Hot off the presses! Jul 28 J Biomech

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Latest Articles Include:

• Editorial Board and Publication Information
  - J Biomech 44(11):IFC (2011)
  
• The role of lubricant entrapment at biological interfaces: Reduction of friction and adhesion in articular cartilage
  - J Biomech 44(11):2015-2020 (2011)
  Friction and adhesion of articular cartilage from high- and low-load-bearing regions of bovine knee joints were examined with a tribometer under various loads and equilibration times. The effect of trapped lubricants was investigated by briefly unloading the cartilage sample before friction testing, to allow fluid to reflow into the contact interface and boundary lubricants to rearrange. Friction and adhesion of high-load-bearing joint regions were consistently lower than those of low-load-bearing regions. This investigation is the first to demonstrate the regional variation in the friction and adhesion properties of articular cartilage. Friction coefficient decreased with increasing contact pressure and decreasing equilibration time. Briefly unloading cartilage before the onset of sliding resulted in significantly lower friction and adhesion and a loss of the friction dependence on contact pressure, suggesting an enhancement of the cartilage tribological properties by! trapped lubricants. The results of this study reveal significant differences in the friction and adhesion properties between high- and low-load-bearing joint regions and elucidate the role of trapped lubricants in cartilage tribology.
• Numerical simulation of blood pulsatile flow in a stenosed carotid artery using different rheological models
  - J Biomech 44(11):2021-2030 (2011)
  Symmetrical 30–60% stenosis in a common carotid artery under unsteady flow condition for Newtonian and six non-Newtonian viscosity models are investigated numerically. Results show power-law model produces higher deviations, in terms of velocity and wall shear stress in comparison with other models while generalized power-law and modified-Casson models are more prone to Newtonian state. Comparing separation length of recirculation region at different critical points of cardiac cycle confirms the necessity of considering blood flow in unsteady mode. Increasing stenosis intensity causes flow patterns more disturbed downstream of the stenosis and WSS appear to develop remarkably at the stenosis throat.
• A mathematical model of force transmission from intrafascicularly terminating muscle fibers
  - J Biomech 44(11):2031-2039 (2011)
  Many long skeletal muscles are comprised of fibers that terminate intrafascicularly. Force from terminating fibers can be transmitted through shear within the endomysium that surrounds fibers or through tension within the endomysium that extends from fibers to the tendon; however, it is unclear which pathway dominates in force transmission from terminating fibers. The purpose of this work was to develop mathematical models to (i) compare the efficacy of lateral (through shear) and longitudinal (through tension) force transmission in intrafascicularly terminating fibers, and (ii) determine how force transmission is affected by variations in the structure and properties of fibers and the endomysium. The models demonstrated that even though the amount of force that can be transmitted from an intrafascicularly terminating fiber is dependent on fiber resting length (the unstretched length at which passive stress is zero), endomysium shear modulus, and fiber volume fraction ! (the fraction of the muscle cross-sectional area that is occupied by fibers), fibers that have values of resting length, shear modulus, and volume fraction within physiologic ranges can transmit nearly all of their peak isometric force laterally through shearing of the endomysium. By contrast, the models predicted only limited force transmission ability through tension within the endomysium that extends from the fiber to the tendon. Moreover, when fiber volume fraction decreases to unhealthy ranges (less than 50%), the force-transmitting potential of terminating fibers through shearing of the endomysium decreases significantly. The models presented here support the hypothesis that lateral force transmission through shearing of the endomysium is an effective mode of force transmission in terminating fibers.
• Fluid flow induced calcium response in osteoblasts: Mathematical modeling
  - J Biomech 44(11):2040-2046 (2011)
  Fluid flow in the bone lacuno–canalicular network can induce dynamic fluctuation of intracellular calcium concentration ([Ca2+]i) in osteoblasts, which plays an important role in bone remodeling. There has been limited progress in the mathematical modeling of this process probably due to its complexity, which is controlled by various factors such as Ca2+ channels and extracellular messengers. In this study we developed a mathematical model to describe [Ca2+]i response induced by fluid shear stress (SS) by integrating the major factors involved and analyzed the effects of different experimental setups (e.g. [Ca2+]i baseline, pretreatment with ATP). In this model we considered the ATP release process and the activities of multiple ion channels and purinergic receptors. The model was further verified quantitatively by comparing the simulation results with experimental data reported in literature. The results showed that: (i) extracellular ATP concentration has more sign! ificant effect on [Ca2+]i baseline (73% increase in [Ca2+]i with extracellular ATP concentration varying between 0 and 10 μM), as compared to that induced by SS (25% variation in [Ca2+]i with SS varying from 0 to 3.5 Pa); (ii) Pretreatment with ATP-medium results in different [Ca2+]i response as compared to the control group (ATP-free medium) under SS; (iii) Relative [Ca2+]i fluctuation over baseline is more reliable to show the [Ca2+]i response process than the absolute [Ca2+]i response peak. The developed model may improve the experimental design and facilitate our understanding of the mechanotransduction process in osteoblasts.
• Monitoring micrometer-scale collagen organization in rat-tail tendon upon mechanical strain using second harmonic microscopy
  - J Biomech 44(11):2047-2052 (2011)
  We continuously monitored the microstructure of a rat-tail tendon during stretch/relaxation cycles. To that purpose, we implemented a new biomechanical device that combined SHG imaging and mechanical testing modalities. This multi-scale experimental device enabled simultaneous visualization of the collagen crimp morphology at the micrometer scale and measurement of macroscopic strain–stress response. We gradually increased the ultimate strain of the cycles and showed that preconditioning mostly occurs in the first stretching. This is accompanied by an increase of the crimp period in the SHG image. Our results indicate that preconditioning is due to a sliding of microstructures at the scale of a few fibrils and smaller, that changes the resting length of the fascicle. This sliding can reverse on long time scales. These results provide a proof of concept that continuous SHG imaging performed simultaneously with mechanical assay allows analysis of the relationship betwe! en macroscopic response and microscopic structure of tissues.
• Molecular dynamics simulations of pore formation dynamics during the rupture process of a phospholipid bilayer caused by high-speed equibiaxial stretching
  - J Biomech 44(11):2053-2058 (2011)
  Rupture of a phospholipid bilayer under mechanical stresses is triggered by pore formation in an intact bilayer. To understand the molecular details of the dynamics of pore formation we perform molecular dynamics simulations of a phospholipid bilayer under two different equibiaxial stretching conditions: first, unsteady stretching with various stretching speeds in the range of 0.1–1.0 m/s, and second, quasistatic stretching. We analyze (i) patterns of pore formation, (ii) the critical area where a pore forms, (iii) the deformation of the bilayer, and (iv) the apparent breaking force. With stretching, the bilayer deforms anisotropically due to lipid chain packing and water penetrating into the hydrophilic region of the bilayer, and when the area exceeds a critical value, water filled pore structure penetrating the bilayer forms and develops into a large pore, resulting in rupture. For a high stretching speed, small pores (multipore) can temporarily form in a small are! a. It has been statistically determined that the probability of the multipore formation, the critical areal strain, and the apparent breaking force increase with the stretching speed in the range of 0–50%, 0.8–2.0, and 250–400 pN, respectively. The results qualitatively agree with the experimental and other simulation results, and rationalize the leakage of hemoglobin from erythrocytes in shock wave experiments.
• Knee and ankle joint torque–angle relationships of multi-joint leg extension
  - J Biomech 44(11):2059-2065 (2011)
  The force–length-relation (F–l-r) is an important property of skeletal muscle to characterise its function, whereas for in vivo human muscles, torque–angle relationships (T–a-r) represent the maximum muscular capacity as a function of joint angle. However, since in vivo force/torque–length data is only available for rotational single-joint movements the purpose of the present study was to identify torque–angle-relationships for multi-joint leg extension. Therefore, inverse dynamics served for calculation of ankle and knee joint torques of 18 male subjects when performing maximum voluntary isometric contractions in a seated leg press. Measurements in increments of 10° knee angle from 30° to 100° knee flexion resulted in eight discrete angle configurations of hip, knee and ankle joints. For the knee joint we found an ascending–descending T–a-r with a maximum torque of 289.5°±43.3 N m, which closely matches literature data from rotational knee extensi! on. In comparison to literature we observed a shift of optimum knee angle towards knee extension. In contrast, the T–a-r of the ankle joint vastly differed from relationships obtained for isolated plantar flexion. For the ankle T–a-r derived from multi-joint leg extension subjects operated over different sections of the force–length curve, but the ankle T–a-r derived from isolated joint efforts was over the ascending limb for all subjects. Moreover, mean maximum torque of 234.7±56.6 N m exceeded maximal strength of isolated plantar flexion (185.7±27.8 N m). From these findings we conclude that muscle function between isolated and more physiological multi-joint tasks differs. This should be considered for ergonomic and sports optimisation as well as for modelling and simulation of human movement.
• Cross-correlations of center of mass and center of pressure displacements reveal multiple balance strategies in response to sinusoidal platform perturbations
  - J Biomech 44(11):2066-2076 (2011)
  Compared to static balance, dynamic balance requires a more complex strategy that goes beyond keeping the center of mass (COM) within the base of support, as established by the range of foot center of pressure (COP) displacement. Instead, neuromechanics must accommodate changing support conditions and inertial effects. Therefore, because they represent body's position and changes in applied moments, relative COM and COP displacements may also reveal dynamic postural strategies. To investigate this concept, kinetics and kinematics were recorded during three 12 cm, 1.25 Hz, sagittal perturbations. Forty-one individual trials were classified according to averaged cross-correlation lag between COM and COP displacement (lagCOM:COP) and relative head-to-ankle displacement (Δhead/Δankle) using a k-means analysis. This process revealed two dominant patterns, one for which the lagCOM:COP was positive (Group 1 (n=6)) and another for which it was negative (Group 2 (n=5)) . Grou! p 1 (G1) absorbed power from the platform over most of the cycle, except during transitions in platform direction. Conversely, Group 2 (G2) participants applied power to the platform to maintain a larger margin between COM and COP position and also had larger knee flexion and ankle dorsiflexion, resulting in a lower stance. By the third repetition, the only kinematic differences were a slightly larger G2 linear knee displacement (p=0.008) and an antiphasic relationship of pelvis (linear) and trunk (angular) displacements. Therefore, it is likely that the strategy differences were detected by including COP in the initial screening method, because it reflects the pattern of force application that is not detectable by tracking body movements.
• Stress and strain analysis of contractions during ramp distension in partially obstructed guinea pig jejunal segments
  - J Biomech 44(11):2077-2082 (2011)
  Previous studies have demonstrated morphological and biomechanical remodeling in the intestine proximal to an obstruction. The present study aimed to obtain stress and strain thresholds to initiate contraction and the maximal contraction stress and strain in partially obstructed guinea pig jejunal segments. Partial obstruction and sham operations were surgically created in mid-jejunum of male guinea pigs. The animals survived 2, 4, 7 and 14 days. Animals not being operated on served as normal controls. The segments were used for no-load state, zero-stress state and distension analyses. The segment was inflated to 10 cmH2O pressure in an organ bath containing 37 °C Krebs solution and the outer diameter change was monitored. The stress and strain at the contraction threshold and at maximum contraction were computed from the diameter, pressure and the zero-stress state data. Young's modulus was determined at the contraction threshold. The muscle layer thickness in obstru! cted intestinal segments increased up to 300%. Compared with sham-obstructed and normal groups, the contraction stress threshold, the maximum contraction stress and the Young's modulus at the contraction threshold increased whereas the strain threshold and maximum contraction strain decreased after 7 days obstruction (P<0.05 and 0.01). In conclusion, in the partially obstructed intestinal segments, a larger distension force was needed to evoke contraction likely due to tissue remodeling. Higher contraction stresses were produced and the contraction deformation (strain) became smaller.
• Low pulse pressure with high pulsatile external left ventricular power: Influence of aortic waves
  - J Biomech 44(11):2083-2089 (2011)
  Elevated pulse pressure (pp) is considered to be a risk factor for adverse cardiovascular events since it is directly related to an elevated myocardial workload. Information about both pressure and flow wave must be provided to assess hemodynamic complexity and true level of external left ventricular power (ELVP). pp value as a single feature of aortic waves cannot identify true level of ELVP. However, it is generally presumed that ELVP (and consequently LV workload) is positively correlated with pp. This study examined this positive correlation. The aim of this study was to test the hypothesis that aortic wave dynamics can create destructive hemodynamic conditions that increase the ELVP even though pp appears to be normal. To test this hypothesis, a computational model of the aorta with physiological properties was used. A Finite Element Method with fluid–structure interaction was employed to solve the equations of the solid and fluid. The aortic wall was assumed to! be elastic and isotropic. The blood was assumed to be an incompressible Newtonian fluid. Simulations were performed for various heart rates (HR) and different aortic compliances while keeping the shape of the inlet flow and peripheral resistance constant. As expected, in most of the cases studied here, higher pp was associated with higher LV power demand. However, for a given cardiac output, mean pressure, and location of total reflection site, we have found cases where the above-mentioned trend does not hold. Our results suggest that using pp as a single index can result in an underestimation of the LV power demand under certain conditions related to the altered wave dynamics. Hence, in hypertensive patients, a full analysis of aortic wave dynamics is essential for the prevention and management of left ventricular hypertrophy (LVH) and congestive heart failure.
• On a phenomenological model for active smooth muscle contraction
  - J Biomech 44(11):2090-2095 (2011)
  This paper presents a three-dimensional phenomenological model for the description of smooth muscle activation. A strain energy function is proposed as sum of the strain energy stored in the passive tissue, consisting of elastin and collagen, and an active calcium-driven energy related to the chemical contraction of the smooth muscle cells. Further, the proposed model includes the dispersions of the orientations of smooth muscle cells and collagen. These dispersions, measured in experiments, can be directly inserted into the model. The approach is implemented into the framework of the finite element method. Consequently, beside a validation with experiments the modelling concept is used for a three-dimensional numerical study.
• Accuracy of generic musculoskeletal models in predicting the functional roles of muscles in human gait
  - J Biomech 44(11):2096-2105 (2011)
  Biomechanical assessments of muscle function are often performed using a generic musculoskeletal model created from anatomical measurements obtained from cadavers. Understanding the validity of using generic models to study movement biomechanics is critical, especially when such models are applied to analyze the walking patterns of persons with impaired mobility. The aim of this study was to evaluate the accuracy of scaled-generic models in determining the moment arms and functional roles of the lower-limb muscles during gait. The functional role of a muscle was described by its potential to contribute to the acceleration of a joint or the acceleration of the whole-body center of mass. A muscle's potential acceleration was defined as the acceleration induced by a unit of muscle force. Dynamic simulations of walking were generated for four children with cerebral palsy and five age-matched controls. Each subject was represented by a scaled-generic model and a model dev! eloped from magnetic resonance (MR) imaging. Calculations obtained from the scaled-generic model of each subject were evaluated against those derived from the corresponding MR-based model. Substantial differences were found in the muscle moment arms computed using the two models. These differences propagated to calculations of muscle potential accelerations, but predictions of muscle function (i.e., the direction in which a muscle accelerated a joint or the center of mass and the magnitude of the muscle's potential acceleration relative to that of other muscles) were consistent between the two modeling techniques. Our findings suggest that scaled-generic models and image-based models yield similar assessments of muscle function in both normal and pathological gait.
• Short range stiffness elastic limit depends on joint velocity
  - J Biomech 44(11):2106-2112 (2011)
  Muscles behave as elastic springs during the initial strain phase, indicated as short range stiffness (SRS). Beyond a certain amount of strain the muscle demonstrates a more viscous behavior. The strain at which the muscle transits from elastic- to viscous-like behavior is called the elastic limit and is believed to be the result of breakage of cross-bridges between the contractile filaments. The aim of this study was to test whether the elastic limit, measured in vivo at the wrist joint, depended on the speed of lengthening. Brief extension rotations were imposed to the wrist joint (n=8) at four different speeds and at three different levels of voluntary torque using a servo controlled electrical motor. Using a recently published identification scheme, we quantified the elastic limit from measured joint angle and torque. The results showed that the elastic limit significantly increased with speed in a linear way, indicating to a constant time of approximately 30 ms be! fore cross-bridges break. The implications for movement control of the joint are discussed.
• On the derivation of passive 3D material parameters from 1D stress–strain data of hydrostats
  - J Biomech 44(11):2113-2117 (2011)
  The present paper offers a novel equivalent-pressure approach to the derivation of isotropic passive muscle parameters from 1D stress–strain data sets. The approach aims specifically at the identification of material parameters in hydrostats, in which case the equivalent-force approach that is common for skeletal muscle generates suboptimal results. Instead, an equivalent-pressure hypothesis is formulated which provides more adequate boundary conditions for the concluding curve-fitting procedure. The choice of an appropriate constitutive description is decisive for the quality of the deduced parameter sets. Here, a Yeoh material law is chosen for the model of a squid tentacle. Parameters derived by both, equivalent-force and equivalent-pressure algorithms, are compared, illustrating the applicability limits of either. They are implemented in a finite element model of the tentacle. A prey-capture strike is simulated and compared to data from literature. The hydrostat-specific interpretation of the equivalent-pressure hypothesis is shown to match the reference very well.
• Effect of chemical cross-linking on the mechanical properties of elastomeric peptides studied by single molecule force spectroscopy
  - J Biomech 44(11):2118-2122 (2011)
  Mechanical properties of animal tissues are mainly provided by the assembly of single elastomeric proteins into a complex network of filaments. Even if the overall elastic properties of such a reticulated structure depend on the mechanical characteristics of the constituents, it is not the only aspect to be considered. In addition, the aggregation mechanism has to be clarified to attain a full knowledge of the molecular basis of the elastic properties of natural nanostructured materials. This aim is even more crucial in the process of rational design of biomaterials with selected mechanical properties, in which not only the mechanics of single molecules but also of their assemblies has to be cared of. In this study, this aspect was approached by means of single molecule stretching experiments. In particular, the effect of chemical cross-linking on the mechanical properties of a naturally inspired elastomeric peptide was investigated. Accordingly, we observed that, in o! rder to preserve the elastic properties of the single filament, the two strands of the dimer have to interact with each other. The results thus confirm that the influence of the aggregation process on the mechanical properties of a molecular assembly cannot be neglected.
• A structurally optimal control model for predicting and analyzing human postural coordination
  - J Biomech 44(11):2123-2128 (2011)
  This paper proposes a closed-loop optimal control model predicting changes between in-phase and anti-phase postural coordination during standing and related supra-postural activities. The model allows the evaluation of the influence of body dynamics and balance constraints onto the adoption of postural coordination. This model minimizes the instantaneous norm of the joint torques with a controller in the head space, in contrast with classical linear optimal models used in the postural literature and defined in joint space. The balance constraint is addressed with an adaptive ankle torque saturation. Numerical simulations showed that the model was able to predict changes between in-phase and anti-phase postural coordination modes and other non-linear transient dynamics phenomena.
• In-vivo determination of 3D muscle architecture of human muscle using free hand ultrasound
  - J Biomech 44(11):2129-2135 (2011)
  Muscle architecture is an important parameter affecting the muscle function. Most of the previous studies on in-vivo muscle architecture have used in 2D ultrasound. The importance of the third dimension has not been much explored due to lack of appropriate methods. DT-MRI has been used to study muscle architecture in 3D, however, due to long scan times of about 15 min DT-MRI has not been suitable to study active muscle contractions. The purpose of this study was to develop and validate methods to determine in-vivo muscle fascicle orientations in 3D using ultrasound. We have used 2D ultrasound and a 3D position tracker system to find the 3D fascicle orientation in 3D space. 2D orientations were obtained by using automated methods developed in our previous studies and we have extended these in the current study to obtain the 3D muscle fascicle orientation in 3D space. The methods were validated using the physical phantom and we found that the mean error in the measuremen! t was less than 0.5° in each of the three co-ordinate planes. These methods can be achieved with short scan times (less than 2 min for the gastrocnemii) and will thus enable future studies to quantify 3D muscle architecture during sub-maximal voluntary contractions.
• Development of a novel intraoral measurement device to determine the biomechanical characteristics of the human periodontal ligament
  - J Biomech 44(11):2136-2143 (2011)
  Periodontal diseases like gingivitis and periodontitis have damaging effects on the periodontium and commonly affect the mechanical properties of the periodontal ligament (PDL), which in the end might lead to loss of teeth. Monitoring tooth mobility and changes of the material properties of the PDL might help in early diagnosis of periodontal diseases and improve their prognosis. It was the aim of this study to develop a novel intraoral device to determine the biomechanical characteristics of the periodontal ligament. This includes the measurement of applied forces and resulting tooth displacement in order to investigate the biomechanical behaviour of the periodontium with varying loading protocols with respect to velocity and tooth displacement. The developed device uses a piezoelectric actuator to apply a displacement to a tooth's crown, and the resulting force is measured by an integrated force sensor. To measure the tooth displacement independently and non-invasive! ly, two magnets are fixed on the teeth. The change in the magnetic field caused by the movement of the magnets is measured by a total of 16 Hall sensors. The displacement of the tooth is calculated from the movement of the magnets. The device was tested in vitro on premolars of four porcine mandibular segments and in vivo on two volunteers. The teeth were loaded with varying activation curves. Comparing the force progression of different activation velocities, the forces decreased with decreasing velocity. Intensive testing demonstrated that the device fulfils all requirements. After acceptance of the ethical committee, further testing in clinical measurements is planned.
• Sarcomere overextension reduces stretch-induced tension loss in myofibrils of rabbit psoas
  - J Biomech 44(11):2144-2149 (2011)
  Stretch-induced damage to skeletal muscles results in loss of isometric tension. Although there is no direct evidence, loss of tension has been implicitly assumed to be the consequence of permanent loss of myofilament overlap in some sarcomeres ('sarcomere overextension'). Using isolated myofibrils of rabbit psoas muscle (n=38; 6 control and 32 test specimens) at 12–15 °C, we directly tested the idea that loss of tension following stretch is caused by sarcomere overextension. Experimental myofibrils were maximally activated at the edge of the descending limb (sarcomere length 2.9 μm) of the sarcomere length-tension relationship and then stretched by 1 μm sarcomere–1 at a constant speed of 0.1 μm s–1 sarcomere–1 to result in an average strain of 33.6±0.9% (mean±1 SE). Myofibrils were immediately returned to the original lengths and relaxed. Isometric tension measured in a subsequent re-activation 3–5 min later was reduced by 24.6±1.5% from its origi! nal value. In 22 out of the 32 test specimens, all sarcomeres maintained myofilament overlap, while in 10 myofibrils one or two sarcomeres were stretched permanently beyond myofilament overlap (>4.0 μm), and thus exhibited overextended sarcomeres. Loss of tension following stretch was significantly smaller in myofibrils with overextended sarcomeres compared to myofibrils with no overextended sarcomeres (19.5±2.3% and 27.1±1.8%, respectively; p=0.017). Combined, these results suggest that the loss of tension associated with stretch-induced damage can occur in the absence of sarcomere overextension and that sarcomere overextension limits rather than causes stretch-induced tension loss.
• Biomechanical effects of flow and coculture on human aortic and cord blood-derived endothelial cells
  - J Biomech 44(11):2150-2157 (2011)
  Human endothelial cells derived from umbilical cord blood (hCB-ECs) represent a promising cell source for endothelialization of tissue engineered blood vessels. hCB-ECs cultured directly above human aortic smooth muscle cells (SMCs), which model native and tissue engineered blood vessels, produce a confluent endothelium that responds to flow like normal human aortic endothelial cells (HAECs). The objective of this study was to quantify the elastic modulus of hCB-ECs cocultured with SMCs under static and flow conditions using atomic force microscopy (AFM). Cytoskeleton structures were assessed by AFM cell surface imaging and immunofluorescence of F-actin. The elastic moduli of hCB-ECs and HAECs were similar and significantly smaller than the value for SMCs in monoculture under static conditions (p<0.05). In coculture, hCB-ECs and HAECs became significantly stiffer with moduli 160–180% larger than their corresponding values in monoculture. While the moduli of hCB-ECs a! nd HAECs almost doubled in monoculture and flow condition, their corresponding values in coculture declined after exposure to flow. Both the number and diameter of cortical stress fiber per cell width increased in coculture and/or flow conditions, whereas the subcortical stress fiber density throughout the cell interior increased by a smaller amount. These findings indicate that changes to biomechanical properties in coculture and/or exposure to flow are correlated with changes in the cortical stress fiber density. For ECs, fluid shear stress appeared to have greater effect on the elastic modulus than the presence of SMCs and changes to the elastic modulus in coculture may be due to EC–SMC communication.
• Entrapment of adult fingers between window glass and seal entry of a motor vehicle side door: An experimental study for investigation of the force at the subjective pain threshold
  - J Biomech 44(11):2158-2161 (2011)
  In modern motor vehicles with automatic power windows, a potential hazard exists for jam events of fingers between the window glass and seal entry. This study determined entrapment forces acting on adult fingers at the subjective maximum pain threshold during entrapment in such windows. The length and the girth of the proximal and distal interphalangeal joints of the triphalangeal fingers of the right hands of 109 participants (60 men, 49 women) were measured; the diameter was calculated from girth, which was assumed to be circular. The automatic power window system of a motor vehicle side door was changed to a mechanical system. During entrapment the force distributed across the four proximal interphalangeal joints (PIPs), and separately on the proximal interphalangeal (iPIP) and then the distal interphalangeal (iDIP) joints of the index finger was measured using a customized force sensor. The maximum bearable entrapment force was 97.2±51.8 N for the PIPs, 43.4±19.9 N for the iPIP, and 36.9±17.8 N for the iDIP. The positive correlation between finger diameter and maximum entrapment force was significant. Particularly with regard to the risk to children's fingers, the 100 N statutory boundary value for closing force of electronic power windows should be reduced.
• An upper extremity inverse dynamics model for pediatric Lofstrand crutch-assisted gait
  - J Biomech 44(11):2162-2167 (2011)
  The objective of this study was to develop an instrumented Lofstrand crutch system, which quantifies three-dimensional (3-D) upper extremity (UE) kinematics and kinetics using an inverse dynamics model. The model describes the dynamics of the shoulders, elbows, wrists, and crutches and is compliant with the International Society of Biomechanics (ISB) recommended standards. A custom designed Lofstrand crutch system with four, six-degree-of-freedom force transducers was implemented with the inverse dynamics model to obtain triaxial UE joint reaction forces and moments. The crutch system was validated statically and dynamically for accuracy of computing joint reaction forces and moments during gait. The root mean square (RMS) error of the system ranged from 0.84 to 5.20%. The system was demonstrated in children with diplegic cerebral palsy (CP), incomplete spinal cord injury (SCI), and type I osteogenesis imperfecta (OI). The greatest joint reaction forces were observed i! n the posterior direction of the wrist, while shoulder flexion moments were the greatest joint reaction moments. The subject with CP showed the highest forces and the subject with SCI demonstrated the highest moments. Dynamic quantification may help to elucidate UE joint demands in regard to pain and pathology in long-term assistive device users.
• Effect of muscle contraction levels on the force–length relationship of the human Achilles tendon during lengthening of the triceps surae muscle–tendon unit
  - J Biomech 44(11):2168-2171 (2011)
  Findings from animal experiments are sometimes contradictory to the idea that the tendon structure is a simple elastic spring in series with muscle fibers, and suggest influence of muscle contraction on the tendon mechanical properties. The purpose of the present study was to investigate the influence of muscle contraction levels on the force–length relationship of the human Achilles tendon during lengthening of the triceps surae muscle–tendon unit. For seven subjects, ankle dorsiflexion was performed without (passive condition) and with contraction of plantar flexor muscles (eccentric conditions, at 3 contraction levels) on an isokinetic dynamometer. Deformation of the Achilles tendon during each trial was measured using ultrasonography. The Achilles tendon force corresponding to the tendon elongation of 10 mm in the passive condition was significantly smaller than those in the eccentric conditions (p<0.05 or p<0.01). Within the eccentric conditions, the Achilles ! tendon force corresponding to the tendon elongation of 10 mm was significantly greater in the maximal contraction level than those in submaximal eccentric conditions (p<0.05 or p<0.01). In addition, the tendon stiffness was greater in higher contraction levels (p<0.05 or p<0.01). Present results suggest that the human tendon structure is not a simple elastic spring in series with muscle fibers.
• An algorithm to correct for camera vibrations in optical motion tracking systems
  - J Biomech 44(11):2172-2176 (2011)
  Recording and reconstruction of 3D motion capturing data relies on fixed, static camera positions with given inter-camera distances in a laboratory frame. To overcome this limitation, we present a correction algorithm that allows us to address camera movements in moving camera setups. Camera vibrations are identified by comparison of specialized target positions in dynamic measurements with their respective positions in static trials. This results in a 2D shift vector with which the individual camera streams are corrected. The capabilities of this vibration reduction procedure are demonstrated in a test setup of four cameras that are (i) separately and (ii) simultaneously perturbed while capturing a static test object. In the former case, the correction algorithm is capable of reducing the reconstruction residuals to the order of the calibrations residual and enables reconstruction in the latter case, which is impossible without any correction. This approach extends t! he application of marker-based infrared motion tracking to moving and even accelerated camera setups.