model-image registration was performed using the radiographs and the three-dimensional implant models, and three-dimensional joint angles were determ! ined. The implanted ankles showed 18.1±8.6° (mean±standard deviation) of plantarflexion, 0.1±0.7° of inversion, 1.2±2.0° of internal rotation, and 0.8±0.6 mm of posterior translation of the talar component in the non-weightbearing activity, and 17.8±7.5° of plantarflexion, 0.4±0.5° of inversion, 1.8±2.0° of internal rotation, and 0.7±0.5 mm of posterior translation in the weightbearing activity. There were no significant differences between the non-weightbearing and weightbearing kinematics except for the plantarflexion angle. Incongruency of the articular surface occurred in more than 75% of the ankles. Our observations will provide useful data against which kinematics of other implant designs, such as three-component total ankle arthroplasty, can be compared.
• A model of a radially expanding and contracting lymphangion
  - J Biomech 44(6):1001-1007 (2011)
  The lymphatic system is an extensive vascular network featuring valves and contractile walls that pump interstitial fluid and plasma proteins back to the main circulation. Immune function also relies on the lymphatic system's ability to transport white blood cells. Failure to drain and pump this excess fluid results in edema characterized by fluid retention and swelling of limbs. It is, therefore, important to understand the mechanisms of fluid transport and pumping of lymphatic vessels. Unfortunately, there are very few studies in this area, most of which assume Poiseuille flow conditions. In vivo observations reveal that these vessels contract strongly, with diameter changes of the order of magnitude of the diameter itself over a cycle that lasts typically 2–3 s. The radial velocity of the contracting vessel is on the order of the axial fluid velocity, suggesting that modeling flow in these vessels with a Poiseuille model is inappropriate. In this paper, we describ! e a model of a radially expanding and contracting lymphatic vessel and investigate the validity of assuming Poiseuille flow to estimate wall shear stress, which is presumably important for lymphatic endothelial cell mechanotransduction. Three different wall motions, periodic sinusoidal, skewed sinusoidal and physiologic wall motions, were investigated with steady and unsteady parabolic inlet velocities. Despite high radial velocities resulting from the wall motion, wall shear stress values were within 4% of quasi-static Poiseuille values. Therefore, Poiseuille flow is valid for the estimation of wall shear stress for the majority of the lymphangion contractile cycle.
• Digital image correlation analysis of the load transfer by implant-supported restorations
  - J Biomech 44(6):1008-1013 (2011)
  This study compared splinted and non-splinted implant-supported prosthesis with and without a distal proximal contact using a digital image correlation method. An epoxy resin model was made with acrylic resin replicas of a mandibular first premolar and second molar and with threaded implants replacing the second premolar and first molar. Splinted and non-splinted metal–ceramic screw-retained crowns were fabricated and loaded with and without the presence of the second molar. A single-camera measuring system was used to record the in-plane deformation on the model surface at a frequency of 1.0 Hz under a load from 0 to 250 N. The images were then analyzed with specialist software to determine the direct (horizontal) and shear strains along the model. Not splinting the crowns resulted in higher stress transfer to the supporting implants when the second molar replica was absent. The presence of a second molar and an effective interproximal contact contributed to lower s! tress transfer to the supporting structures even for non-splinted restorations. Shear strains were higher in the region between the molars when the second molar was absent, regardless of splinting. The opposite was found for the region between the implants, which had higher shear strain values when the second molar was present. When an effective distal contact is absent, non-splinted implant-supported restorations introduce higher direct strains to the supporting structures under loading. Shear strains appear to be dependent also on the region within the model, with different regions showing different trends in strain changes in the absence of an effective distal contact.
• Design optimization of a total knee replacement for improved constraint and flexion kinematics
  - J Biomech 44(6):1014-1020 (2011)
  Total knee replacement (TKR) constraint and flexion range of motion can be limiting factors in terms of kinematics performance and cause for revision. These characteristics are closely related to the shape of the implant components. No previous studies have used a rigorous and systematic design optimization method to determine the optimal shape of TKR components. Previous studies have failed to define a quantifiable objective function for optimization, have not used any optimization algorithms, and have only considered a limited design space (4 or less design variables). This study addresses these limitations and determines the optimum shape of the femoral component and ultra high molecular weight polyethylene (UHMWPE) insert in terms of kinematics. The constraint characteristics with respect to those of the natural knee, the importance of the posterior cruciate ligament, and the flexion range of motion were all considered. The kinematics optimized design featured smal! l femoral radii of curvature in the frontal and sagittal planes, but asymmetric with slightly larger radii of curvature for the lateral condyle. This condyle was also less conforming than the medial side. Compared to a commercially available TKR design, the kinematics performance (based on constraint and flexion range of motion) was improved by 81%, with constraint characteristics generally closer to those of the natural knee and a 12.6% increase in the flexion range of motion (up to 143°). The results yielded a new TKR design while demonstrating the feasibility of design optimization in TKR design.
• Effect of rocker shoe radius on oxygen consumption rate in young able-bodied persons
  - J Biomech 44(6):1021-1024 (2011)
  We studied oxygen consumption rate of eleven young able-bodied persons walking at self-selected speed with five different pairs of shoes: one regular pair without rocker soles (REG) and four pairs with uniform hardness (35–40 shore A durometer) rocker soles of different radii (25% of leg length (LL) (R25), 40% LL (R40), 55% LL (R55), and infinite radius (FLAT)). Rocker soled shoes in the study were developed to provide similar vertical lift (three inches higher than the REG shoes condition). Oxygen consumption rate was significantly affected by the use of the different shoes (p<0.001) and pairwise comparisons indicated that persons consumed significantly less oxygen (per minute per kilogram of body mass) when walking on the R40 shoes when compared with both the FLAT (p<0.001) and REG (p=0.021) shoe conditions. Oxygen consumption was also significantly less for the R25 shoes compared with the FLAT shoes (p=0.005) and for the R55 shoes compared with FLAT shoes (p=0.027! ). The three-inch lift on the FLAT shoe did not cause a significant change in oxygen consumption compared to the shoe without the lift (REG).
• Ultrasound assessment of the anatomical validity of T3 and L4 as sEMG recording sites
  - J Biomech 44(6):1025-1030 (2011)
  The accuracy of surface EMG measurement is dependent upon minimizing potential crosstalk from other muscles. Although they are deeply situated, in places the erector spinae are covered with electrically silent aponeuroses rather than active muscle tissue. Theoretically these aponeuroses can serve as windows for sEMG recordings. A recent anatomical study concluded that T3 and L4 are ideal sites for recording the ES because the superficial muscle aponeuroses are wide at these sites. The aim of this prospective study was to investigate these sites in vivo using real time ultrasound. Ultrasound images from 20 subjects (10<30 years and 10>70 years; equal numbers of males and females in each group) were acquired during rest and in prone extension with the arms in three different positions. The most superficial aponeurosis widths were measured. The mean T3 aponeurosis width reduced significantly in extension from 4.4±4.7 mm at rest to 1.8±2.6 mm in extension (p<0.0001). Mal! es had significantly smaller T3 aponeurosis widths than females (p=0.049). The mean L4 aponeurosis width also significantly decreased in extension from 35.5±7.0 mm at rest to 29.9±7.2 mm in extension (p<0.0001) due to 'doming' of the aponeurosis. Our results demonstrate that T3 is not a reliable site over which to record the ES because the aponeurosis width is too narrow. L4 is a good site if the electrodes are placed no more than 20 mm from the midline.
• Structural and hydrodynamic simulation of an acute stenosis-dependent thrombosis model in mice
  - J Biomech 44(6):1031-1039 (2011)
  Platelet activation under blood flow is thought to be critically dependent on the autologous secretion of soluble platelet agonists (chemical activators) such as ADP and thromboxane. However, recent evidence challenging this model suggests that platelet activation can occur independent of soluble agonist signalling, in response to the mechanical effects of micro-scale shear gradients. A key experimental tool utilized to define the effect of shear gradients on platelet aggregation is the murine intravital microscopy model of platelet thrombosis under conditions of acute controlled arteriolar stenosis. This paper presents a computational structural and hydrodynamic simulation of acute stenotic blood flow in the small bowel mesenteric vessels of mice. Using a homogeneous fluid at low Reynolds number (0.45) we investigated the relationship between the local hydrodynamic strain-rates and the severity of arteriolar stensosis. We conclude that the critical rates of blood flow! acceleration and deceleration at sites of artificially induced stenosis (vessel side-wall compression or ligation) are a function of tissue elasticity. By implementing a structural simulation of arteriolar side wall compression, we present a mechanistic model that provides accurate simulations of stenosis in vivo and allows for predictions of the effects on local haemodynamics in the murine small bowel mesenteric thrombosis model.
• Experimental and computational biomechanical characterisation of the tracheo-bronchial tree of the bottlenose dolphin (Tursiops truncatus) during diving
  - J Biomech 44(6):1040-1045 (2011)
  Dolphins have adapted their anatomic structures to survive in the water environment and so far, the behaviour of their respiratory system during diving has not been fully understood, since they being protected species cannot be subjected to invasive analysis. Aim of this work is to model the tracheo-bronchial tree of the bottlenose dolphin Tursiops truncatus to study its behaviour during diving by coupling experimental in vitro mechanical characterisation of airways tissues to finite element computational analyses. Furthermore, a comparison was performed between the mechanical behaviour of tracheo-bronchial trees of dolphins and that of the goat, a terrestrial mammal, whose conformation of the upper airways is similar to that of the human, to determine how different structures respond to pressure in a controlled experimental set-up. The comparison between the goat and dolphin airways' mechanical behaviour highlights a lower collapsibility of the dolphin structure due to higher stiffness, lack of musculature and irregular shape of cartilaginous rings. Our data showed that the air entrapped into the airways plays a key role in avoiding the collapse. This effect is enhanced when accounting for the air flow escaping the alveoli that start to collapse during descent, even at depth as shallow as 10 m of sea water. The comparison between airways behaviour of marine and terrestrial mammals may help in shedding a light on the biomechanical behaviour of human airways during breath-holding diving.
• Finite element analysis of the effects of focal adhesion mechanical properties and substrate stiffness on cell migration
  - J Biomech 44(6):1046-1050 (2011)
  The attachment of cells to the extracellular matrix (ECM) is achieved by the specific binding of cell-surface receptors to ligands present in the ECM. These interactions are important for many biological processes, including cell migration, cancer development, and wound healing. Our objective was to develop a computational model to investigate how focal adhesion mechanical properties, substrate stiffness, and intracellular stresses affect cell–matrix interactions during cell migration on a flat substrate. In our model, the cell–substrate traction was proportional to the bound receptor concentration, relative velocity between the cell and substrate, and the cell–substrate friction coefficient. Simulation results showed that even if the receptor number and ligand density were fixed, the mechanical properties of the focal adhesions still affected cell–ECM interactions. In fact, the cell–substrate traction was biphasic with respect to the friction coefficient, a ! parameter that can be used to quantify focal adhesion properties. In contrast, the cell speed was a monotonically decreasing function with respect to this parameter. Furthermore, tractions showed greater increases when the maximum intracellular stress was increased from 400 to 600 Pa than when substrate stiffness was increased from 0.5 to 100 kPa. This mathematical model is able to quantify the effects of focal adhesion mechanical properties, extracellular stiffness, and intracellular stresses on cell–ECM interactions, and should be beneficial to research in cancer development.
• Variability observed in mechano-regulated in vivo tissue differentiation can be explained by variation in cell mechano-sensitivity
  - J Biomech 44(6):1051-1058 (2011)
  Computational simulations of tissue differentiation have been able to capture the main aspects of tissue formation/regeneration observed in animal experiments—except for the considerable degree of variability reported. Understanding and modelling the source of this variability is crucial if computational tools are to be developed for clinical applications. The objective of this study was to test the hypothesis that differences in cell mechano-sensitivity between individuals can explain the variability of tissue differentiation patterns observed experimentally. Simulations of an experiment of tissue differentiation in a mechanically loaded bone chamber were performed. Finite element analysis was used to determine the biophysical environment, and a lattice-modelling approach was used to simulate cell activity. Differences in cell mechano-sensitivity among individuals were modelled as differences in cell activity rates, with the activation of cell activities regulated b! y the mechanical environment. Predictions of the tissue distribution in the chambers produced the two different classes of results found experimentally: (i) chambers with a layer of bone across the chamber covered by a layer of cartilage on top and (ii) chambers with almost no bone, mainly fibrous tissue and small islands of cartilage. This indicates that the differing cellular response to the mechanical environment (i.e., subject-specific mechano-sensitivity) could be a reason for the different outcomes found when implants (or tissue engineered constructs) are used in a population.
• Global biomechanical model for dental implants
  - J Biomech 44(6):1059-1065 (2011)
  The osseointegration of titanium dental implants is a complex process and there is a need for systematization of the factors influencing anchoring of implant. A common way of analyzing the strength of the fixation in bone is by measuring the torque required to remove the implants after healing. In this paper, a global biomechanical model is introduced and derived for removal torque situations. In this model, a gap is allowed to form between the bone and the implant and the size of the gap at fracture is a function of the surface roughness and can be shown to be directly related to the mean slope of the surface. The interfacial shear strength increases almost linearly with the mean slope and was also found to increase with an increase in the 2D surface roughness parameter, Ra. Besides the surface roughness, the design of the implant, the bone anatomy and the bone quality were shown to influence the interfacial shear strength. The Global biomechanical model can be used a! s a tool for optimizing the implant design and the surface topography to obtain high anchoring strength.
• The effect of holding time on nanoindentation measurements of creep in bone
  - J Biomech 44(6):1066-1072 (2011)
  Viscoelasticity may affect both the elastic and fracture characteristics of bone. Nanoindentation can be used to measure the creep behavior of bone by fitting the depth vs. time data at constant load to rheological models. However, the creep data may be influenced by latent effects arising during the loading phase of indentation. As such, the loading protocol, particularly the holding time, may affect the measured creep time constants. To characterize the effect of holding time on the measures, four cortical bone samples were prepared from four bovine femora and subjected to nanoindentation to measure the creep behavior. The creep time constants were found by fitting the indentation depth vs. time curve to three different rheological models: the standard linear solid, Burgers model, and a two-dashpot Kelvin model. All three models provided good fits to the data, which were relatively insensitive to the initial parameter estimates. The calculated creep time constants in! creased monotonically with increasing holding time for all three models. However, the relative differences between measurements within a single osteon, within a single sample, and between samples were maintained for creep holding times over 16 s. Hence, while the creep time constants measured by nanoindentation with hold times up to 30 s may not provide accurate property measurements, comparisons between samples are valid if all are assessed at the same holding time. Considering the long-term viscosity of bone tissue, Burgers model provided the best performance in terms of stability and goodness of fit, and is recommended for future studies.
• Conjugate momentum estimate using non-linear dynamic model of the sit-to-stand correlates well with accelerometric surface data
  - J Biomech 44(6):1073-1077 (2011)
  The purpose of this study was the development of a non-linear double inverted constrained pendulum model for the analysis of the movement of sit-to-stand (STS) transition. Ten able-bodied subjects perform five trials in their natural speed. Kinematics, kinetics as well as body worn accelerometer data were collected during the STS task using optoelectronic motion capture, force plate and inertial measurement unit, respectively. The conjugate momentum for the whole body which includes linear and angular motion correlates well with the accelerometric surface spanned by the accelerometer data. The partitioning of the conjugate momentum indicates a clear coordination between upper and lower limb after seat-off period. Moreover, the normalization procedure indicates a clear minimal and somehow invariant threshold value of the conjugate momentum to approximately 0.3 (body mass×body length) to perform the sit-to-stand for able-bodied subject. This threshold correlates well wi! th the data obtained from accelerometeric index. The proposed accelerometric index is relevant to assess STS performance and to detect failed STS in clinics and outside a laboratory for patients with reduced mobility.
• Compression behavior of porcine spinal cord white matter
  - J Biomech 44(6):1078-1082 (2011)
  Spinal cord injury often results from a compressive load; however, the compression behavior of spinal cord white matter has not been clearly established. Quantifying the compression behavior is important for advancing our understanding of spinal cord injury mechanics and facilitating the use of finite element models to study injury. The objective of this study was to characterize the unconfined compression behavior of isolated white matter segments and determine the constitutive model which best captured the stress–strain behavior. Spinal cord white matter samples were harvested immediately following sacrifice from juvenile Yorkshire pigs (n=104). The samples were compressed to 40% strain at four strain rates (0.005, 0.05, 0.5, and 5.0/s) and allowed to relax for 60 s. The effects of preload, peak strain, sample aspect ratio, and time post mortem on peak stress, and constitutive model parameters were also examined. Strain rate had a significant effect on peak stress ! (p<0.001). A first-order Ogden model best captured the loading response of spinal cord white matter (R2=0.99) and a viscoelastic material model combining a first-order Ogden model with a 3-term Prony series effectively captured the effect of strain rate and the relaxation response. This study showed spinal cord white matter to be less stiff than previously estimated by inverse finite element methods, which will have a significant effect on finite element model predictions of the magnitude and distribution of stresses and strains in the spinal cord. This study is the first to quantify the unconfined compression response of spinal cord white matter.
• Estimation of minimum ground clearance (MGC) using body-worn inertial sensors
  - J Biomech 44(6):1083-1088 (2011)
  Objective assessment of balance and mobility in elderly populations using body-worn sensors has recently become a prevalent theme in falls-related research. Recent research by the authors identified mean absolute-valued vertical angular velocity measured using shank mounted inertial sensors during a timed-up-and-go test as having a strong association with falls history in a group of elderly adults. This study aimed to investigate the clinical relevance of this parameter by exploring the relationship between it and minimum ground clearance (MGC) measured with an optical motion capture system. MGC is an important variable when considering trip-related falls risk. This paper also presents a method of estimating properties of MGC during walking, across a range of speeds and gait patterns, using body-worn inertial sensors. We found that mean MGC and coefficient of variation (CV) MGC are correlated with mean absolute-valued vertical angular velocity and acceleration as measu! red by shank or foot mounted inertial sensors. Regression models generated using inertial sensor derived variables were used to robustly estimate the mean MGC and CV MGC measured by an optical marker-tracking system. Foot-mounted sensors were found to yield slightly better results than sensors on the shank. Different walking speeds and gait patterns were not found to influence the accuracy of the models. We conclude that these findings have the potential to evaluate a walking trial using body-worn inertial sensors, which could then be used to identify individuals with increased risk of unprovoked collisions with the ground during locomotion.
• The influence of boundary conditions on wall shear stress distribution in patients specific coronary trees
  - J Biomech 44(6):1089-1095 (2011)
  Patient specific geometrical data on human coronary arteries can be reliably obtained multislice computer tomography (MSCT) imaging. MSCT cannot provide hemodynamic variables, and the outflow through the side branches must be estimated. The impact of two different models to determine flow through the side branches on the wall shear stress (WSS) distribution in patient specific geometries is evaluated. Murray's law predicts that the flow ratio through the side branches scales with the ratio of the diameter of the side branches to the third power. The empirical model is based on flow measurements performed by Doriot et al. (2000) in angiographically normal coronary arteries. The fit based on these measurements showed that the flow ratio through the side branches can best be described with a power of 2.27. The experimental data imply that Murray's law underestimates the flow through the side branches. We applied the two models to study the WSS distribution in 6 coronary artery trees. Under steady flow conditions, the average WSS between the side branches differed significantly for the two models: the average WSS was 8% higher for Murray's law and the relative difference ranged from −5% to +27%. These differences scale with the difference in flow rate. Near the bifurcations, the differences in WSS were more pronounced: the size of the low WSS regions was significantly larger when applying the empirical model (13%), ranging from −12% to +68%. Predicting outflow based on Murray's law underestimates the flow through the side branches. Especially near side branches, the regions where atherosclerotic plaques preferentially develop, the differences are significant and application of Murray's law underestimates the size of the low WSS region.
• Insights gained into the interpretation of surface electromyograms from the gastrocnemius muscles: A simulation study
  - J Biomech 44(6):1096-1103 (2011)
  Interpretation of surface electromyograms (EMG) is usually based on the assumption that the surface representation of action potentials does not change during their propagation. This assumption does not hold for muscles whose fibers are oblique to the skin. Consequently, the interpretation of surface EMGs recorded from pinnate muscles unlikely prompts from current knowledge. Here we present a complete analytical model that supports the interpretation of experimental EMGs detected from muscles with oblique architecture. EMGs were recorded from the medial gastrocnemius muscle during voluntary and electrically elicited contractions. Preliminary indications obtained from simulated and experimental signals concern the spatial localization of surface potentials and the myoelectric fatigue. Specifically, the spatial distribution of surface EMGs was localized about the fibers superficial extremity. Strikingly, this localization increased with the pinnation angle, both for the ! simulated EMGs and the recorded M-waves. Moreover, the average rectified value (ARV) and the mean frequency (MNF) of interference EMGs increased and decreased with simulated fatigue, respectively. The degree of variation in ARV and MNF did not depend on the pinnation angle simulated. Similar variations were observed for the experimental EMGs, although being less evident for a higher fiber inclination. These results are discussed on a physiological context, highlighting the relevance of the model proposed here for the interpretation of gastrocnemius EMGs and for conceiving future experiments on muscles with pinnate geometry.
• Changes in running mechanics and spring–mass behavior induced by a mountain ultra-marathon race
  - J Biomech 44(6):1104-1107 (2011)
  Changes in running mechanics and spring–mass behavior due to fatigue induced by a mountain ultra-marathon race (MUM, 166 km, total positive and negative elevation of 9500 m) were studied in 18 ultra-marathon runners. Mechanical measurements were undertaken pre- and 3 h post-MUM at 12 km h−1 on a 7 m long pressure walkway: contact (tc), aerial (ta) times, step frequency (f), and running velocity (v) were sampled and averaged over 5–8 steps. From these variables, spring–mass parameters of peak vertical ground reaction force (Fmax), vertical downward displacement of the center of mass (Δz), leg length change (ΔL), vertical (kvert) and leg (kleg) stiffness were computed. After the MUM, there was a significant increase in f (5.9±5.5%; P<0.001) associated with reduced ta (−18.5±17.4%; P<0.001) with no change in tc, and a significant decrease in both Δz and Fmax (−11.6±10.5 and −6.3±7.3%, respectively; P<0.001). kvert increased by 5.6±11.7% (P=0.053), an! d kleg remained unchanged. These results show that 3 h post-MUM, subjects ran with a reduced vertical oscillation of their spring–mass system. This is consistent with (i) previous studies concerning muscular structure/function impairment in running and (ii) the hypothesis that these changes in the running pattern could be associated with lower overall impact (especially during the braking phase) supported by the locomotor system at each step, potentially leading to reduced pain during running.
• Computational wear prediction of artificial knee joints based on a new wear law and formulation
  - J Biomech 44(6):1108-1116 (2011)
  Laboratory joint wear simulator testing has become the standard means for preclinical evaluation of wear resistance of artificial knee joints. Recent simulator designs have been advanced and become successful at reproducing the wear patterns observed in clinical retrievals. However, a single simulator test can be very expensive and take a long time to run. On the other hand computational wear modelling is an alternative attractive solution to these limitations. Computational models have been used extensively for wear prediction and optimisation of artificial knee designs. However, all these models have adopted the classical Archard's wear law, which was developed for metallic materials, and have selected wear factors arbitrarily. It is known that such an approach is not generally true for polymeric bearing materials and is difficult to implement due to the high dependence of the wear factor on the contact pressure. Therefore, these studies are generally not independent! and lack general predictability. The objective of the present study was to develop a new computational wear model for the knee implants, based on the contact area and an independent experimentally determined non-dimensional wear coefficient. The effects of cross-shear and creep on wear predictions were also considered. The predicted wear volume was compared with the laboratory simulation measurements. The model was run under two different kinematic inputs and two different insert designs with curved and custom designed flat bearing surfaces. The new wear model was shown to be capable of predicting the difference of the wear volume and wear pattern between the two kinematic inputs and the two tibial insert designs. Conversely, the wear factor based approach did not predict such differences. The good agreement found between the computational and experimental results, on both the wear scar areas and volumetric wear rates, suggests that the computational wear modelling based o! n the new wear law and the experimentally calculated non-dimen! sional wear coefficient should be more reliable and therefore provide a more robust virtual modelling platform.
• An elasto-hydrodynamical model of friction for the locomotion of Caenorhabditis elegans
  - J Biomech 44(6):1117-1122 (2011)
  Caenorhabditis elegans (C. elegans) is one of the most studied organisms by biologists. Composed of around one thousand cells, easy to culture and to modify genetically, it is a good model system to address fundamental physiological questions and in particular to investigate neuromuscular processes. Many C. elegans mutants can be distinguished by their locomotion phenotype and it then important to understand the biomechanics of their locomotion and in particular the mechanics of their undulating crawling motion on agar aqueous gels where they are commonly grown and observed. In this article, we present a mechanical model of the friction of the worms on their substrate where we have included capillarity (which pins the worm of the gel), the hydrodynamics of the lubrication film (between worm and gel) and the substrate/body elasticity. We determine the ratio of the transverse to longitudinal friction coefficients of the worm body on the culture gel as a function of a con! trol parameter which describes the relative role of the deformation of the gel and the viscous dissipation in the lubrication film. Experimentally this ratio is – for soft gels – larger than the maximal value predicted by our model (this maximum is equal to 2, the value for an infinite cylinder in bulk liquid) and we propose to include the plasticity of the gel (i.e. the dissipation of the deformation of the gel) for a better description of the worm/gel interaction.
• Effect of non-Newtonian and pulsatile blood flow on mass transport in the human aorta
  - J Biomech 44(6):1123-1131 (2011)
  To investigate the effects of both non-Newtonian behavior and the pulsation of blood flow on the distributions of luminal surface LDL concentration and oxygen flux along the wall of the human aorta, we numerically compared a non-Newtonian model with the Newtonian one under both steady flow and in vivo pulsatile flow conditions using a human aorta model constructed from MRI images. The results showed that under steady flow conditions, although the shear thinning non-Newtonian nature of blood could elevate wall shear stress (WSS) in most regions of the aorta, especially areas with low WSS, it had little effect on luminal surface LDL concentration (cw) in most regions of the aorta. Nevertheless, it could significantly enhance cw in areas with high luminal surface LDL concentration through the shear dependent diffusivity of LDLs. For oxygen transport, the shear thinning non-Newtonian nature of blood could slightly reduce oxygen flux in most regions of the aorta, but this e! ffect became much more apparent in areas with already low oxygen flux. The pulsation of blood flow could significantly reduce cw and enhance oxygen flux in these disturbed places. In most other regions of the aorta, the oxygen flux was also significantly higher than that for the steady flow simulation. In conclusion, the shear shining non-Newtonian nature of blood has little effect on LDL and oxygen transport in most regions of the aorta, but in the atherogenic-prone areas where luminal surface LDL concentration is high and oxygen flux is low, its effect is apparent. Similar is for the effect of pulsatile flow on the transport of LDLs. But, the pulsation of blood flow can apparently affect oxygen flux in the aorta, especially in areas with low oxygen flux.
• Determination of the relationship between collagen cross-links and the bone–tissue stiffness in the porcine mandibular condyle
  - J Biomech 44(6):1132-1136 (2011)
  Although bone–tissue stiffness is closely related to the degree to which bone has been mineralized, other determinants are yet to be identified. We, therefore, examined the extent to which the mineralization degree, collagen, and its cross-links are related to bone–tissue stiffness. A total of 50 cancellous and cortical bone samples were derived from the right mandibular condyles of five young and five adult female pigs. The degree of mineralization of bone (DMB) was assessed using micro-computed tomography. Using high-performance liquid chromatography, we quantified the collagen content and the number of cross-links per collagen molecule of two enzymatic cross-links: hydroxylysylpyridinoline (HP) and lysylpyridinoline (LP), and one non-enzymatic cross-link: pentosidine (Pen). Nanoindentation was used to assess bone–tissue stiffness in three directions, and multiple linear regressions were used to calculate the correlation between collagen properties and bone–tissue stiffness, with the DMB as first predictor. Whereas the bone–tissue stiffness of cancellous bone did not differ between the three directions of nanoindentation, or between the two age groups, cortical bone–tissue stiffness was higher in the adult tissue. After correction for DMB, the cross-links studied did not increase the explained variance. In the young group, however, LP significantly improved the explained variance in bone–tissue stiffness. Approximately half of the variation in bone–tissue stiffness in cancellous and cortical bone was explained by the DMB and the LP cross-links and thus they cannot be considered the sole determinants of the bone–tissue stiffness.
• The ventilation distribution of helium–oxygen mixtures and the role of inertial losses in the presence of heterogeneous airway obstructions
  - J Biomech 44(6):1137-1143 (2011)
  The regional distribution of inhaled gas within the lung is affected in part by normal variations in airway geometry or by obstructions resulting from disease. In the present work, the effects of heterogeneous airway obstructions on the distribution of air and helium–oxygen were examined using an in vitro model, the two compartments of a dual adult test lung. Breathing helium–oxygen resulted in a consistently more uniform distribution, with the gas volume delivered to a severely obstructed compartment increased by almost 80%. An engineering approach to pipe flow was used to analyze the test lung and was extrapolated to a human lung model to show that the in vitro experimental parameters are relevant to the observed in vivo conditions. The engineering analysis also showed that helium–oxygen can decrease the relative weight of the flow resistance due to obstructions if they are inertial in nature (i.e., density dependent) due to either turbulence or laminar convect! ive losses.
• Relative movements between the tibia and femur induced by external plantar shocks are controlled by muscle forces in vivo
  - J Biomech 44(6):1144-1148 (2011)
  The purpose of this study was to investigate the role of muscle activation on the relative motion between tibia and femur. Impacts were initiated under the heels of four volunteers in three different activation levels of muscles crossing the extended knee joint: 0%, 30% and 60% of previously performed maximal voluntary isometric contractions. Impact forces were measured and tibial and femoral accelerations and displacements were determined by means of accelerometry. The accelerometers were mounted on the protruding ends of intracortical pins, inserted into the distal aspect of the femur and proximal aspect of the tibia. Under the 0%-condition the impact force (475±64 N) led to 2.3±1.2 mm knee compression and to 2.4±1.9 mm medio-lateral and 4.4±1.1 mm antero-posterior shear. The impact forces increased significantly with higher activation levels (619±33 N (30%), 643±147 N (60%)), while the knee compression (1.5±1.2, 1.4±1.3 mm) and both medio-lateral shear (1.8�! �1.4, 1.5±1.1 mm) and antero-posterior shear (2.6±1.3, 1.5±1.1 mm) were significantly reduced. This study indicated that muscles are effective in controlling the relative motion between tibia and femur when the knee is subjected to external forces.
• In vivo dynamic strains of the ovine anterior mitral valve leaflet
  - J Biomech 44(6):1149-1157 (2011)
  Understanding the mechanics of the mitral valve is crucial in terms of designing and evaluating medical devices and techniques for mitral valve repair. In the current study we characterize the in vivo strains of the anterior mitral valve leaflet. On cardiopulmonary bypass, we sew miniature markers onto the leaflets of 57 sheep. During the cardiac cycle, the coordinates of these markers are recorded via biplane fluoroscopy. From the resulting four-dimensional data sets, we calculate areal, maximum principal, circumferential, and radial leaflet strains and display their profiles on the averaged leaflet geometry. Average peak areal strains are 13.8±6.3%, maximum principal strains are 13.0±4.7%, circumferential strains are 5.0±2.7%, and radial strains are 7.8±4.3%. Maximum principal strains are largest in the belly region, where they are aligned with the circumferential direction during diastole switching into the radial direction during systole. Circumferential strain! s are concentrated at the distal portion of the belly region close to the free edge of the leaflet, while radial strains are highest in the center of the leaflet, stretching from the posterior to the anterior commissure. In summary, leaflet strains display significant temporal, regional, and directional variations with largest values inside the belly region and toward the free edge. Characterizing strain distribution profiles might be of particular clinical significance when optimizing mitral valve repair techniques in terms of forces on suture lines and on medical devices.
• Measurement of the hyperelastic properties of ex vivo brain tissue slices
  - J Biomech 44(6):1158-1163 (2011)
  The elastic and hyperelastic properties of brain tissue are of interest to the medical research community as there are several applications where accurate characterization of these properties is crucial for an accurate outcome. The linear response is applicable to brain elastography, while the non-linear response is of interest for surgical simulation programs. Because of the biological differences between gray and white matter, it is reasonable to expect a difference in their mechanical properties. The goal of this work is to characterize the elastic and hyperelastic properties of the brain gray and white matter. In this method, force–displacement data of these tissues were acquired from 25 different brain samples using an indentation apparatus. These data were processed with an inverse problem algorithm using finite element method as the forward problem solver. Young's modulus and the hyperelastic parameters corresponding to the commonly used Polynomial, Yeoh, Ar! ruda–Boyce, and Ogden models were obtained. The parameters characterizing the linear and non-linear mechanical behavior of gray and white matters were found to be significantly different. Young's modulus was 1787±186 and 1195±157 Pa for white matter and gray matter, respectively. Among hyperelastic models, due to its accuracy, fewer parameters and shorter computational time requirements, Yeoh model was found to be the most suitable. Due to the significant differences between the linear and non-linear tissue response, we conclude that incorporating these differences into brain biomechanical models is necessary to increase accuracy.
• Damage accumulation in vertebral trabecular bone depends on loading mode and direction
  - J Biomech 44(6):1164-1169 (2011)
  Osteoporotic vertebral fractures constitute a major clinical problem in ageing societies. A third of all vertebral fractures is caused by falls, 15% by lifting heavy loads or traffic accidents and over 50% are not relatable to a traumatic event. In the latter case vertebrae show sinter processes which indicate the accumulation of damage and permanent deformation. Accumulated damage may not be visible on radiographs but increases the risk of fracture and could lead to vertebral collapse. Clear understanding of the accumulation of damage and residual strains and their dependence on loading mode and direction is important for understanding vertebral fractures. Altogether, 251 cylindrical samples (8×18–25 mm) were obtained from 50 male and 54 female fresh frozen human vertebrae (T1–L3) of 65 (21–94) years. Vertebrae were randomly assigned to three groups cranial–caudal, anterior–posterior and latero-lateral. Specimens were mechanically loaded in compression, tension or torsion in five load steps at a strain rate of 0.2%/s. Three conditioning cycles were driven per load step. Stress–strain curves were reconstructed from the force–displacement or from the moment–twist angle curves. Damage accumulated from 0 to 86% in compression, from 0 to 76% in tension and from 0 to 86% in torsion through the five load steps. Residual strains accumulated from 0 to −0.008 mm/mm in compression, 0 to 0.006 mm/mm in tension and 0 to 0.026 rad/rad in torsion. Significantly less damage (p<0.05) but not residual strains accumulated in transverse directions. This study provides detailed experimental insights into the damage behaviour of vertebral trabecular bone under various loads occurring in vivo. Damage but not residual strain evolution seems to be anisotropic. Both seem to evolve differently under different loading modes. The results could be of importance in understanding vertebral fractures.
• Joint forces and torques when walking in shallow water
  - J Biomech 44(6):1170-1175 (2011)
  This study reports for the first time an estimation of the internal net joint forces and torques on adults' lower limbs and pelvis when walking in shallow water, taking into account the drag forces generated by the movement of their bodies in the water and the equivalent data when they walk on land. A force plate and a video camera were used to perform a two-dimensional gait analysis at the sagittal plane of 10 healthy young adults walking at comfortable speeds on land and in water at a chest-high level. We estimated the drag force on each body segment and the joint forces and torques at the ankle, knee, and hip of the right side of their bodies using inverse dynamics. The observed subjects' apparent weight in water was about 35% of their weight on land and they were about 2.7 times slower when walking in water. When the subjects walked in water compared with walking on land, there were no differences in the angular displacements but there was a significant reduction i! n the joint torques which was related to the water's depth. The greatest reduction was observed for the ankle and then the knee and no reduction was observed for the hip. All joint powers were significantly reduced in water. The compressive and shear joint forces were on average about three times lower during walking in water than on land. These quantitative results substantiate the use of water as a safe environment for practicing low-impact exercises, particularly walking.
• In vitro indentation to determine the mechanical properties of epidermis
  - J Biomech 44(6):1176-1181 (2011)
  The lack of understanding of the mechanical behavior of the human skin layers makes the development of drug delivery using microneedles or microjets a challenging task. In particular, the key mechanical properties of the epidermis composed of stratum corneum and viable epidermis should be better understood. Micro-indentation experiments were applied, using a spherical tip with a large diameter to the sample thickness ratio. The Young's moduli were derived via an analytical and a numerical method. The tests showed that the analytical method was not appropriate to assess the Young's moduli. That is why a numerical model was used to obtain the correct stiffness. When loaded perpendicularly, the stiffness of both the epidermis and stratum corneum vary between 1 and 2 MPa. No significant differences in stiffness between the stratum corneum and viable epidermis were observed.
• Effects of soft tissue artifacts on the calculated kinematics and kinetics of the knee during stair-ascent
  - J Biomech 44(6):1182-1188 (2011)
  Biomechanics of the knee during stair-ascent has mostly been studied using skin-marker-based motion analysis techniques, but no study has reported a complete assessment of the soft tissue artifacts (STA) and their effects on the calculated joint center translation, angles and moments at the knee in normal subjects during this activity. This study aimed to bridge the gap. Twelve young adults walked up a three-step stair while data were acquired simultaneously from a three-dimensional motion capture system, a force plate and a dynamic fluoroscopy system. The "gold standards" of poses of the knee were obtained using a 3D fluoroscopy method. The STA of the markers on the thigh and shank were then calculated, together with their effects on the calculated joint center translations, angles and moments at the knee. The STA of the thigh markers were greater than those on the shank, leading to significantly underestimated flexion and extensor moments, but overestimated joint! center translations during the first half of the stance phase. The results will be useful for a better understanding of the normal biomechanics of the knee during stair-ascent, as a baseline for future clinical applications and for developing a compensation method to correct for the effects of STA.
• Whole body vibration increases area and stiffness of the flexor carpi ulnaris tendon in the rat
  - J Biomech 44(6):1189-1191 (2011)
  Whole body vibration (WBV) has been extensively studied as an anabolic stimulus for bone and muscle. Therapeutic WBV delivers low magnitude, high frequency vibrations to tissues, eliciting biological and structural responses. This study investigated the effect of 0.3 G (Peak-to-Peak), 30 Hz sinusoidal vibration on intact flexor carpi ulnaris tendons in rats. Experimental rats were subjected to twenty minutes of WBV daily for five days a week for a total of five weeks. The tendon cross-sectional area and the structural properties of the muscle–tendon–bone unit under tensile loading to failure were evaluated. Initial body weights were similar between the groups and the mean change in body weight of the animals of each group did not differ. The cross-sectional area of the tendons of the vibrated animals was found to be 32% greater (P<0.05) than the controls and the structural stiffness of the vibrated tendons was found to be 41% greater (P<0.05) than the controls. For! specimens that failed in the midsubstance of the tendon, a trend (P=0.087) for increased ultimate load was observed in the vibrated tendons compared to the controls. No differences in material properties were observed except for the strain to ultimate load, which was reduced 22% in the vibrated group. These initial findings suggest that vibration may serve as an anabolic stimulus to tendon similar to its effects on bone and muscle. These findings are important as they open the potential that low magnitude, high frequency vibration might serve as a means to accelerate tendon healing.
• The effect of the conjoined tendon of the short head of the biceps and coracobrachialis on shoulder stability and kinematics during in-vitro simulation
  - J Biomech 44(6):1192-1195 (2011)
  The kinematics and stability of the shoulder during in-vitro simulation are affected by the muscles chosen for simulation and their loads. Existing simulators have commonly actuated the rotator cuff and deltoids; however, the contribution of secondary muscles, such as those which form the conjoined tendon, are not well understood. The conjoined tendon consists of the origins of the short head of the biceps and coracobrachialis (SH&C), and is thought to produce an anterior stabilizing effect. This study investigated the effect of SH&C tension at four loading levels: 0, 5, 10, 15 N. Our primary outcome variable was glenohumeral stiffness for anterior loading but internal/external rotation and extension ranges of motion were also measured. Four joint configurations were tested: adduction and 90° combined abduction, each in neutral and maximal external rotation. Increasing SH&C load resulted in a significant trend of increased glenohumeral stiffness across the average of ! all joint configurations (p=0.008). In abduction, neutral rotation differences were found between the stiffness at 10 and 15 N compared to 0 N (p=0.038 and 0.043, respectively); however, no differences were found for the three other joint configurations. There was a tendency for a decrease in the range of shoulder extension with increasing SH&C load, but this did not achieve significance (p=0.065). These findings demonstrate that the SH&C provides a stabilizing barrier effect, but only in configurations when it wraps directly anterior to the humeral head. Thus SH&C loading is likely critical to in-vitro simulation due to its effect on joint stability and kinematics.
• Effect of geometric variations on pressure loss for a model bifurcation of the human lung airway
  - J Biomech 44(6):1196-1199 (2011)
  Characteristics of pressure loss (ΔP) in human lung airways were numerically investigated using a realistic model bifurcation. Flow equations were numerically solved for the steady inspiratory condition with the tube length, the branching angle and flow velocity being varied over a wide range. In general, the ΔP coefficient K showed a power-law dependence on Reynolds number (Re) and length-to-diameter ratio with a different exponent for Re≥100 than for Re<100. The effect of different branching angles on pressure loss was very weak in the smooth-branching airways.
• Computation of continuous relative phase and modulation of frequency of human movement
  - J Biomech 44(6):1200-1204 (2011)
  Continuous relative phase measures have been used to quantify the coordination between different body segments in several activities. Our aim in this study was to investigate how the methods traditionally used to compute the continuous phase of human rhythmic movement are affected by modulations of frequency. We compared the continuous phase computed method with the traditional method derived from the position–velocity phase plane and with the Hilbert Transform. The methods were tested using sinusoidal signals with a modulation of frequency between or within cycles. Our results showed that the continuous phase computed with the first method results in oscillations in the phase time-series not expected for a sinusoidal signal and that the continuous phase is overestimated with the Hilbert Transform. We proposed a new method that produces a correct estimation of continuous phase by using half-cycle estimations of frequency to normalize the phase planes prior to calcula! ting phase angles. The findings of the current study have important implications for computing continuous relative phase when investigating human movement coordination.
• Major and minor centroidal axes serve as objective, automatable reference points to test mechanobiological hypotheses using histomorphometry
  - J Biomech 44(6):1205-1208 (2011)
  Recent studies show that minor and major centroidal axes (CA) of long bone cross sections provide valuable predictions of prevailing loading patterns in age and treatment matched cohorts of animals. Furthermore, using CA, we recently showed that the degree of mineralization and area of woven bone laid down in the first two weeks after creation of a critical sized bone defect relate inversely and correlate significantly to loading patterns. Here, we aim to determine how closely independent measures of the spatial distribution of bone apposition determined using the major and minor CA as reference points correlate to those using anatomically defined axes as reference points. In histological sections from the previous study, we found no statistically significant difference between the anatomical and centroidal axes with respect to the centroid location or axis rotation, but there is a significant albeit small difference in the average distance between centroids. Outcome m! easures calculated in areas of bone defined by 15°, 30°, 45°, 60°, or 90° sectors when using the CA differ less than 5% from those calculated using anatomical axes as reference points. Hence, the major and minor CA provide objective reference points for comparison of mechanobiological outcome measures between animals in matched cohorts. Calculation of major and minor CA is automated, which reduces the potential for observer bias. A major advantage of using the major and minor CA as reference points is that it allows for direct relation of outcome measures to loading patterns in age and treatment matched cohorts, ultimately providing a tool to test mechanobiological hypotheses on histological cross sections of bone.
• New interpretation of arterial stiffening due to cigarette smoking using a structurally motivated constitutive model
  - J Biomech 44(6):1209-1211 (2011)
  Cigarette smoking is the leading self-inflicted risk factor for cardiovascular diseases; it causes arterial stiffening with serious sequelea including atherosclerosis and abdominal aortic aneurysms. This work presents a new interpretation of arterial stiffening caused by smoking based on data published for rat pulmonary arteries. A structurally motivated "four fiber family" constitutive relation was used to fit the available biaxial data and associated best-fit values of material parameters were estimated using multivariate nonlinear regression. Results suggested that arterial stiffening caused by smoking was reflected by consistent increase in an elastin-associated parameter and moreover by marked increase in the collagen-associated parameters. That is, we suggest that arterial stiffening due to cigarette smoking appears to be isotropic, which may allow simpler phenomenological models to capture these effects using a single stiffening parameter similar to the appr! oach in isotropic continuum damage mechanics. There is a pressing need, however, for more detailed histological information coupled with more complete biaxial mechanical data for a broader range of systemic arteries.
• Design and validation of surface-marker clusters for the quantification of joint rotations in general movements in early infancy
  - J Biomech 44(6):1212-1215 (2011)
  Lack of complexity in general movements in early infancy is an important marker of potential motor disorders of neurological origin, such as cerebral palsy. Quantitative approaches to characterising this complexity are hampered by experimental difficulties in recording from infants in their first few months of life. The aim of this study was to design and validate bespoke surface-marker clusters to facilitate data acquisition and enable full quantification of joint rotations. The clusters were validated by recording the controlled movements of a soft-body dummy doll simultaneously with an optical (Qualisys) and inertial (XSens) motion capture system. The angles estimated from the optical system were compared with those measured by the inertial system. We demonstrate that the surface-marker based approach compares well with the use of an inertial system to obtain "direct" readings of the rotations whilst alleviating the issues associated with the use of an optical m! otion capture system. We briefly report use of this technique in 1–5 month old infants. By enabling full quantification of joint rotation, use of the custom made markers could pave the way for early diagnosis of movement disorders.
• Scaling of adult human body mass with height
  - J Biomech 44(6):1216 (2011)
  
• Erratum to "Optimization-based prediction of asymmetric human gait" [J. Biomech. 44 (4) (2011) 683–693]
  - J Biomech 44(6):1217 (2011)