Journal of Biomechanics - Articles in Press

Predicting the metabolic cost of incline walking from muscle activity and walking mechanics - Corrected Proof

Amy Silder, Thor Besier, Scott L. Delp — 2012-05-14

Abstract: The goal of this study was to identify which muscle activation patterns and gait features best predict the metabolic cost of inclined walking. We measured muscle activation patterns, joint kinematics and kinetics, and metabolic cost in sixteen subjects during treadmill walking at inclines of 0%, 5%, and 10%. Multivariate regression models were developed to predict the net metabolic cost from selected groups of the measured variables. A linear regression model including incline and the squared integrated electromyographic signals of the soleus and vastus lateralis explained 96% of the variance in metabolic cost, suggesting that the activation patterns of these large muscles have a high predictive value for metabolic cost. A regression model including only the peak knee flexion angle during stance phase, peak knee extension moment, peak ankle plantarflexion moment, and peak hip flexion moment explained 89% of the variance in metabolic cost; this finding indicates that kinematics and kinetics alone can predict metabolic cost during incline walking. The ability of these models to predict metabolic cost from muscle activation patterns and gait features points the way toward future work aimed at predicting metabolic cost when gait is altered by changes in neuromuscular control or the use of an assistive technology.

Role of gastrocnemius–soleus muscle in forefoot force transmission at heel rise — A 3D finite element analysis - Corrected Proof

Wen-Ming Chen, Jaeyoung Park, Seung-Bum Park, Victor Phyau-Wui Shim, Taeyong Lee — 2012-05-14

Abstract: The functions of the gastrocnemius–soleus (G–S) complex and other plantar flexor muscles are to stabilize and control major bony joints, as well as to provide primary coordination of the foot during the stance phase of gait. Geometric positioning of the foot and transferring of plantar loads can be adversely affected when muscular control is abnormal (e.g., equinus contracture). Although manipulation of the G–S muscle complex by surgical intervention (e.g., tendo-Achilles lengthening) is believed to be effective in restoring normal plantar load transfer in the foot, there is lack of quantitative data supporting that notion. Thus, the objective of this study is to formulate a three-dimensional musculoskeletal finite element model of the foot to quantify the precise role of the G–S complex in terms of biomechanical response of the foot.The model established corresponds to a muscle-demanding posture during heel rise, with simulated activation of major extrinsic plantar flexors. In the baseline (reference) case, required muscle forces were determined from what would be necessary to generate the targeted resultant ground reaction forces. The predicted plantar load transfer through the forefoot plantar surface, as indicated by plantar pressure distribution, was verified by comparison with experimental observations. This baseline model served as a reference for subsequent parametric analysis, where muscle forces applied by the G–S complex were decreased in a step-wise manner.Adaptive changes of the foot mechanism, in terms of internal joint configurations and plantar stress distributions, in response to altered muscular loads were analyzed. Movements of the ankle and metatarsophalangeal joints, as well as forefoot plantar pressure peaks and pressure distribution under the metatarsal heads (MTHs), were all found to be extremely sensitive to reduction in the muscle load in the G–S complex. A 40% reduction in G–S muscle stabilization can result in dorsal-directed rotations of 8.81° at the ankle, and a decreased metatarsophalangeal joint extension of 4.65°. The resulting peak pressure reductions at individual MTHs, however, may be site-specific and possibly dependent on foot structure, such as intrinsic alignment of the metatarsals. The relationships between muscular control, internal joint movements, and plantar load distributions are envisaged to have important clinical implications on tendo-Achilles lengthening procedures, and to provide surgeons with an understanding of the underlying mechanism for relieving forefoot pressure in diabetic patients suffering from ankle equinus contracture.

A three-compartment muscle fatigue model accurately predicts joint-specific maximum endurance times for sustained isometric tasks - Corrected Proof

Laura A. Frey-Law, John M. Looft, Jesse Heitsman — 2012-05-11

Abstract: The development of localized muscle fatigue has classically been described by the nonlinear intensity–endurance time (ET) curve (). These empirical intensity–ET relationships have been well-documented and vary between joint regions. We previously proposed a three-compartment biophysical fatigue model, consisting of compartments (i.e. states) for active (MA), fatigued (MF), and resting (MR) muscles, to predict the decay and recovery of muscle force (). The purpose of this investigation was to determine optimal model parameter values, fatigue (F) and recovery (R), which define the “flow rate” between muscle states and to evaluate the model's accuracy for estimating expected intensity–ET curves. Using a grid-search approach with modified Monte Carlo simulations, over 1 million F and R permutations were used to predict the maximum ET for sustained isometric tasks at 9 intensities ranging from 10% to 90% of maximum in 10% increments (over 9 million simulations total). Optimal F and R values ranged from 0.00589 (Fankle) and 0.0182 (Rankle) to 0.00058 (Fshoulder) and 0.00168 (Rshoulder), reproducing the intensity–ET curves with low mean RMS errors: shoulder (2.7s), hand/grip (5.6s), knee (6.7s), trunk (9.3s), elbow (9.9s), and ankle (11.2s). Testing the model at different task intensities (15–95% maximum in 10% increments) produced slightly higher errors, but largely within the 95% prediction intervals expected for the intensity–ET curves. We conclude that this three-compartment fatigue model can be used to accurately represent joint-specific intensity–ET curves, which may be useful for ergonomic analyses and/or digital human modeling applications.

Deposition of micrometer particles in pulmonary airways during inhalation and breath holding - Corrected Proof

Yohsuke Imai, Takahito Miki, Takuji Ishikawa, Takayuki Aoki, Takami Yamaguchi — 2012-05-07

Abstract: We investigated how breath holding increases the deposition of micrometer particles in pulmonary airways, compared with the deposition during inhalation period. A subject-specific airway model with up to thirteenth generation airways was constructed from multi-slice CT images. Airflow and particle transport were simulated by using GPU computing. Results indicate that breath holding effectively increases the deposition of 5μm particles for third to sixth generation (G3–G6) airways. After 10s of breath holding, the particle deposition fraction increased more than 5 times for 5μm particles. Due to a small terminal velocity, 1μm particles only showed a 50% increase in the most efficient case. On the other hand, 10μm particles showed almost complete deposition due to high inertia and high terminal velocity, leading to an increase of 2 times for G3–G6 airways. An effective breath holding time for 5μm particle deposition in G3–G6 airways was estimated to be 4–6s, for which the deposition amount reached 75% of the final deposition amount after 10s of breath holding.

Functional foot symmetry and its relation to lower extremity physical performance in older adults: The Framingham Foot Study - Corrected Proof

J.L. Riskowski, T.J. Hagedorn, A.B. Dufour, M.T. Hannan — 2012-05-07

Abstract: Background: While many studies use gait symmetry as a marker of healthy gait, the evidence that gait symmetry exists is limited. Because gait symmetry is thought to arise through laterality (i.e., limb preference) and affects gait retraining efforts, it is important to understand if symmetry exists during gait in older adults. Therefore, the purpose of this study was to evaluate foot and gait symmetry in the population-based Framingham Foot Study as well as to determine the effects of vertical force symmetry on physical performance measures.Methods: Members of the Framingham Foot Study were included in this analysis (N=1333). Foot function and force data were collected using the Tekscan Matscan during self-selected gait, with symmetry evaluated using the symmetry index. The short physical performance battery (SPPB) measures of balance, chair stands and gait speed assessed lower extremity physical function. Participants were evaluated using quartiles of gait speed and foot symmetry to determine the effects of symmetry on lower extremity physical function.Results: Individuals with faster gait speed displayed greater foot function asymmetry; individuals with −3.0% to −9.5% asymmetry in foot function performed better on the short physical performance battery (SPPB). Further, with aging, the degree of asymmetry was reduced.Conclusions: While this research suggests that a moderate degree of foot asymmetry is associated with better lower extremity function, the causes of vertical force asymmetry are unknown. Future studies should evaluate the causes of foot asymmetry and should track the changes in symmetry that occur with aging.

Prediction of trabecular bone principal structural orientation using quantitative ultrasound scanning - Corrected Proof

Liangjun Lin, Jiqi Cheng, Wei Lin, Yi-Xian Qin — 2012-05-07

Abstract: Bone has the ability to adapt its structure in response to the mechanical environment as defined as Wolff's Law. The alignment of trabecular structure is intended to adapt to the particular mechanical milieu applied to it. Due to the absence of normal mechanical loading, it will be extremely important to assess the anisotropic deterioration of bone during the extreme conditions, i.e., long term space mission and disease orientated disuse, to predict risk of fractures. The propagation of ultrasound wave in trabecular bone is substantially influenced by the anisotropy of the trabecular structure. Previous studies have shown that both ultrasound velocity and amplitude is dependent on the incident angle of the ultrasound signal into the bone sample. In this work, seven bovine trabecular bone balls were used for rotational ultrasound measurement around three anatomical axes to elucidate the ability of ultrasound to identify trabecular orientation. Both ultrasound attenuation (ATT) and fast wave velocity (UV) were used to calculate the principal orientation of the trabecular bone. By comparing to the mean intercept length (MIL) tensor obtained from μCT, the angle difference of the prediction by UV was 4.45°, while it resulted in 11.67° angle difference between direction predicted by μCT and the prediction by ATT. This result demonstrates the ability of ultrasound as a non-invasive measurement tool for the principal structural orientation of the trabecular bone.

Evaluation of the influence of strain rate on Colles' fracture load - Corrected Proof

Ani Ural, Peter Zioupos, Drew Buchanan, Deepak Vashishth — 2012-05-07

Abstract: Colles' fracture, a transverse fracture of the distal radius bone, is one of the most frequently observed osteoporotic fractures resulting from low energy or traumatic events, associated with low and high strain rates, respectively. Although experimental studies on Colles' fracture were carried out at various loading rates ranging from static to impact loadings, there is no systematic study in the literature that isolates the influence of strain rate on Colles' fracture load. In order to provide a better understanding of fracture risk, the current study combines experimental material property measurements under varying strain rates with computational modeling and presents new information on the effect of strain rate on Colles' fracture. The simulation results showed that Colles' fracture load decreased with increasing strain rate with a steeper change in lower strain rates. Specifically, strain rate values (0.29s−1) associated with controlled falling without fracture corresponded to a 3.7% reduction in the fracture load. On the other hand, the reduction in the fracture load was 34% for strain rate of 3.7s−1 reported in fracture inducing impact cadaver experiments. Further increase in the strain rate up to 18s−1 led to an additional 22% reduction. The most drastic reduction in fracture load occurs at strain rates corresponding to the transition from controlled to impact falling. These results are particularly important for the improvement of fracture risk assessment in the elderly because they identify a critical range of loading rates (10–50mm/s) that can dramatically increase the risk of Colles' fracture.

OxLDL and substrate stiffness promote neutrophil transmigration by enhanced endothelial cell contractility and ICAM-1 - Corrected Proof

Kimberly M. Stroka, Irena Levitan, Helim Aranda-Espinoza — 2012-05-04

Abstract: Elevated levels of oxLDL in the bloodstream and increased vasculature stiffness are both associated with cardiovascular disease in patients. However, it is not known how oxLDL and subendothelial matrix stiffness together regulate an immune response. Here, we used an in vitro model of the vascular endothelium to explore the combined effects of oxLDL and subendothelial matrix stiffening on neutrophil transmigration. We prepared fibronectin-coated polyacrylamide gels of varying stiffness and plated human umbilical vein endothelial cells (ECs) onto the gels. We observed that oxLDL treatment of the endothelium promoted neutrophil transmigration (from <1% to 26% on soft 0.87kPa substrates), with stiffer substrates further promoting transmigration (54% on 5kPa and 41% on 280kPa). OxLDL exposure enhanced intercellular adhesion molecule-1 (ICAM-1) expression on the endothelium, which was likely responsible for the oxLDL-induced transmigration. Importantly, inhibition of MLCK-mediated EC contraction reduced transmigration to ∼9% on all substrates and eliminated the effects of subendothelial matrix stiffness. In addition, large holes, thousands of square microns in size, formed in monolayers on stiff substrates following transmigration, indicating that oxLDL treatment and subsequent neutrophil transmigration caused serious damage to the endothelium. Our results reveal that an interplay between ICAM-1 and MLCK-dependent contractile forces mediates neutrophil transmigration through oxLDL-treated endothelium. Thus, microvasculature stiffness, which likely varies depending on tissue location and health, is an important regulator of the transmigration step of the immune response in the presence of oxLDL.

Leg stiffness can be maintained during reactive hopping despite modified acceleration conditions - Corrected Proof

A. Kramer, R. Ritzmann, M. Gruber, A. Gollhofer — 2012-05-03

Abstract: Aim: The aim of the present study was to evaluate reactive hops under systematically modified acceleration conditions. It was hypothesized that a high preactivity of the leg extensors and phase-specific adjustments of the leg muscle activation would compensate the alterations caused by the various acceleration levels in order to maintain a high leg stiffness, thus enabling the jumper to perform truly reactive jumps with short ground contact times despite the unaccustomed acceleration conditions.Methods: Ground reaction forces (GRF), kinematic and electromyographic data of 20 healthy subjects were recorded during reactive hopping in a special sledge jump system for seven different acceleration levels: three acceleration levels with lower than normal gravity (0.7g, 0.8g, 0.9g), one with gravitational acceleration (1g) and three with higher acceleration (1.1g, 1.2g, 1.3g).Results: The increase of the acceleration from 0.7g to 1.3g had no significant effect on the preactivity of the leg extensors, the leg stiffness and the rate of force development. However, it resulted in increased peak GRF (+15%), longer ground contact time (+10%) and increased angular excursion at the ankle and knee joints (+3°).Discussion: Throughout a wide acceleration range, the subjects were able to maintain a high leg stiffness and perform reactive hops by keeping the preactivity constantly high and adjusting the muscle activity in the later phases. In consequence, it can be concluded that the neuromuscular system can cope with different acceleration levels, at least in the acceleration range used in this study.

Improved prediction of disturbed flow via hemodynamically-inspired geometric variables - Corrected Proof

Payam B. Bijari, Luca Antiga, Diego Gallo, Bruce A. Wasserman, David A. Steinman — 2012-05-01

Abstract: Arterial geometry has long been considered as a pragmatic alternative for inferring arterial flow disturbances, and their impact on the natural history and treatment of vascular diseases. Traditionally, definition of geometric variables is based on convenient shape descriptors, with only superficial consideration of their influence on flow and wall shear stress patterns. In the present study we demonstrate that a more studied consideration of the actual (cf. nominal) local hemodynamics can lead to substantial improvements in the prediction of disturbed flow by geometry. Starting from a well-characterized computational fluid dynamics (CFD) dataset of 50 normal carotid bifurcations, we observed that disturbed flow tended to be confined proximal to the flow divider, whereas geometric variables previously shown to be significant predictors of disturbed flow included features distal to the flow divider in their definitions. Flaring of the bifurcation leading to flow separation was redefined as the maximum relative expansion of the common carotid artery (CCA), proximal to the flow divider. The beneficial effect of primary curvature on flow inertia, via suppression of flow separation, was characterized by the in-plane tortuosity of CCA as it enters the flare region. Multiple linear regressions of these redefined geometric variables against various metrics of disturbed flow revealed R2 values approaching 0.6, better than the roughly 0.3 achieved using the conventional shape-based variables, while maintaining their demonstrated real-world reproducibility. Such a hemodynamically-inspired approach to the definition of geometric variables may reap benefits for other applications where geometry is used as a surrogate marker of local hemodynamics.

Correlation between plantarflexor moment arm and preferred gait velocity in slower elderly men - Corrected Proof

Sabrina S.M. Lee, Stephen J. Piazza — 2012-05-01

Abstract: In this study, the relationship between musculoskeletal architecture of the lateral gastrocnemius muscle and gait velocity in elderly individuals was investigated using ultrasonography and standardized tests of physical performance in 20 older adult males. Musculoskeletal architecture parameters included moment arm, fascicle length, pennation angle, and muscle thickness. The Six Minute Walk Test (6MIN) and Four Metre Walk Velocity Test (4METRE) were used to determine preferred and maximum gait velocity, respectively. Only weak correlations were found for all 20 subjects taken together. After subjects were separated into faster and slower subgroups by preferred velocity using cluster analysis; however, a strong correlation was found between plantarflexion moment arm and 6MIN velocity in the slower group (R2=0.669, p=0.004). Examination of subgroup differences revealed that the slow subgroup was significantly older than the fast subgroup (p=0.034), and had average body mass (p=0.021) and body mass index (p=0.011) that were significantly greater. The strength of the correlation between plantarflexion moment arm and 6MIN velocity found for slower subjects is much greater than those previously reported for correlations between ankle strength or power and walking velocity. Further investigation is necessary to determine if a link exists between plantarflexor moment arm and gait velocity in older and heavier adults.

Construction of healthy arteries using computed tomography and virtual histology intravascular ultrasound - Corrected Proof

Hong Sun Ryou, Seungwook Kim, Sang Wook Kim, Seong Wook Cho — 2012-04-30

Abstract: Vessel geometry for numerical analysis is generally obtained by computed tomography (CT) or magnetic resonance imaging (MRI) and intravascular ultrasound (IVUS). Most medical imaging is obtained from patients for hemodynamic analysis due to the properties of vascular disease and the difficulties in angiography. To predict the site where plaque occurs and understand the progression of the lesion, however, it is necessary to take into consideration not only the diseased artery, but also the blood flow characteristics of healthy artery. In order to simulate healthy vessels prior to lesion formation, we performed CT and virtual histology intravascular ultrasound (VH-IVUS) on three actual patients and this data was used to develop criteria for healthy vessel construction, a method that virtually removes all intravascular plaque. The lumen of a vessel generated by CT and the lumen from VH-IVUS were compared, and the cross-sectional areas of plaque components (fibrous, fibrofatty, dense calcium, and necrotic) and the lumen from VH-IVUS were analyzed. Geometric differences in the healthy vessel and diseased vessel were analyzed, and flow characteristics of the healthy vessel and diseased vessel were compared through computational fluid dynamics simulation. Low average wall shear stress (AWSS) was distributed in the site where plaque was removed from the healthy vessel, and a high oscillatory shear index (OSI) was observed in the region proximal to the site where plaque previously existed. Low AWSS and high OSI are widely accepted indicators of plaque formation or the direction of plaque progression. A numerical model that effectively predicts lesion forming sites was also generated based on the healthy vessel construction method presented in this study.

A direct comparison of spine rotational stiffness and dynamic spine stability during repetitive lifting tasks - Corrected Proof

Ryan B. Graham, Stephen H.M. Brown — 2012-04-30

Abstract: Stability of the spinal column is critical to bear loads, allow movement, and at the same time avoid injury and pain. However, there has been a debate in recent years as to how best to define and quantify spine stability, with the outcome being that different methods are used without a clear understanding of how they relate to one another. Therefore, the goal of the present study was to directly compare lumbar spine rotational stiffness, calculated with an EMG-driven biomechanical model, to local dynamic spine stability calculated using Lyapunov analyses of kinematic data, during a series of continuous dynamic lifting challenges. Twelve healthy male subjects performed 30 repetitive lifts under three varying load and three varying rate conditions. With an increase in the load lifted (constant rate) there was a significant increase in mean, maximum, and minimum spine rotational stiffness (p<0.001) and a significant increase in local dynamic stability (p<0.05); both stability measures were moderately to strongly related to one another (r=−0.55 to −0.71). With an increase in lifting rate (constant load), there was also a significant increase in mean and maximum spine rotational stiffness (p<0.01); however, there was a non-significant decrease in the minimum rotational stiffness and a non-significant decrease in local dynamic stability (p>0.05). Weak linear relationships were found for the varying rate conditions (r=−0.02 to −0.27). The results suggest that spine rotational stiffness and local dynamic stability are closely related to one another, as they provided similar information when movement rate was controlled. However, based on the results from the changing lifting rate conditions, it is evident that both models provide unique information and that future research is required to completely understand the relationship between the two models. Using both techniques concurrently may provide the best information regarding the true effects of (in) stability under different loading and movement scenarios, and in comparing healthy and clinical populations.

Kinematic adaptations to a variable stiffness shoe: Mechanisms for reducing joint loading - Corrected Proof

K.A. Boyer, P. Federolf, C. Lin, B.M. Nigg, T.P. Andriacchi — 2012-04-30

Abstract: A recently described variable-stiffness shoe has been shown to reduce the adduction moment and pain in patients with medial-compartment knee osteoarthritis. The mechanism associated with how this device modifies overall gait patterns to reduce the adduction moment is not well understood. Yet this information is important for applying load modifying intervention for the treatment of knee osteoarthritis.A principal component analysis (PCA) was used to test the hypothesis that there are differences in the frontal plane kinematics that are correlated with differences in the ground reaction forces (GRFs) and center of pressure (COP) for a variable-stiffness compared to a constant-stiffness control shoe. Eleven healthy adults were tested in a constant-stiffness control shoe and a variable-stiffness shoe while walking at self-selected speeds. The PCA was performed on trial vectors consisting of all kinematic, GRF and COP data.The projection of trial vectors onto the linear combination of four PCs showed there were significant differences between shoes. The interpretation of the PCs indicated an increase in the ankle eversion, knee abduction and adduction, decreases in the hip adduction and pelvic obliquity angles and reduced excursion of both the COP and peak medial–lateral GRFs for the variable-stiffness compared to the control shoe.The variable-stiffness shoe produced a unique dynamic change in the frontal plane motion of the ankle, hip and pelvis that contributed to changes in the GRF and COP and thus reduced the adduction moment at a critical instant during gait suggesting a different mechanism that was seen with fixed interventions (e.g. wedges).

Interdependency of stress relaxation and afferent nerve discharge in rat small intestine - Corrected Proof

2012-04-30

Abstract: Background and aims: To be able to characterize intestinal mechano-electrical transduction, i.e. the mechanoreceptor behaviour, quantitative nerve studies with controlled and quantified stimulus are needed. This study aimed to determine the relationship between mechanical stress relaxation and afferent discharge adaptation evoked by fast isovolumetric bag distensions in the rat jejunum.Methods: Multiunit afferent activity was recorded in vivo from jejunum afferents from five male Wistar rats. The jejunum was distended via a bag at a distension speed of 0.5ml/s to volumes of 0.2, 0.25, 0.3 and 0.4ml, respectively. The distension was stopped and the volume was kept constant for 2min to induce stress relaxation. The pressure in the bag, the afferent discharge (spike rate) and the diameter of the segment during the relaxation time were recorded simultaneously.Results: The afferent discharge responses to distension showed a pattern with a peak during the sudden loading followed by decreasing activity with time. At distension volumes of 0.2, 0.25, 0.3 and 0.4ml, the afferent discharge declined faster and to a greater extent (94%, 91%,96% and 87%) than the stress decreased (55%, 45%, 59% and 56%) during stress relaxation (p<0.001). Both the stress and the afferent discharge during the constant volume distension were independent of the distension volumes (p>0.5). The stress and the afferent discharge during the distension can be described mathematically on the basis of the quasi-linear theory of viscoelasticity. The association between the stress and the afferent discharge during the constant volume distension is linear with the same slope under various distension volumes.Conclusions: Intestinal mechanoreceptors were sensitive to the stress stimulus and a linear association between the stress relaxation and afferent discharge adaptation was found. The quasi-linear theory of visco-elasticity can be transferred to analysis of mechanical stimulus evoked afferent discharge.

Computational models for wall shear stress estimation in scaffolds: A comparative study of two complete geometries - Corrected Proof

2012-04-30

Abstract: Fluid mechanical stimuli are known to upregulate cell differentiation and matrix formation. Since wall shear stress plays an important role various studies tried to estimate the scaffold fluid dynamic environment. However, because of the geometrical complexity, nearly all studies created their CFD model based on a submodel of the entire scaffold assuming that the model covers heterogeneity sufficiently. However to the authors’ knowledge no study exist providing guidelines in this matter.In a previous study we demonstrated that submodels are influenced by the boundary conditions, inevitable when flow channels are chopped off. For the current study we therefore developed μCT based models of two complete scaffold geometries (one titanium and one hydroxyapatite). Imposing a 0.04ml/min flow rate resulted in a surface area averaged wall shear stress of 1.41mPa for titanium and 1.09mPa for hydroxyapatite.In order to get insight in required model size we subdivided the domain in regions of different size. From our results we propose a model size between 6 and 10 times the average pore size. The wall shears stress should be calculated on a region at least one pore size away from the boundaries.These guidelines could be of use for computationally more costly simulations where it is not possible to simulate the complete scaffold domain.

A novel 3D shape context method based strain analysis on a rat stomach model - Corrected Proof

Donghua Liao, Jingbo Zhao, Hans Gregersen — 2012-04-30

Abstract: The stomach has the ability to change its geometry and volume during digestion. Thus, the stomach shape changes dynamically due to changes in contents and due to pressure from adjacent organs. Full-field strain analysis is therefore important for accurate estimation of the true deformation in this highly non-homogeneous, anisotropic organ. The aim of this study is to introduce a modified non-rigid image registration based 3D shape context method combined with a full-field strain analysis method to describe a distension-induced 3D gastric deformation. The geometry of a normal rat stomach at distension pressures from 0.05kPa to 0.8kPa were obtained by ultrasonic scanning. The full-field strain distribution of the 3D gastric model between the reference state and the distended state were computed on the basis of the improved 3D shape context method and full-field strain analysis method. The registered surface showed a good agreement with the real deformed surface for all distension states. However, the errors increased with the distension pressure due to increasing dissimilarity between the deformed and the reference surface. The strain distributions on the stomach surface were non-uniform with the largest deformation in the non-glandular part and the greater and lesser curvature when the pressure was higher than 0.2kPa. The wall stiffness of the non-glandular part was softer than that of the glandular part. The modelling analysis method which is closely allied with the non-rigid image registration and strain analysis provides a kinematically possible deformation mode of the gastric wall. This method can be potentially used for clinical data estimating the kinematical properties of the human visceral organs in health and disease.

The superposition principle applied to grasping an object producing moments outside anatomically-defined axes - Corrected Proof

Jason W. Robertson, Jamie A. Johnston — 2012-04-30

Abstract: The superposition principle suggests that motor commands can be divided into individually controlled components that summate to produce complex motor actions. Previous studies have examined the validity of this principle in human grasping by changing moments acting on an object about a single anatomically-defined axis and asking subjects to hold the object while their forearm was constrained. Superposition was reflected as separate control of the grip force and moments required to prevent object slip and maintain orientation. The objective of this study was to examine the robustness of this principle by: 1) expanding the range of tasks to include those where moments act on an object with respect to moment arms not necessarily in line with the anatomically-defined axes; 2) asking subjects to hold the object in an unconstrained manner. Ten subjects were asked to lift and hold an object vertically under eighteen moment conditions. Force and moment data from all digits were analysed using principal components analysis (PCA). Different PCAs were run for variable sets corresponding to moments about the long axis of the forearm (Mx), the vertical (My) and grip (Mz) axes, and for the entire dataset (M3D). The PCA showed grip force and moment variables on separate PCs for the Mx, My, and M3D variable sets. The M3D PCA also showed a separation of variables corresponding to moments about each anatomically-defined axis. Thus, the present results show that the superposition principle holds during natural manipulation of an object experiencing external moments outside the anatomically-defined axes.

Older adults have unstable gait kinematics during weight transfer - Corrected Proof

2012-04-30

Abstract: The present article investigates gait stability of healthy older persons during weight transfer. Ten healthy older persons and ten younger persons walked 10min each on a treadmill at 3 different gait speeds. The intra-stride change in gait stability was defined by the local divergence exponent λ(t) estimated by a newly developed method. The intra-stride changes in λ(t) during weight transfer were identified by separating each stride into a single and double support phase. The intra-stride changes in λ(t) were also compared to changes in the variation of the gait kinematics, i.e., SD(t). The healthy older persons walked at the same preferred walking speed as the younger persons. However, they exhibited significantly larger λ(t) (p<0.001) during weight transfer in the double support phase. Local divergence was closely related to intra-stride changes in SD(t) of the feet in the anterior–posterior direction. Furthermore, a high correlation was found between local divergence and the variation in step length and step width for both older (R>0.67, p<0.05) and younger persons (R>0.67, p<0.05). The present results indicate that the gait kinematics of older adults are more dynamical unstable during the weight transfer compared to younger persons. Furthermore, a close relationship exists between intra-stride changes in dynamical stability and variation in step length and step width. Further work will validate the results of the present study using real-life perturbations of the gait kinematics of both younger and older adults.

Influence of hydration on nanoindentation induced energy expenditure of dentin - Corrected Proof

Luiz Eduardo Bertassoni, Michael Vincent Swain — 2012-04-27

Abstract: Improved understanding of the effects of hydration and drying in mineralized tissues is highly desirable, particularly for physiologically hydrated biological materials such as dentin. We investigated the influence of hydration on the nanomechanical properties of healthy dentin and hypothesized that drying leads to an increase in indentation induced energy expenditure and hardness. Hydrated and dry dentin were tested with a UMIS set up with a Berkovich indenter at a maximum load of 50mN. Values representative of the energy expenditure behavior were presented as dissipated energy, Ud, recovered energy, Ue, normalized energy expenditure index, ψ, and hardness, H. Energy expenditure index results, which normalize the energy expenditure for each test and describe the relative energy dissipation–recovery behavior of a material, suggested that, for the relatively severe contact strains about a sharp Berkovich indenter, dissipation dominates the mechanical response of both the hydrated and dry dentin. In support of our initial hypothesis, dry dentin presented a significantly higher energy expenditure index than hydrated dentin (p<0.0001). These results were primarily associated with a lower Ue that was found upon drying. Hydration also decreased H significantly (p<0.0001). In summary, this study presents the first direct measurements of the energy expenditure behavior of hydrated and dry dentin using instrumented nanoindentation.

Dynamic material properties of the pregnant human uterus - Corrected Proof

Sarah J. Manoogian, Jill A. Bisplinghoff, Andrew R. Kemper, Stefan M. Duma — 2012-04-27

Abstract: Given that automobile crashes are the largest single cause of death for pregnant females, scientists are developing advanced computer models of pregnant occupants. The purpose of this study is to quantify the dynamic material properties of the human uterus in order to increase the biofidelity of these models. A total of 19 dynamic tension tests were performed on pregnant human uterus tissues taken from six separate donors. The tissues were collected during full term Cesarean style deliveries and tested within 36h of surgery. The tissues were processed into uniform coupon sections and tested at 1.5strains/s using linear motors. Local stress and strain were determined from load data and optical markers using high speed video. The experiments resulted in a non-linear stress versus strain curves with an overall average peak failure true strain of 0.32±0.112 and a corresponding peak failure true stress of 656.3±483.9kPa. These are the first data available for the dynamic response of pregnant human uterus tissues, and it is anticipated they will increase the accuracy of future pregnant female computational models.

Strain-induced damage reduces echo intensity changes in tendon during loading - Corrected Proof

Sarah Duenwald-Kuehl, Roderic Lakes, Ray Vanderby — 2012-04-27

Abstract: Tendon functionality is related to its mechanical properties. Tendon damage leads to a reduction in mechanical strength and altered biomechanical behavior, and therefore leads to compromised ability to carry out normal functions such as joint movement and stabilization. Damage can also accumulate in the tissue and lead to failure. A noninvasive method with which to measure such damage potentially could quantify structural compromise from tendon injury and track improvement over time. In this study, tendon mechanics are measured before and after damage is induced by “overstretch” (strain exceeding the elastic limit of the tissue) using a traditional mechanical test system while ultrasonic echo intensity (average gray scale brightness in a B-mode image) is recorded using clinical ultrasound. The diffuse damage caused by overstretch lowered the stress at a given strain in the tissue and decreased viscoelastic response. Overstretch also lowered echo intensity changes during stress relaxation and cyclic testing. As the input strain during overstretch increased, stress levels and echo intensity changes decreased. Also, viscoelastic parameters and time-dependent echo intensity changes were reduced.

Residual force enhancement following eccentric induced muscle damage - Corrected Proof

Geoffrey A. Power, Charles L. Rice, Anthony A. Vandervoort — 2012-04-27

Abstract: During lengthening of an activated skeletal muscle, the force maintained following the stretch is greater than the isometric force at the same muscle length. This is termed residual force enhancement (RFE), but it is unknown how muscle damage following repeated eccentric contractions affects RFE. Using the dorsiflexors, we hypothesised muscle damage will impair the force generating sarcomeric structures leading to a reduction in RFE. Following reference maximal voluntary isometric contractions (MVC) in 8 young men (26.5±2.8y) a stretch was performed at 30°/s over a 30° ankle excursion ending at the same muscle length as the reference MVCs (30° plantar flexion). Surface electromyography (EMG) of the tibialis anterior and soleus muscles was recorded during all tasks. The damage protocol involved 4 sets of 25 isokinetic (30°/s) lengthening contractions. The same measures were collected at baseline and immediately post lengthening contractions, and for up to 10min recovery. Following the lengthening contraction task, there was a 30.3±6.4% decrease in eccentric torque (P<0.05) and 36.2±9.7% decrease in MVC (P<0.05) compared to baseline. Voluntary activation using twitch interpolation and RMS EMG amplitude of the tibialis anterior remained near maximal without increased coactivation for MVC. Contrary to our hypothesis, RFE increased (∼100–250%) following muscle damage (P<0.05). It appears stretch provided a mechanical strategy for enhanced muscle function compared to isometric actions succeeding damage. Thus, active force of cross-bridges is decreased because of impaired excitation–contraction coupling but force generated during stretch remains intact because force contribution from stretched sarcomeric structures is less impaired.

Does screw–bone interface modelling matter in finite element analyses? - Corrected Proof

Alisdair R. MacLeod, Pankaj Pankaj, A. Hamish R.W. Simpson — 2012-04-27

Abstract: The effect of screw–bone interface modelling strategies was evaluated in the setting of a tibial mid-shaft fracture stabilised using locking plates. Three interface models were examined: fully bonded interface; screw with sliding contact with bone; and screw with sliding contact with bone in an undersized pilot hole. For the simulation of the last interface condition we used a novel thermal expansion approach to generate the pre-stress that the bone would be exposed to during screw insertion. The study finds that the global load-deformation response is not influenced by the interface modelling approach employed; the deformation varied by less than 1% between different interaction models. However, interface modelling is found to have a considerable impact on the local stress–strain environment within the bone in the vicinity of the screws. Frictional and tied representations did not have significantly different peak strain values (<5% difference); the frictional interface had higher peak compressive strains while the tied interface had higher tensile strains. The undersized pilot hole simulation produced the largest strains. The peak minimum principal strains for the frictional interface were 26% of those for the undersized pilot hole simulation at a load of 770 N. It is concluded that the commonly used tie constraint can be used effectively when the only interest is the global load-deformation behaviour. Different contact interface models, however, alter the mechanical response around screw holes leading to different predictions for screw loosening, bone damage and stress shielding.

Can martial arts techniques reduce fall severity? An in vivo study of femoral loading configurations in sideways falls - Corrected Proof

2012-04-26

Abstract: Sideways falls onto the hip are a major cause of femoral fractures in the elderly. Martial arts (MA) fall techniques decrease hip impact forces in sideways falls. The femoral fracture risk, however, also depends on the femoral loading configuration (direction and point of application of the force). The purpose of this study was to determine the effect of fall techniques, landing surface and fall height on the impact force and the loading configuration in sideways falls.Twelve experienced judokas performed sideways MA and Block (‘natural’) falls on a force plate, both with and without a judo mat on top. Kinematic and force data were analysed to determine the hip impact force and the loading configuration.In falls from a kneeling position, the MA technique reduced the impact force by 27%, but did not change the loading configuration. The use of the mat did not change the loading configuration. Falling from a standing changed the force direction. In all conditions, the point of application was distal and posterior to the greater trochanter, but it was less distal and more posterior in falls from standing than from kneeling position.The present decrease in hip impact force with an unchanged loading configuration indicates the potential protective effect of the MA technique on the femoral fracture risk. The change in loading configuration with an increased fall height warrant further studies to examine the effect of MA techniques on fall severity under more natural fall circumstances.

Anisotropic material behaviours of soft tissues in human trachea: An experimental study - Corrected Proof

2012-04-26

Abstract: Background: Human trachea is a multi-component structure composed of cartilage, trachealis muscle, mucosa and submucosa membrane and adventitial membrane. Its mechanical properties are essential for an accurate prediction of tracheal deformation, which has a significant clinic relevance. Efforts have been made in quantifying the material behaviour of tracheal cartilage and trachealis muscle. However, the material behaviours of other components have been least investigated.Methods: Three human cadaveric trachea specimens were used in this study. Trachealis muscle, mucosa and submucosa membrane and adventitia membrane were excised to perform the uniaxial test in axial and circumferential directions. In total, 72 tissue strips were prepared and tested. Tangent modulus was used to quantified the stiffness of each tissue strip at various stretch levels.Results: The obtained results indicated that all types of tracheal soft tissues were highly non-linear and anisotropic. Trachealis muscle in the circumferential direction had the most excellent extensibility; and the adventitial collagen membrane in the circumferential direction was the stiffest.Conclusion: This study is helpful in understanding the material behaviour of trachea. Obtained results can be used for computational and analytic modelling to quantify the tracheal deformation.

Influence of pole plant time on the performance of a special jump and plant exercise in the pole vault - Corrected Proof

Falk Schade, Adamantios Arampatzis — 2012-04-25

Abstract: The purpose of this study was to examine the effect of the timing of the pole plant during the stance phase of the jump on the energy level of the vaulter/pole system at take-off for a special pole vault take-off exercise (Jagodin). We hypothesised that an earlier pole plant would increase the pole energy at take-off compared to the energy decrease of the vaulter during the jump and plant complex and so lead to a higher total energy of the vaulter/pole system at take-off. Six male pole vaulters experienced three Jagodins each with different pole plant time building three groups of vaults (early, intermediate, late pole plant). Kinematic data of vaulter and pole were recorded, as were ground reaction forces measured at the end of the pole under the planting box and under the take-off foot. These measurements allowed the energy exchange between the vaulter and pole to be determined. We found neither statistical significant differences in the mechanical energy level of the vaulter/pole system during take-off between the three groups nor a relationship between the timing of the pole plant and the energy level of the vaulter–pole system during take-off. We conclude that although the timing of the pole plant influences the interactions between the vaulter, the pole, and the ground, it does not affect the athlete's performance. Although a late pole plant decreases the loss of energy by the vaulter during the take-off, this is counterbalanced by a decrease in the energy stored in the pole at take-off.

Mechanical interaction of center of pressure and force direction in the upright human - Corrected Proof

Kreg G. Gruben, Wendy L. Boehm — 2012-04-23

Abstract: Humans maintain upright bipedal posture by producing appropriate force against the environment through the interaction of neural controlled muscle force with the mechanics of the skeletal system. Characterizing these mechanics facilitates understanding of the neural control. We used a mechanical model of an upright human to analyze how the mechanical linkage aspects of the human body affect the force between the feet and the ground (F). Key parameters of F that directly regulate upright body posture are the direction of F (θF) and its point of application (xCP, anterior–posterior position of the center of pressure). Instantaneous analysis of the equations of motion demonstrated that θF varied systematically with xCP such that the F vectors intersected at a point called the Posture-specific force Intersection point or PI (Π). The Π was located above the center of mass when the hip and knee joints were modeled as rigid and was located near the knee when the hip and knee torques were held constant. Limb posture and the knee torque affected the location of Π. This Π behavior quantifies the purely mechanical effect of anterior–posterior center of pressure shifts on the direction of F, which has consequences for the control of whole body posture.

Acoustic emission signals can discriminate between compressive bone fractures and tensile ligament injuries in the spine during dynamic loading - Corrected Proof

C. Van Toen, J. Street, T.R. Oxland, P.A. Cripton — 2012-04-23

Abstract: Acoustic emission (AE) sensors are a reliable tool in detecting fracture; however they have not been used to differentiate between compressive osseous and tensile ligamentous failures in the spine. This study evaluated the effectiveness of AE data in detecting the time of injury of ligamentum flavum (LF) and vertebral body (VB) specimens tested in tension and compression, respectively, and in differentiating between these failures. AE signals were collected while LF (n=7) and VB (n=7) specimens from human cadavers were tested in tension and compression (0.4m/s), respectively. Times of injury (time of peak AE amplitude) were compared to those using traditional methods (VB: time of peak force, LF: visual evidence in high speed video). Peak AE signal amplitudes and frequencies (using Fourier and wavelet transformations) for the LF and VB specimens were compared. In each group, six specimens failed (VB, fracture; LF, periosteal stripping or attenuation) and one did not. Time of injury using AE signals for VB and LF specimens produced average absolute differences to traditional methods of 0.7 (SD=0.2)ms and 2.4 (SD=1.5)ms (representing 14% and 20% of the average loading time), respectively. AE signals from VB fractures had higher amplitudes and frequencies than those from LF failures (average peak amplitude 87.7 (SD=6.9)dB vs. 71.8 (SD=9.8)dB for the inferior sensor, p<0.05; median characteristic frequency from the inferior sensor 97 (interquartile range, IQR, 41)kHz vs. 31 (IQR 2)kHz, p<0.05). These findings demonstrate that AE signals could be used to delineate complex failures of the spine.

Dynamic plantar loading index: Understanding the benefit of custom foot orthoses for painful pes cavus - Corrected Proof

Bijan Najafi, Elizabeth Barnica, James S. Wrobel, Joshua Burns — 2012-04-20

Abstract: The purpose of this study was to evaluate a new method showing how custom foot orthoses (CFO) improve dynamics of plantar loading. The method is based on the probability distribution of peak pressure time series and is quantified using the Regression Factor (RF). RF is a least square regression slope between the experimentally observed plantar pressure magnitude probability distribution and a modeled Gaussian shape. Plantar pressure data from a randomized controlled trial of 154 participants with painful Pes Cavus were retrospectively re-analyzed. The participants were randomized to an active treatment group given CFO or a control group given sham orthoses. The location of 2nd Peak pressure as a percentage of stance time (PLoc2) and its magnitude (PM2) was also calculated. In addition, plantar pressure data were collected on 23 healthy volunteers with normal foot alignment and no foot pain. Results demonstrated Pes Cavus had a significantly lower RF than healthy participants (0.30 v. 0.51; p<10−7). PM2 was reduced in both active and control groups. However, RF and the PLoc2 were only changed in the active group (p<0.005) without any significant change in the control group (p>0.5). This study suggests that painful Pes Cavus alters the shape of probability distribution of plantar loading during walking and CFO are an effective therapeutic solution that can significantly improve it. Further use of the RF index and 2nd peak pressure location as an outcome measure for treatment of foot and ankle deformities is suggested.

Musculotendon lengths and moment arms for a three-dimensional upper-extremity model - Corrected Proof

Jeffery W. Rankin, Richard R. Neptune — 2012-04-20

Abstract: Generating muscle-driven forward dynamics simulations of human movement using detailed musculoskeletal models can be computationally expensive. This is due in part to the time required to calculate musculotendon geometry (e.g., musculotendon lengths and moment arms), which is necessary to determine and apply individual musculotendon forces during the simulation. Modeling upper-extremity musculotendon geometry can be especially challenging due to the large number of multi-articular muscles and complex muscle paths. To accurately represent this geometry, wrapping surface algorithms and/or other computationally expensive techniques (e.g., phantom segments) are used. This paper provides a set of computationally efficient polynomial regression equations that estimate musculotendon length and moment arms for thirty-two (32) upper-extremity musculotendon actuators representing the major muscles crossing the shoulder, elbow and wrist joints. Equations were developed using a least squares fitting technique based on geometry values obtained from a validated public-domain upper-extremity musculoskeletal model that used wrapping surface elements (). In general, the regression equations fit well the original model values, with an average root mean square difference for all musculotendon actuators over the represented joint space of 0.39mm (1.1% of peak value). In addition, the equations reduced the computational time required to simulate a representative upper-extremity movement (i.e., wheelchair propulsion) by more than two orders of magnitude (315 versus 2.3s). Thus, these equations can assist in generating computationally efficient forward dynamics simulations of a wide range of upper-extremity movements.

Passive elastic properties of the rat ankle - Corrected Proof

Mengnan (Mary) Wu, Dinesh K. Pai, Matthew C. Tresch, Thomas G. Sandercock — 2012-04-20

Abstract: Passive properties of muscles and tendons, including their elasticity, have been suggested to influence motor control. We examine here the potential role of passive elastic muscle properties at the rat ankle joint, focusing on their potential to specify an equilibrium position of the ankle. We measured the position-dependent passive torques at the rat ankle before and after sequential cuts of flexor (a.k.a. dorsiflexor) and extensor (a.k.a. plantarflexor) ankle muscles. We found that there was a passive equilibrium position of the ankle that shifted systematically with the cuts, demonstrating that the passive torques produced by ankle flexor and extensor muscles work in opposition in order to maintain a stable equilibrium. The mean equilibrium position of the intact rat ankle ranged from 9.3° to 15.7° in extension relative to the orthogonal position, depending on the torque metric. The mean shift in equilibrium position due to severing extensors ranged from 4.4° to 7.7°, and the mean shift due to severing flexors was smaller, ranging from 0.9° to 2.5°. The restoring torques generated by passive elasticity are large enough (approximately 1.5–5mNm for displacements of 18° from equilibrium) to affect ankle movement during the swing phase of locomotion, and the asymmetry of larger extension vs. flexion torques is consistent with weight support, demonstrating the importance of accounting for passive muscle properties when considering the neural control of movement.

Cortical bone failure mechanisms during screw pullout - Corrected Proof

Emer M. Feerick, J. Patrick McGarry — 2012-04-20

Abstract: An experimental and computational study of screw pullout from cortical bone has been conducted. A novel modification of standard pullout tests providing real time image capture of damage mechanisms during screw pullout was developed. Pullout forces, measured using the novel test rig, have been validated against standard pullout tests. Pullout tests were conducted, considering osteon alignment, to investigate the effect of osteons aligned parallel to the axis of the orthopaedic screw (longitudinal pullout) as well as the effect of osteons aligned perpendicular to the axis of the screw (transverse pullout). Distinctive alternate failure mechanisms, for longitudinally and transversely orientated cortical bone during screw pullout, were uncovered. Vertical crack propagation, parallel to the axis of the screw, was observed for a longitudinal pullout. Horizontal crack propagation, perpendicular to the axis of the screw, was observed for a transverse pullout. Finite element simulation of screw pullout, incorporating material damage and crack propagation, was also performed. Simulations revealed that a homogenous material model for cortical bone predicts vertical crack propagation patterns for both longitudinal and transverse screw pullout. A bi-layered composite model representing cortical bone microstructure was developed. A unique set of material and damage properties was used for both transverse and longitudinal pullout simulations, with only layer orientations being changed. Simulations predicted: (i) higher pullout forces for transverse pullout; (ii) horizontal crack paths perpendicular to screw axis for transverse pullout, whereas vertical crack paths were computed for longitudinal pullout. Computed results agreed closely with experimental observations in terms of pullout force and crack propagation.

Experimental validation of a pulse wave propagation model for predicting hemodynamics after vascular access surgery - Corrected Proof

2012-04-20

Abstract: Hemodialysis patients require a vascular access that is, preferably, surgically created by connecting an artery and vein in the arm, i.e. an arteriovenous fistula (AVF). The site for AVF creation is chosen by the surgeon based on preoperative diagnostics, but AVFs are still compromised by flow-associated complications. Previously, it was shown that a computational 1D-model is able to describe pressure and flow after AVF surgery. However, predicted flows differed from measurements in 4/10 patients. Differences can be attributed to inaccuracies in Doppler measurements and input data, to neglecting physiological mechanisms or to an incomplete physical description of the pulse wave propagation after AVF surgery. The physical description can be checked by validating against an experimental setup consisting of silicone tubes mimicking the aorta and arm vasculature both before and after AVF surgery, which is the aim of the current study. In such an analysis, the output uncertainty resulting from measurement uncertainty in model input should be quantified. The computational model was fed by geometrical and mechanical properties collected from the setup. Pressure and flow waveforms were simulated and compared with experimental waveforms. The precision of the simulations was determined by performing a Monte Carlo study. It was concluded that the computational model was able to simulate mean pressures and flows accurately, whereas simulated waveforms were less attenuated than experimental ones, likely resulting from neglecting viscoelasticity. Furthermore, it was found that in the analysis output uncertainties, resulting from input uncertainties, cannot be neglected and should thus be considered.

A hierarchy of computationally derived surgical and patient influences on metal on metal press-fit acetabular cup failure - Corrected Proof

S.G. Clarke, A.T.M. Phillips, A.M.J. Bull, J.P. Cobb — 2012-04-18

Abstract: The impact of anatomical variation and surgical error on excessive wear and loosening of the acetabular component of large diameter metal-on-metal hip arthroplasties was measured using a multi-factorial analysis through 112 different simulations. Each surgical scenario was subject to eight different daily loading activities using finite element analysis. Excessive wear appears to be predominantly dependent on cup orientation, with inclination error having a higher influence than version error, according to the study findings. Acetabular cup loosening, as inferred from initial implant stability, appears to depend predominantly on factors concerning the area of cup–bone contact, specifically the level of cup seating achieved and the individual patient's anatomy. The extent of press fit obtained at time of surgery did not appear to influence either mechanism of failure in this study.

Removal of the cortical endplates has little effect on ultimate load and damage distribution in QCT-based voxel models of human lumbar vertebrae under axial compression - Corrected Proof

Ghislain Maquer, Enrico Dall'Ara, Philippe K. Zysset — 2012-04-16

Abstract: Every year, 500,000 osteoporotic vertebral compression fractures occur in Europe. Quantitative computed tomography (QCT)-based finite element (FE) voxel models predict ultimate force whether they simulate vertebral bodies embedded in polymethylmethacrylate (PMMA) or vertebral sections with both endplates removed. To assess the effect of endplate removal in those predictions, non-linear FE analyses of QCT-based voxel models of human vertebral bodies were performed. High resolution pQCT images of 11 human lumbar vertebrae without posterior elements were coarsened to clinical resolution and bone volume fraction was used to determine the elastic, plastic and damage behavior of bone tissue. Three model boundary conditions (BCs) were chosen: the endplates were cropped (BC1, BC2) or voxel layers were added on the intact vertebrae to mimic embedding (BC3). For BC1 and BC3, the bottom nodes were fully constrained and the top nodes were constrained transversely while both node sets were freed transversely for BC2. Axial displacement was prescribed to the top nodes. In each model, we compared ultimate force and damage distribution during post-yield loading. The results showed that ultimate forces obtained with BC3 correlated perfectly with those computed with BC1 (R2=0.9988) and BC2 (R2=0.9987), but were in average 3.4% lower and 6% higher respectively. Moreover, good correlation of damage distribution calculated for BC3 was found with those of BC1 (R2=0.92) and BC2 (R2=0.73). This study demonstrated that voxel models of vertebral sections provide the same ultimate forces and damage distributions as embedded vertebral bodies, but with less preprocessing and computing time required.

Patellofemoral joint compression forces in backward and forward running - Corrected Proof

Paulien E. Roos, Nick Barton, Robert W.M. van Deursen — 2012-04-16

Abstract: Patellofemoral pain (PFP) is a common injury and increased patellofemoral joint compression forces (PFJCF) may aggravate symptoms. Backward running (BR) has been suggested for exercise with reduced PFJCF.The aims of this study were to (1) investigate if BR had reduced peak PFJCF compared to forward running (FR) at the same speed, and (2) if PFJCF was reduced in BR, to investigate which biomechanical parameters explained this. It was hypothesized that (1) PFJCF would be lower in BR, and (2) that this would coincide with a reduced peak knee moment caused by altered ground reaction forces (GRFs).Twenty healthy subjects ran in forward and backward directions at consistent speed. Kinematic and ground reaction force data were collected; inverse dynamic and PFJCF analyses were performed.PFJCF were higher in FR than BR (4.5±1.5; 3.4±1.4BW; p<0.01). The majority of this difference (93.1%) was predicted by increased knee moments in FR compared to BR (157±54; 124±51Nm; p<0.01). 54.8% of differences in knee moments could be predicted by the magnitude of the GRF (2.3±0.3; 2.4±0.2BW), knee flexion angle (44±6; 41±7) and center of pressure location on the foot (25±11; 12±6%) at time of peak knee moment. Results were not consistent in all subjects.It was concluded that BR had reduced PFJCF compared to FR. This was caused by an increased knee moment, due to differences in magnitude and location of the GRF vector relative to the knee. BR can therefore be used to exercise with decreased PFJCF.

Fracture mechanics analysis of vertical root fracture from condensation of gutta-percha - Corrected Proof

Herzl Chai, Aviad Tamse — 2012-04-16

Abstract: A two-dimensional fracture mechanics analysis of vertical root fracture (VRF) in single-canal roots from apical condensation of gutta-percha (gp) is developed. The resulting analytic relation for apical load causing VRF agrees with major trends reported in in-vitro tests on roots subjected to either continuous or, the more clinically relevant, repeating vertical condensation of gp. The model explicitly exposes the role of root canal morphology and dentin fracture toughness on VRF. Ovoid and irregular canals are prone to fracture while the effect of mean root canal radius is modest. Canal taper and instrumentation details may affect VRF only marginally and indirectly. The model predicts dentinal cracks to occur following root canal instrumentation and obturation, which may pose long-term threats to tooth integrity.

Shoe midsole hardness, sex and age effects on lower extremity kinematics during running - Corrected Proof

Benno M. Nigg, Jennifer Baltich, Christian Maurer, Peter Federolf — 2012-04-16

Abstract: Previous studies investigating the effects of shoe midsole hardness on running kinematics have often used male subjects from within a narrow age range. It is unknown whether shoe midsole hardness has the same kinematic effect on male and female runners as well as runners from different age categories. As sex and age have an effect on running kinematics, it is important to understand if shoe midsole hardness affects the kinematics of these groups in a similar fashion. However, current literature on the effects of sex and age on running kinematics are also limited to a narrow age range distribution in their study population. Therefore, this study tested the influence of three different midsole hardness conditions, sex and age on the lower extremity kinematics during heel-toe running. A comprehensive analysis approach was used to analyze the lower-extremity kinematic gait variables for 93 runners (male and female) aged 16–75 years. Participants ran at 3.33±0.15m/s on a 30m-long runway with soft, medium and hard midsoles. A principal component analysis combined with a support vector machine showed that running kinematics based on shoe midsole hardness, sex, and age were separable and classifiable. Shoe midsole hardness demonstrated a subject-independent effect on the kinematics of running. Additionally, it was found that age differences affected the more dominant movement components of running compared to differences due to the sex of a runner.

Biomechanical evaluation of the relationship between postural control and body mass index - Corrected Proof

P.X. Ku, N.A. Abu Osman, A. Yusof, W.A.B. Wan Abas — 2012-04-16

Abstract: Postural stability is crucial in maintaining body balance during quiet standing, locomotion, and any activities that require a high degree of balance performance, such as participating in sports and dancing. Research has shown that there is a relationship between stability and body mass. The aims of this study were to examine the impact that two variables had on static postural control: body mass index (BMI) and gender. Eighty healthy young adults (age=21.7±1.8yr; height=1.65±0.09m; mass=67.5±19.0kg) participated in the study and the static postural control was assessed using the Biodex Balance System, with a 20Hz sampling rate in the bipedic stance (BLS) and unipedic stance (ULS) for 30s. Five test evaluations were performed for each balance test. Postural control was found to be negatively correlated with increased adiposity, as the obese BMI group performed significantly poorer than the underweight, normal weight and overweight groups during BLS and ULS tests. The underweight, normal weight and overweight groups exhibited greater anterior–posterior stability in postural control during quiet stance. In addition, female displayed a trend of having a greater postural sway than male young adults, although it was evidenced in only some BMI groups. This study revealed that BMI do have an impact on postural control during both BLS and ULS. As such, BMI and gender-specific effects should be taken into consideration when selecting individuals for different types of sporting activities, especially those that require quiet standing.

Letter to the editor regarding “Effect of low pass filtering on joint moments from inverse dynamics: implications for injury prevention” - Corrected Proof

Timothy E. Hewett, Gregory D. Myer, Benjamin D. Roewer, Kevin R. Ford — 2012-04-05

A recent study performed by Kristianslund et al. highlights the importance of selecting appropriate filtering cutoff frequencies when analyzing kinematic and kinetic data collected using modern three-dimensional motion capture systems (). The authors posit that the choice of cutoff frequency significantly influences the magnitude of the peak knee abduction moment (KAM) measured during a running sidestep-cut (run-cut) task with particular implication on the validity of existing anterior cruciate ligament (ACL) injury prediction paradigms. How one decides to filter and analyze motion data is both an art and science that requires careful consideration of both the tasks being analyzed and the outcome variables of interest. For these reasons, there are several subtleties and some possible flaws in Kristianslund's study that warrant clarification.

Authors' reply regarding “Effect of low pass filtering on joint moments from inverse dynamics: Implications for injury prevention” - Corrected Proof

Eirik Kristianslund, Tron Krosshaug, Antonie J. van den Bogert — 2012-03-26

We appreciate the comments from to our recent study () and welcome their initiative to debate some potential implications of our findings. It would appear that we agree that force and marker data should be filtered at the same cut-off frequency when performing inverse dynamics. This understanding has gained increasing acceptance in the biomechanics community, and in our paper we have described the theoretical background and demonstrated the effect a difference in cut-off frequencies can have for the joint moments calculated for a side-step cutting task. The main point of our paper was that a general consensus is needed to ensure the same cut-off frequency is chosen for the filtering of force and movement data in studies employing inverse dynamics. This will ensure confidence in the validity of results and allow comparison of findings across studies.

Response to the Comment by Gröning and Fagan on “The effects of modeling simplifications on craniofacial finite element models: The alveoli (tooth sockets) and periodontal ligaments” (volume 44, issue 10, pages 1831–1838) - Corrected Proof

I.R. Grosse, S.A. Wood, D.S. Strait, E.R. Dumont, C.F. Ross — 2012-02-21

Dear Editor, We welcome the comments by Drs. Gröning and Fagan on our paper (). They make some excellent points and clarified some of our misunderstandings regarding their work (). In our work we found that modeling the teeth and bone as continuous (i.e. omitting the PDL from our model and fusing the tooth roots to the cranium bone) had only a local over-stiffening effect in the alveolar process of the cranium. However, we recognize that the mandible is a significantly different structure than the cranium, and the size of the tooth roots relative the height of the mandible could make this affect significant on a global level.

Comment on “The effects of modelling simplifications on craniofacial finite element models: The alveoli (tooth sockets) and periodontal ligaments” (volume 44, issue 10, pages 1831–1838) - Corrected Proof

F. Gröning, M.J. Fagan — 2012-02-20

Dear Editor, We read with interest the recent article “The effects of modelling simplifications on craniofacial finite element models: The alveoli (tooth sockets) and periodontal ligaments” by . Using a finite element (FE) model of a primate cranium the authors investigated the effects of filling empty tooth sockets with bone material and changing the mechanical properties of the periodontal ligament (PDL). At the end of their paper the authors discuss the previous findings from similar studies with mandibles. Unfortunately, we identified some errors and misunderstandings in this discussion. Two of these refer to one of our recent papers () and we therefore see the need for a correction.

Poisson's ratios in anisotropic materials at finite strains; comment on short communication by Smith et al. (2011) - Corrected Proof

Peter P. Purslow — 2012-02-16

provide interesting information on the change in volume of bundles of muscle fibers and single muscle fibers on being passively stretched along the fiber direction to approximately twice rest length. The authors have kindly confirmed that the horizontal axis of their Fig. 2 is erroneously marked “strain” but actually indicates values of extension ratio λ (i.e. stretched length/original length) rather than strain (i.e. change in length/original length, or λ−1).

Response to Letter to the Editor: ‘Poisson's ratios in anisotropic materials at finite strains; comment on short communication by Smith et al. (2011)’ - Corrected Proof

Lucas R. Smith, Lewis Fowler-Gerace, Richard L. Lieber — 2012-02-15

We appreciate the careful examination of our study by Dr. Purslow in his letter to the editor (). Dr. Purslow has made significant contributions to the field of muscle extracellular matrix and is more than qualified to provide this constructive critique.

Direct measurement of power during one single sprint on treadmill - Corrected Proof

J.B. Morin, P. Samozino, R. Bonnefoy, P. Edouard, A. Belli — 2010-06-14

Abstract: We tested the validity of an instrumented treadmill dynamometer for measuring maximal propulsive power during sprint running, and sought to verify whether this could be done over one single sprint, as shown during sprint cycling. The treadmill dynamometer modified towards sprint use (constant motor torque) allows vertical and horizontal forces to be measured at the same location as velocity, i.e. at the foot, which is novel compared to existing methods in which power is computed as the product of belt velocity and horizontal force measured by transducers placed in the tethering system. Twelve males performed 6s sprints against default, high and low loads set from the motor torque necessary to overcome the friction due to subjects’ weight on the belt (default load), and 20% higher and lower motor torque values. Horizontal ground reaction force, belt velocity, propulsive power and linear force–velocity relationships were compared between the default load condition and when taking all conditions together. Force and velocity traces and values were reproducible and consistent with the literature, and no significant difference was found between maximal power and force–velocity relationships obtained in the default load condition only vs. adding data from all conditions. The presented method allows one to measure maximal propulsive power and calculate linear force–velocity relationships from one single sprint data. The main novelties are that both force and velocity are measured at the same location, and that instantaneous values are averaged over one contact period, and not over a constant arbitrary time-window.

Generalized n-dimensional biomechanical field analysis using statistical parametric mapping - Corrected Proof

Todd C. T.C. Pataky — 2010-05-03

Abstract: A variety of biomechanical data are sampled from smooth n-dimensional spatiotemporal fields. These data are usually analyzed discretely, by extracting summary metrics from particular points or regions in the continuum. It has been shown that, in certain situations, such schemes can compromise the spatiotemporal integrity of the original fields. An alternative methodology called statistical parametric mapping (SPM), designed specifically for continuous field analysis, constructs statistical images that lie in the original, biomechanically meaningful sampling space. The current paper demonstrates how SPM can be used to analyze both experimental and simulated biomechanical field data of arbitrary spatiotemporal dimensionality. Firstly, 0-, 1-, 2-, and 3-dimensional spatiotemporal datasets derived from a pedobarographic experiment were analyzed using a common linear model to emphasize that SPM procedures are (practically) identical irrespective of the data's physical dimensionality. Secondly two probabilistic finite element simulation studies were conducted, examining heel pad stress and femoral strain fields, respectively, to demonstrate how SPM can be used to probe the significance of field-wide simulation results in the presence of uncontrollable or induced modeling uncertainty. Results were biomechanically intuitive and suggest that SPM may be suitable for a wide variety of mechanical field applications. SPM's main theoretical advantage is that it avoids problems associated with a priori assumptions regarding the spatiotemporal foci of field signals. SPM's main practical advantage is that a unified framework, encapsulated by a single linear equation, affords comprehensive statistical analyses of smooth scalar fields in arbitrarily bounded n-dimensional spaces.

Automated muscle wrapping using finite element contact detection - Corrected Proof

Philippe Favre, Christian Gerber, Jess G. Snedeker — 2010-05-03

Abstract: Realistic muscle path representation is essential to musculoskeletal modeling of joint function. Algorithms predicting these muscle paths typically rely on a labor intensive predefinition of via points or underlying geometries to guide wrapping for given joint positions. While muscle wrapping using anatomically precise three-dimensional (3D) finite element (FE) models of bone and muscle has been achieved, computational expense and pre-processing associated with this approach exclude its use in applications such as subject-specific modeling. With the intention of combining advantageous features of both approaches, an intermediate technique relying on contact detection capabilities of commercial FE packages is presented. We applied the approach to the glenohumeral joint, and validated the method by comparison against existing experimental data. Individual muscles were modeled as a straight series of deformable beam elements and bones as anatomically precise 3D rigid bodies. Only the attachment locations and a default orientation of the undeformed muscle segment were pre-defined. The joint was then oriented in a static position of interest. The muscle segment free end was then moved along the shortest Euclidean path to its origin on the scapula, wrapping the muscle along bone surfaces by relying on software contact detection. After wrapping for a given position, the resulting moment arm was computed as the perpendicular distance from the line of action vector to the humeral head center of rotation.This approach reasonably predicted muscle length and moment arm for 27 muscle segments when compared to experimental measurements over a wide range of shoulder motion. Artificial via points or underlying contact geometries were avoided, contact detection and multiobject wrapping on the bone surfaces were automatic, and low computational cost permitted wrapping of individual muscles within seconds on a standard desktop PC. These advantages may be valuable for both general and subject-specific musculoskeletal modeling.

A novel gait platform to measure isolated plantar metatarsal forces during walking - Corrected Proof

2010-05-03

Abstract: A new gait platform described in this report allows an isolated measurement of the vertical and shear forces under an individual metatarsal head during barefoot walking. The apparatus incorporated a customized tactile force sensor and a high-speed camera system, which enabled easy identification of a single anatomical landmark at the forefoot’s plantar surface that is in contact with the sensor throughout stance. After calibration, the measured peak forces under the 2nd MTH showed variability of 3.7%, 9.2%, and 8.9% in vertical, anterior–posterior, and medial–lateral directions, respectively. The device therefore provides information about the magnitude and timing of such local metatarsal forces, and has been shown to be of significant research and clinical interest. Its ability to achieve this with a high degree of accuracy ensures its potential as a valuable research tool.