Hot off the presses! Jul 07 J Biomech

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Latest Articles Include:

• Editorial Board and Publication Information
  - J Biomech 44(10):IFC (2011)
  
• The effects of modeling simplifications on craniofacial finite element models: The alveoli (tooth sockets) and periodontal ligaments
  - J Biomech 44(10):1831-1838 (2011)
  Several finite element models of a primate cranium were used to investigate the biomechanical effects of the tooth sockets and the material behavior of the periodontal ligament (PDL) on stress and strain patterns associated with feeding. For examining the effect of tooth sockets, the unloaded sockets were modeled as devoid of teeth and PDL, filled with teeth and PDLs, or simply filled with cortical bone. The third premolar on the left side of the cranium was loaded and the PDL was treated as an isotropic, linear elastic material using published values for Young's modulus and Poisson's ratio. The remaining models, along with one of the socket models, were used to determine the effect of the PDL's material behavior on stress and strain distributions under static premolar biting and dynamic tooth loading conditions. Two models (one static and the other dynamic) treated the PDL as cortical bone. The other two models treated it as a ligament with isotropic, linear elastic m! aterial properties. Two models treated the PDL as a ligament with hyperelastic properties, and the other two as a ligament with viscoelastic properties. Both behaviors were defined using published stress–strain data obtained from in vitro experiments on porcine ligament specimens. Von Mises stress and strain contour plots indicate that the effects of the sockets and PDL material behavior are local. Results from this study suggest that modeling the sockets and the PDL in finite element analyses of skulls is project dependent and can be ignored if values of stress and strain within the alveolar region are not required.
• Can Achilles tendon moment arm be predicted from anthropometric measures in pre-pubescent children?
  - J Biomech 44(10):1839-1844 (2011)
  Muscle–tendon moment arm magnitudes are essential variables for accurately calculating muscle forces from joint moments. Their measurement requires specialist knowledge and expensive resources. Research has shown that the patellar tendon moment arm length is related to leg anthropometry in children. Here, we asked whether the Achilles tendon moment arm (MAAT) can be accurately predicted in pre-pubescent children from surface anthropometry. Age, standing height, mass, foot length, inter-malleolar ankle width, antero-posterior ankle depth, tibial length, lower leg circumference, and distances from the calcaneus to the distal head of the 1st metatarsal and medial malleolus were determined in 49 pre-pubescent children. MAAT was calculated at three different ankle positions (neutral, 10° plantarflexion, and 10° dorsiflexion) by differentiating tendon excursion, measured via ultrasonography, with respect to ankle angle change using seven different differentiation techniq! ues. Backwards stepwise regression analyses were performed to identify predictors of MAAT. When all variables were included, the regression analysis accounted for a maximum of 49% of MAAT variance at the neutral ankle angle when a third-order polynomial was used to differentiate tendon excursion with respect to ankle angle. For this condition, foot length and the distance between calcaneus and 1st metatarsal were the only significant predictors, accounting for 47% of the variance (p<0.05). The absolute error associated with this regression model was 3.8±4.4 mm, which would result in significant error (mean=14.5%) when estimating muscle forces from joint moments. We conclude that MAAT cannot be accurately predicted from anthropometric measures in children.
• The effects of single-leg landing technique on ACL loading
  - J Biomech 44(10):1845-1851 (2011)
  Anterior Cruciate Ligament (ACL) injury is one of the most serious and costly injuries of the lower extremity, occurring more frequently in females than males. Injury prevention training programs have reported the ability to reduce non-contact ACL injury occurrence. These programs have also been shown to alter an athletes' lower extremity position at initial contact with the ground and throughout the deceleration phase of landing. The purpose of this study was to determine the influence of single-leg landing technique on ACL loading in recreationally active females. Participants were asked to perform "soft" and "stiff" drop landings. A series of musculoskeletal models were then used to estimate muscle, joint, and ACL forces. Dependent t-tests were conducted to investigate differences between the two landing techniques (p<0.05). Instructing participants to land 'softly' resulted in a significant decrease in peak ACL force (p=0.05), and a significant increase! in hip and knee flexion both at initial contact (IC) and the time of peak ACL force (FPACL). These findings suggest that altering landing technique using simple verbal instruction may result in lower extremity alignment that decreases the resultant load on the ACL. Along with supporting the findings of reduced ACL force with alterations in sagittal plane landing mechanics in the current literature, the results of this study suggest that simple verbal instruction may reduce the ACL force experienced by athletes when landing.
• Nanoindentation testing and finite element simulations of cortical bone allowing for anisotropic elastic and inelastic mechanical response
  - J Biomech 44(10):1852-1858 (2011)
  Anisotropy is one of the most peculiar aspects of cortical bone mechanical behaviour, and the numerical approach can be successfully used to investigate aspects of bone tissue mechanics that analytical methods solve in approximate way or do not cover. In this work, nanoindentation experimental tests and finite element simulations were employed to investigate the elastic–inelastic anisotropic mechanical properties of cortical bone. The model allows for anisotropic elastic and post-yield behaviour of the tissue. A tension-compression mismatch and direction-dependent yield stresses are allowed for. Indentation experiments along the axial and transverse directions were simulated with the purpose to predict the indentation moduli and hardnesses along multiple orientations. Results showed that the experimental transverse-to-axial ratio of indentation moduli, equal to 0.74, is predicted with a 3% discrepancy regardless the post-yield material behaviour; whereas, the transve! rse-to-axial hardness ratio, equal to 0.86, can be correctly simulated (discrepancy 6% w.r.t. the experimental results) only employing an anisotropic post-elastic constitutive model. Further, direct comparison between the experimental and simulated indentation tests evidenced a good agreement in the loading branch of the indentation curves and in the peak loads for a transverse-to-axial yield stress ratio comparable to the experimentally obtained transverse-to-axial hardness ratio. In perspective, the present work results strongly support the coupling between indentation experiments and FEM simulations to get a deeper knowledge of bone tissue mechanical behaviour at the microstructural level. The present model could be used to assess the effect of variations of constitutive parameters due to age, injury, and/or disease on bone mechanical performance in the context of indentation testing.
• Shoulder muscle function depends on elbow joint position: An illustration of dynamic coupling in the upper limb
  - J Biomech 44(10):1859-1868 (2011)
  Shoulder muscle function has been documented based on muscle moment arms, lines of action and muscle contributions to contact force at the glenohumeral joint. At present, however, the contributions of individual muscles to shoulder joint motion have not been investigated, and the effects of shoulder and elbow joint position on shoulder muscle function are not well understood. The aims of this study were to compute the contributions of individual muscles to motion of the glenohumeral joint during abduction, and to examine the effect of elbow flexion on shoulder muscle function. A three-dimensional musculoskeletal model of the upper limb was used to determine the contributions of 18 major muscles and muscle sub-regions of the shoulder to glenohumeral joint motion during abduction. Muscle function was found to depend strongly on both shoulder and elbow joint positions. When the elbow was extended, the middle and anterior deltoid and supraspinatus were the greatest contrib! utors to angular acceleration of the shoulder in abduction. In contrast, when the elbow was flexed at 90°, the anterior deltoid and subscapularis were the greatest contributors to joint angular acceleration in abduction. This dependence of shoulder muscle function on elbow joint position is explained by the existence of dynamic coupling in multi-joint musculoskeletal systems. The extent to which dynamic coupling affects shoulder muscle function, and therefore movement control, is determined by the structure of the inverse mass matrix, which depends on the configuration of the joints. The data provided may assist in the diagnosis of abnormal shoulder function, for example, due to muscle paralysis or in the case of full-thickness rotator cuff tears.
• Computation of hemodynamics in the left coronary artery with variable angulations
  - J Biomech 44(10):1869-1878 (2011)
  The purpose of this study was to investigate the hemodynamic effect of variations in the angulations of the left coronary artery, based on simulated and realistic coronary artery models. Twelve models consisting of four realistic and eight simulated coronary artery geometries were generated with the inclusion of left main stem, left anterior descending and left circumflex branches. The simulated models included various coronary artery angulations, namely, 15°, 30°, 45°, 60°, 75°, 90°, 105° and 120°. The realistic coronary angulations were based on selected patient's data with angles ranging from narrow angles of 58° and 73° to wide angles of 110° and 120°. Computational fluid dynamics analysis was performed to simulate realistic physiological conditions that reflect the in vivo cardiac hemodynamics. The wall shear stress, wall shear stress gradient, velocity flow patterns and wall pressure were measured in simulated and realistic models during the cardiac c! ycle. Our results showed that a disturbed flow pattern was observed in models with wider angulations, and wall pressure was found to reduce when the flow changed from the left main stem to the bifurcated regions, based on simulated and realistic models. A low wall shear stress gradient was demonstrated at left bifurcations with wide angles. There is a direct correlation between coronary angulations and subsequent hemodynamic changes, based on realistic and simulated models. Further studies based on patients with different severities of coronary artery disease are required to verify our results.
• Experimental and numerical estimations into the force distribution on an occlusal surface utilizing a flexible force sensor array
  - J Biomech 44(10):1879-1884 (2011)
  This study presents a novel flexible force sensor array for measuring the distribution of the force distribution over the first molar. The developed force sensor array is composed of a flexible polyimide electrode and barium-titanate-based multilayer ceramic capacitors (MLCCs). The piezoelectric and material properties of industrial-grade MLCCs are ideal for measuring large-force loadings. The sensors are cheap and easy to integrate with automated manufacturing processes. Prior to experimental measurements, the force responses for the MLCC sensor cells were systematically measured and evaluated, confirming their high fracture strength and good sensing properties. Finite element (FE) simulations were used to calculate the force distribution over the tooth crown from the measurement results of the 3×3 force sensor array. Results indicate that the sensor has great sensitivity and linearity under a high-speed cycle loading of 500 N/s conducted to simulate normal chewing. ! The total force measured using the developed sensor array within the artificial tooth had an error of less than 5%. In addition, the force distributions over the molar crown obtained using a numerical method of FE analysis agree well with those obtained from experiments. The developed flexible force sensor array thus has potential for in-situ bite force measurements that are low-cost and reliable.
• Globographic visualisation of three dimensional joint angles
  - J Biomech 44(10):1885-1891 (2011)
  Three different methods for describing three dimensional joint angles are commonly used in biomechanics. The joint coordinate system and Cardan/Euler angles are conceptually quite different but are known to represent the same underlying mathematics. More recently the globographic method has been suggested as an alternative and this has proved particularly attractive for the shoulder joint. All three methods can be implemented in a number of ways leading to a choice of angle definitions. Very recently Rab has demonstrated that the globographic method is equivalent to one implementation of the joint coordinate system. This paper presents a rigorous analysis of the three different methods and proves their mathematical equivalence. The well known sequence dependence of Cardan/Euler is presented as equivalent to configuration dependence of the joint coordinate system and orientation dependence of globographic angles. The precise definition of different angle sets can be easily visualised using the globographic method using analogues of longitude, latitude and surface bearings with which most users will already be familiar. The method implicitly requires one axis of the moving segment to be identified as its principal axis and this can be extremely useful in helping define the most appropriate angle set to describe the orientation of any particular joint. Using this technique different angle sets are considered to be most appropriate for different joints and examples of this for the hip, knee, ankle, pelvis and axial skeleton are outlined.
• An analysis of the measurement principle of smart brackets for 3D force and moment monitoring in orthodontics
  - J Biomech 44(10):1892-1900 (2011)
  Measuring the three-dimensional (3D) force–moment (F/M) systems applied for correcting tooth malposition is highly desirable for accurate spatial control of tooth movement and for reducing traumatic side effects such as irreversible root resorption. To date, suitable tools for monitoring the applied F/M system during therapy are lacking. We have previously introduced a true-scale orthodontic bracket with an integrated microelectronic stress sensor system for 3D F/M measurements on individual teeth with a perspective for clinical application. The underlying theoretical concept assumes a linear correlation between externally applied F/M systems and mechanical stresses induced within the smart bracket. However, in combined applications of F/M components the actual wire-bracket contacts may differ from those caused by separate applications of corresponding individual F/M components, thus violating the principle of linear superposition of mechanical stresses. This study s! ystematically evaluates this aspect using finite element (FE) simulations and measurements with a real smart bracket. The FE analysis indicated that variability in the wire-bracket contacts is a major source for measurement errors. By taking the critical F/M combinations into account in the calibration of the real smart bracket, we were able to reduce the mean measurement error in five of the six F/M components to values <0.12 N and <0.04 N cm. Bucco-lingually directed forces still showed mean errors up to 0.21 N. Improving the force measurement accuracy and integrating components for telemetric energy and data transfer are the next steps towards clinical application of intelligent orthodontic appliances based on smart brackets.
• Multivariable static ankle mechanical impedance with relaxed muscles
  - J Biomech 44(10):1901-1908 (2011)
  Quantitative characterization of ankle mechanical impedance is important to understand how the ankle supports lower-extremity functions during interaction with the environment. This paper reports a novel procedure to characterize static multivariable ankle mechanical impedance. An experimental protocol using a wearable therapeutic robot, Anklebot, enabled reliable measurement of torque and angle data in multiple degrees of freedom simultaneously, a combination of inversion–eversion and dorsiflexion–plantarflexion. The measured multivariable torque–angle relation was represented as a vector field, and approximated using a method based on thin-plate spline smoothing with generalized cross validation. The vector field enabled assessment of several important characteristics of static ankle mechanical impedance, which are not available from prior single degree of freedom studies: the directional variation of ankle mechanical impedance, the extent to which the ankle be! haves as a spring, and evidence of uniquely neural contributions. The method was validated by testing a simple physical "mock-up" consisting of passive elements. Experiments with young unimpaired subjects quantified the behavior of the maximally relaxed human ankle, showing that ankle mechanical impedance is spring-like but strongly direction-dependent, being weakest in inversion. Remarkably, the analysis was sufficiently sensitive to detect a subtle but statistically significant deviation from spring-like behavior if subjects were not fully relaxed. This method may provide new insight about the function of the ankle, both unimpaired and after biomechanical or neurological injury.
• Viscoelastic properties of human cerebellum using magnetic resonance elastography
  - J Biomech 44(10):1909-1913 (2011)
  Background The cerebellum has never been mechanically characterised, despite its physiological importance in the control of motion and the clinical prevalence of cerebellar pathologies. The aim of this study was to measure the linear viscoelastic properties of the cerebellum in human volunteers using Magnetic Resonance Elastography (MRE). Methods Coronal plane brain 3D MRE data was performed on eight healthy adult volunteers, at 80 Hz, to compare the properties of cerebral and cerebellar tissues. The linear viscoelastic storage (G′) and loss moduli (G″) were estimated from the MRE wave images by solving the wave equation for propagation through an isotropic linear viscoelastic solid. Contributions of the compressional wave were removed via application of the curl-operator. Results The storage modulus for the cerebellum was found to be significantly lower than that for the cerebrum, for both white and grey matter. Cerebrum: white matter (mean±SD) G′=2.41±0.23 kPa, grey matter G′=2.34±0.22 kPa; cerebellum: white matter, G′=1.85±0.18 kPa, grey matter G′=1.77±0.24 kPa; cerebrum vs cerebellum, p<0.001. For the viscous behaviour, there were differences in between regions and also by tissue type, with the white matter being more viscous than grey matter and the cerebrum more viscous than the cerebellum. Cerebrum: white matter G″=1.21±0.21 kPa, grey matter G″=1.11±0.03 kPa; cerebellum: white matter G″=1.1±0.23 kPa, grey matter G″=0.94±0.17 kPa. Discussion These data represent the first available data on the viscoelastic properties of cerebellum, which suggest that the cerebellum is less physically stiff than the cerebrum, possibly leading to a different response to mechanical loading. These data will be useful for modelling of the cerebellum for a range of purposes.
• The effect of femoral tunnel placement on ACL graft orientation and length during in vivo knee flexion
  - J Biomech 44(10):1914-1920 (2011)
  Anatomically placed grafts are believed to more closely restore the function of the ACL. This study measured the effect of femoral tunnel placement on graft orientation and length during weight-bearing flexion. Both knees of twelve patients where the graft was placed near the anteroproximal border of the ACL and ten where the graft was placed near the center of the ACL were imaged using MR. These images were used to create 3D models of the reconstructed and intact contralateral knees, including the attachment sites of the native ACL and graft. Next, patients were imaged using biplanar fluoroscopy while performing a quasi-static lunge. The models were registered to the fluoroscopic images to reproduce in vivo knee motion. From the relative motion of the attachment sites on the models, the length and orientation of the graft and native ACL were measured. Grafts placed anteroproximally on the femur were longer and more vertical than the native ACL in both the sagittal and! coronal planes, while anatomically placed grafts more closely mimicked ACL motion. In full extension, the grafts placed anteroproximally were 12.3±5.2° (mean and 95%CI) more vertical than the native ACL in the sagittal plane, whereas the grafts placed anatomically were 2.9±3.7° less vertical. Grafts placed anteroproximally were up to 6±2 mm longer than the native ACL, while the anatomically placed grafts were a maximum of 2±2 mm longer. In conclusion, grafts placed anatomically more closely restored native ACL length and orientation. As a result, anatomic grafts are more likely to restore intact knee kinematics.
• Adaptive feedback potential in dynamic stability during disturbed walking in the elderly
  - J Biomech 44(10):1921-1926 (2011)
  After perturbation of the gait, feedback information may help regaining balance adequately, but it remains unknown whether adaptive feedback responses are possible after repetitive and unexpected perturbations during gait and if there are age-related differences. Prior experience may contribute to improved reactive behavior. Fourteen old (59–73yrs) and fourteen young (22–31yrs) males walked on a walkway which included one covered element. By exchanging this element participants either stepped on hard surface or unexpectedly on soft surface which caused a perturbation in gait. The gait protocol contained 5 unexpected soft trials to quantify the reactive adaptation. Each soft trial was followed by 4–8 hard trials to generate a wash-out effect. The dynamic stability was investigated by using the margin of stability (MoS), which was calculated as the difference between the anterior boundary of the base of support and the extrapolated position of the center of mass in the anterior-posterior direction. MoS at recovery leg touchdown were significantly lower in the unexpected soft trials compared to the baseline, indicating a less stable posture. However, MoS increased (p<0.05) in both groups within the disturbed trials, indicating feedback adaptive improvements. Young and old participants showed differences in the handling of the perturbation in the course of several trials. The magnitude of the reactive adaptation after the fifth unexpected perturbation was significantly different compared to the first unexpected perturbation (old: 49±30%; young: 77±40%), showing a tendency (p=0.065) for higher values in the young participants. Old individuals maintain the ability to adapt to feedback controlled perturbations. However, the locomotor behavior is more conservative compared to the young ones, leading to disadvantages in the reactive adaptation during disturbed walking.
• Glycated collagen alters endothelial cell actin alignment and nitric oxide release in response to fluid shear stress
  - J Biomech 44(10):1927-1935 (2011)
  People with diabetes suffer from early accelerated atherosclerosis, which contributes to morbidity and mortality from myocardial infarction, stroke, and peripheral vascular disease. Atherosclerosis is thought to initiate at sites of endothelial cell injury. Hyperglycemia, a hallmark of diabetes, leads to non-enzymatic glycosylation (or glycation) of extracellular matrix proteins. Glycated collagen alters endothelial cell function and could be an important factor in atherosclerotic plaque development. This study examined the effect of collagen glycation on endothelial cell response to fluid shear stress. Porcine aortic endothelial cells were grown on native or glycated collagen and exposed to shear stress using an in vitro parallel plate system. Cells on native collagen elongated and aligned in the flow direction after 24 h of 20 dynes/cm2 shear stress, as indicated by a 13% decrease in actin fiber angle distribution standard deviation. However, cells on glycated collag! en did not align. Shear stress-mediated nitric oxide release by cells on glycated collagen was half that of cells on native collagen, which correlated with decreased endothelial nitric oxide synthase (eNOS) phosphorylation. Glycated collagen likely inhibited cell shear stress response through altered cell–matrix interactions, since glycated collagen attenuated focal adhesion kinase activation with shear stress. When focal adhesion kinase was pharmacologically blocked in cells on native collagen, eNOS phosphorylation with flow was reduced in a manner similar to that of glycated collagen. These detrimental effects of glycated collagen on endothelial cell response to shear stress may be an important contributor to accelerated atherosclerosis in people with diabetes.
• Elasticity and stress relaxation of a very small vocal fold
  - J Biomech 44(10):1936-1940 (2011)
  Across mammals many vocal sounds are produced by airflow induced vocal fold oscillation. We tested the hypothesis that stress–strain and stress-relaxation behavior of rat vocal folds can be used to predict the fundamental frequency range of the species' vocal repertoire. In a first approximation vocal fold oscillation has been modeled by the string model but it is not known whether this concept equally applies to large and small species. The shorter the vocal fold, the more the ideal string law may underestimate normal mode frequencies. To accommodate the very small size of the tissue specimen, a custom-built miniaturized tensile test apparatus was developed. Tissue properties of 6 male rat vocal folds were measured. Rat vocal folds demonstrated the typical linear stress–strain behavior in the low strain region and an exponential stress response at strains larger than about 40%. Approximating the rat's vocal fold oscillation with the string model suggests that fund! amental frequencies up to about 6 kHz can be produced, which agrees with frequencies reported for audible rat vocalization. Individual differences and time-dependent changes in the tissue properties parallel findings in other species, and are interpreted as universal features of the laryngeal sound source.
• Circumferential variations of mechanical behavior of the porcine thoracic aorta during the inflation test
  - J Biomech 44(10):1941-1947 (2011)
  We developed an extension-inflation experimental apparatus with a stereo vision system and a stress–strain analysis method to determine the regional mechanical properties of a blood vessel. Seven proximal descending thoracic aortas were investigated during the inflation test at a fixed longitudinal stretch ratio of 1.35 over a transmural pressure range from 1.33 to 21.33 kPa. Four circumferential regions of each aorta were designated as the anterior (A), left lateral (L), posterior (P), and right lateral (R) regions, and the inflation test was repeated for each region of the aortas. We used continuous functions to approximate the surfaces of the regional aortic wall in the reference configuration and the deformed configuration. Circumferential stretch and stress at the four circumferential regions of the aorta were computed. Circumferential stiffness, defined as the tangent of the stress–stretch curve, and physiological aortic stiffness, named pressure–strain ela! stic modulus, were also computed for each region. In the low pressure range, the stress increased linearly with increased stretch, but the mechanical response became progressively stiffer in the high-pressure range above a transition point. At a transmural pressure of 12.00 kPa, mean values of stiffness were 416±104 kPa (A), 523±99 kPa (L), 634±91 kPa (P), and 489±82 kPa (R). The stiffness of the posterior region was significantly higher than that of the anterior region, but no significant difference was found in pressure–strain elastic modulus.
• Quadriceps strength and weight acceptance strategies continue to improve two years after anterior cruciate ligament reconstruction
  - J Biomech 44(10):1948-1953 (2011)
  The anterior cruciate ligament (ACL) is the most commonly-injured knee ligament during sporting activities. After injury, most individuals experience episodes of the knee giving way during daily activities (non-copers). Non-copers demonstrate asymmetrical quadriceps strength and movement patterns, which could have long-term deleterious effects on the integrity of the knee joint. The purpose of this study was to determine if non-copers resolve their strength and movement asymmetries within two years after surgery. 26 non-copers were recruited to undergo pre-operative quadriceps strength testing and 3-dimensional gait analysis. Subjects underwent surgery to reconstruct the ligament followed by physical therapy focused on restoring normal range of motion, quadriceps strength, and function. Subjects returned for quadriceps strength testing and gait analysis six months and two years after surgery. Acutely after injury, quadriceps strength was asymmetric between limbs, but r! esolved six months after surgery. Asymmetric knee angles, knee moments, and knee and hip power profiles were also observed acutely after injury and persisted six months after surgery despite subjects achieving symmetrical quadriceps strength. Two years after surgery, quadriceps strength in the involved limb continued to improve and most kinematic and kinetic asymmetries resolved. These findings suggest that adequate quadriceps strength does not immediately resolve gait asymmetries in non-copers. They also suggest that non-copers have the capacity to improve their quadriceps strength and gait symmetry long after ACL reconstruction.
• Anisotropic mechanical properties of ovine femoral periosteum and the effects of cryopreservation
  - J Biomech 44(10):1954-1959 (2011)
  The mechanical properties of periosteum are not well characterized. An understanding of these properties is critical to predict the environment of pluripotent and osteochondroprogenitor cells that reside within the periosteum and that have been shown recently to exhibit a remarkably rapid capacity to generate bone de novo. Furthermore, the effects of cryopreservation on periosteal mechanical properties are currently unknown. We hypothesized that the periosteum is pre-stressed in situ and that the periosteum exhibits anisotropic material properties, e.g. the elastic modulus of the periosteum depends significantly on the direction of loading. We measured the change in area, axial length, and circumferential length of anterior, posterior, medial, and lateral fresh periosteal samples removed from underlying bone (t=0–16 h) as well as the average strain in axially and circumferentially oriented anterior periosteal samples subjected to tensile strain (0.004 mm/s) until fai! lure. The elastic modulus was calculated from the resulting stress–strain curves. Tensile testing was repeated with axially aligned samples that had been slowly cryopreserved for comparison to fresh samples. Periosteal samples from all aspects shrank 44–54%, 33–47%, and 9–19% in area, axial length, and circumferential length, respectively. At any given time, the periosteum shrank significantly more in the axial direction than the circumferential direction. Tensile testing showed that the periosteum is highly anisotropic. When loaded axially, a compliant toe region of the stress–strain curve (1.93±0.14 MPa) is followed by a stiffer region until failure (25.67±6.87 MPa). When loaded circumferentially, no toe region is observable and the periosteum remained compliant until failure (4.41±1.21 MPa). Cryopreservation had no significant effect on the elastic modulus of the periosteum. As the periosteum serves as the bounding envelope of the femur, anisotropy in perios! teal properties may play a key role in modulating bone growth,! healing and adaptation, in health, disease, and trauma.
• Wave propagation in protein microtubules modeled as orthotropic elastic shells including transverse shear deformations
  - J Biomech 44(10):1960-1966 (2011)
  Wave propagation along the microtubules is one of the issues of major concern in various microtubule cellular functions. In this study, the general wave propagation behavior in protein microtubules is investigated based on a first-order shear deformation shell theory for orthotropic materials, with particular emphasis on the role of strongly anisotropic elastic properties of microtubules. According to experimental observation, the first-order shear deformation theory is used for the modeling of microtubule walls. A general displacement representation is introduced and a type of coupled polynomial eigenvalue problem is developed. Numerical examples describe the effects of shear deformation and rotary inertia on wave velocities in orthotropic microtubules. Finally, the influences of the microtubule shear modulus, axial external force, effective thickness and material temperature dependency on wave velocities along the microtubule protofilaments, helical pathway and radia! l directions are elucidated. Most results presented in the present investigation have been absent from the literature for the wave propagation in microtubules.
• Modelling tendon excursions and moment arms of the finger flexors: Anatomic fidelity versus function
  - J Biomech 44(10):1967-1973 (2011)
  A detailed musculoskeletal model of the human hand is needed to investigate the pathomechanics of tendon disorders and carpal tunnel syndrome. The purpose of this study was to develop a biomechanical model with realistic flexor tendon excursions and moment arms. An existing upper extremity model served as a starting point, which included programmed movement of the index finger. Movement capabilities were added for the other fingers. Metacarpophalangeal articulations were modelled as universal joints to simulate flexion/extension and abduction/adduction while interphalangeal articulations used hinges to represent flexion. Flexor tendon paths were modelled using two approaches. The first method constrained tendons with control points, representing annular pulleys. The second technique used wrap objects at the joints as tendon constraints. Both control point and joint wrap models were iteratively adjusted to coincide with tendon excursions and moment arms from a anthropom! etric regression model using inputs for a 50th percentile male. Tendon excursions from the joint wrap method best matched the regression model even though anatomic features of the tendon paths were not preserved (absolute differences: mean<0.33 mm, peak<0.74 mm). The joint wrap model also produced similar moment arms to the regression (absolute differences: mean<0.63 mm, peak<1.58 mm). When a scaling algorithm was used to test anthropometrics, the scaled joint wrap models better matched the regression than the scaled control point models. Detailed patient-specific anatomical data will improve model outcomes for clinical use; however, population studies may benefit from simplified geometry, especially with anthropometric scaling.
• Sex-specific differences in gait patterns of healthy older adults: Results from the Baltimore Longitudinal Study of Aging
  - J Biomech 44(10):1974-1979 (2011)
  The effects of normal aging and orthopedic conditions on gait patterns during customary walking have been extensively investigated. Empirical evidence supports the notion that sex differences exist in the gait patterns of young adults but it is unclear as to whether sex differences exist in older adults. The aim of this study was to investigate sex-specific differences in gait among older adults. Study participants were 336 adults (50–96 years; 162 women) enrolled in the Baltimore Longitudinal Study of Aging (BLSA) who completed walking tasks at self-selected speed without assistance. After adjusting for significant covariates, women walked with higher cadence (p=0.01) and shorter stride length (p=0.006) compared to men, while gait speed was not significantly related to sex. Women also had less hip range of motion (ROM; p=0.004) and greater ankle ROM (p<0.001) in the sagittal-plane, and greater hip ROM (p=0.004) in the frontal-plane. Hip absorptive mechanical work ex! penditure (MWE) of the women was greater in the sagittal-plane (p<0.001) and lower in the frontal-plane (p<0.001), compared to men. In summary, women's gait is characterized by greater ankle ROM than men while men tend to have greater hip ROM than women. Characterizing unique gait patterns of women and men with aging may be beneficial for detecting the early stages of gait abnormalities that may lead to pathology.
• Wave front migration of endothelial cells in a bone-implant interface
  - J Biomech 44(10):1980-1986 (2011)
  The neo-vascularization of the host site is crucial for the primary fixation and the long-term stability of the bone-implant interface. Our aim was to investigate the progression of endothelial cell population in the first weeks of healing. We proposed a theoretical reactive model to study the role of initial conditions, random motility, haptotaxis and chemotaxis in interactions with fibronectin factors and transforming angiogenic factors. The application of governing equations concerned a canine experimental implant and numerical experiments based upon statistical designs of experiments supported the discussion. We found that chemotaxis due to transforming angiogenic factors was attracting endothelial cells present into the host bone. Haptotaxis conditioned by fibronectin factors favored cells adhesion to the host bone. The combination of diffusive and reactive effects nourished the wave front migration of endothelial cells from the host bone towards the implant. Angiogenesis goes together with new-formed bone formation in clinics, so the similarity of distribution patterns of mineralized tissue observed in-vivo and the spatio-temporal concentration of endothelial cells predicted by the model, tended to support the reliability of our theoretical approach.
• Moment arms of the human digital flexors
  - J Biomech 44(10):1987-1990 (2011)
  For the extrinsic hand flexors (flexor digitorum profundus, FDP; flexor digitorum superficialis, FDS; flexor pollicis longus, FPL), moment arm corresponds to the tendon's distance from the center of the metacarpalphalangeal (MP), proximal interphalangeal (PIP), or distal interphalangeal (DIP) joint. The clinical value of establishing accurate moment arms has been highlighted for biomechanical modeling, the development of robotic hands, designing rehabilitation protocols, and repairing flexor tendon pulleys ([Brand et al., 1975], [An et al., 1983], [Thompson and Giurintano, 1989], [Deshpande et al., 2010] and [Wu et al., 2010]). In this study, we define the moment arms for all of the extrinsic flexor tendons of the hand across all digital joints for all digits in cadaveric hands.
• Estimation of stride length in level walking using an inertial measurement unit attached to the foot: A validation of the zero velocity assumption during stance
  - J Biomech 44(10):1991-1994 (2011)
  In a variety of applications, inertial sensors are used to estimate spatial parameters by double integrating over time their coordinate acceleration components. In human movement applications, the drift inherent to the accelerometer signals is often reduced by exploiting the cyclical nature of gait and under the hypothesis that the velocity of the sensor is zero at some point in stance. In this study, the validity of the latter hypothesis was investigated by determining the minimum velocity of progression of selected points of the foot and shank during the stance phase of the gait cycle while walking at three different speeds on level ground. The errors affecting the accuracy of the stride length estimation resulting from assuming a zero velocity at the beginning of the integration interval were evaluated on twenty healthy subjects. Results showed that the minimum velocity of the selected points on the foot and shank increased as gait speed increased. Whereas the avera! ge minimum velocity of the foot locations was lower than 0.011 m/s, the velocity of the shank locations were up to 0.049 m/s corresponding to a percent error of the stride length equal to 3.3%. The preferable foot locations for an inertial sensor resulted to be the calcaneus and the lateral aspect of the rearfoot. In estimating the stride length, the hypothesis that the velocity of the sensor can be set to zero sometimes during stance is acceptable only if the sensor is attached to the foot.
• Near-terminal creep damage does not substantially influence fatigue life under physiological loading
  - J Biomech 44(10):1995-1998 (2011)
  Cortical bone specimens were damaged using repeated blocks of tensile creep loading until a near-terminal amount of creep damage was generated (corresponding to a reduction in elastic modulus of 15%). One group of cortical bone specimens was submitted to the near-terminal damage protocol and subsequently underwent fatigue loading in tension with a maximum strain of 2000 με (Damage Fatigue, n=5). A second group was submitted to cyclic fatigue loading but was not pre-damaged (Control Fatigue, n=5). All but one specimen (a damaged specimen) reached run-out (10 million cycles, 7.7 days). No significant differences in microscopic cracks or other tissue damage were observed between the two groups or between either group and additional, completely unloaded specimens. Our results suggest that damage in cortical bone allograft that is not obvious or associated with a stress riser may not substantially affect its fatigue life under physiologic loading.
• Knee kinematics and kinetics during shuttle run cutting: Comparison of the assessments performed with and without the point cluster technique
  - J Biomech 44(10):1999-2003 (2011)
  The differences between the assessments performed with and without the point cluster technique (PCT) for knee joint motions during the high-risk movements associated with non-contact anterior cruciate ligament (ACL) injuries have not been reported. This study aims to examine the differences between PCT and non-PCT assessments for knee joint angles and moments during shuttle run cutting. Fourteen high school athletes performed a maximal effort shuttle run cutting task. Motion data were collected by an 8-camera motion analysis system at 200 Hz, and ground reaction force data were recorded using a force plate at 1000 Hz. In both PCT and non-PCT approaches, the knee joint angles were calculated using Euler angle rotations, and the knee joint moments were obtained by solving the Newton–Euler equations using an inverse dynamics technique. For the extension/flexion angle, good agreement was measured between PCT and non-PCT assessments. The abduction angle obtained in the no! n-PCT assessment was smaller than that obtained with the PCT. An internal rotation angle was obtained in the PCT assessment, whereas a small external rotation angle was obtained in the non-PCT assessment. For the knee joint moments, good agreement between PCT and non-PCT assessments was observed for all the components. The differences in the knee joint angles were attributed in part to the differences in the position of the medial femoral epicondyle. The results suggest that the ACL injury risk during shuttle run cutting is estimated lower in the non-PCT assessment than in the PCT assessment.
• Skin-fixed scapula trackers: A comparison of two dynamic methods across a range of calibration positions
  - J Biomech 44(10):2004-2007 (2011)
  The aim of this study was to establish the optimal methodology for skin-fixed measurement of the scapula during dynamic movement. This was achieved by comparing an optimally positioned Scapula Tracker device (ST) to a previously described palpation device, taken as the true measure of scapular kinematics. These measurements were compared across a range of calibration positions, including the use of multiple calibration positions for a single movement, in order to establish an optimal calibration approach. Ten subjects' scapular motion was measured using this ST and a previously described Acromial Method (AM). The two datasets were compared at a standard, an optimal and a 'multiple' calibration position, thus allowing a direct comparison between two common skin-fixed methods to track the bony kinematics of the scapula across different calibration positions. A comparison was also made with a bone-fixed technique from the literature. At both the standard and optimal c! alibration positions the ST was shown to be the more accurate measure of internal rotation and posterior tilt, particularly above 100° of humerothoracic elevation. The ST errors were found to be acceptable in relation to clinically important levels. Calibration positions have been shown to have a significant effect on the errors of both skin-fixed measurement techniques and therefore the importance of correct calibration is highlighted. It has thus been shown that a ST can be used to accurately quantify scapular motion when appropriately calibrated for the range of motion being measured.
• Identification of factors associated with risk of fall using a force platform and power spectrum analysis technique
  - J Biomech 44(10):2008-2012 (2011)
  The purpose of this retrospective study was to investigate some parameters of neuromuscular performance of the lower limbs in a population cross-section and their relationship to the risk of falls, using a force platform (FP). Individuals from the Lower Franconia population were invited by public advertisement. Out of a total of 1720 invited subjects 50–90 years of age, the successful completion of all tests were achieved by 807 women, age 66.4±9.3, and 442 men, age 64.0±9.2. A novel FP measured the time series of vertical forces over 10 s during 3 kinds of tests: tandem stand with eyes closed, knee bends, and chair rise. Proprietary software captured the peak force and calculated the power density distribution (PSD), intended to characterize balance and power through the FP. Grip strength as a common geriatric force test was dynamometrically measured for comparison. The parameters were related to the number of falls in the past 12 months in both genders. Mean PSD ! showed little age dependency and was not related to falls in tandem stance. Peak forces and power over 10 s knee bends showed a larger age-related decrease in men than in women and these parameters were related to falls (p<0.001), whereas they were not related to falls in the chair rise test. Chair rise time and grip strength was related to falls in women (p<0.01). The PSD obtained from the tandem test with eyes closed did not provide a sensitive parameter associated with falls. Knee bends may be a meaningful FP screening test that justifies further studies of physical performance related to the risk of falls, whereas chair rise and grip measurements provided inferior information in this study.
• Corrigendum to "A study of the viability of obtaining a generic animation of the foot while walking for the virtual testing of footwear using dorsal pressures" [J. Biomech. 42 (2009) 2040–2046]
  - J Biomech 44(10):2013 (2011)