Explore the vast range of titles available.

Existing experimental studies on human balance have primarily focused on biomechanical responses of major lower-limb joints, while the role of hallux and lesser toes of human foot in response to external perturbations has not been fully explored. Although toe grip strength may significantly influence balance performance, a more physiological-relevant toe grip evaluation, has not yet been established. This study investigates the biomechanical grip strength of the hallux and lesser toes during perturbations by quantifying the shear interactions (i.e., horizontal traction forces) at the foot–ground interface. A robotic platform with an instrumented multi-axial force platform was employed to analyze the involvement of the hallux and lesser toes in maintaining standing balance due to random ground perturbations. Our results indicate that hallux and lesser toes demonstrated significant direction-specific shear responses, and the proportion of shear traction force in the hallux and lesser toes significantly increased, particularly for perturbations in the posterior half-plane (p < 0.05). A substantial increase in shear traction force underneath the lesser toes was observed during contralateral perturbation events (p < 0.05). Importantly, the lesser toes could generate substantial shear traction forces by enhancing griping following perturbation, a capability not shown in other foot regions. This study introduced a novel approach for precisely quantifying the grip functions of individual toes at in-vivo perturbing conditions. The information provided is envisaged to have important implications on improved interventions for posture and balance rehabilitation.

Phase-separated biomolecular condensates must respond agilely to biochemical and environmental cues in performing their wide-ranging cellular functions, but our understanding of condensate dynamics is lagging. Ample evidence now indicates biomolecular condensates as viscoelastic fluids, where shear stress relaxes at a finite rate, not instantaneously as in viscous liquids. Yet the fusion dynamics of condensate droplets has only been modeled based on viscous liquids, with fusion time given by the viscocapillary ratio (viscosity over interfacial tension). Here we used optically trapped polystyrene beads to measure the viscous and elastic moduli and the interfacial tensions of four types of droplets. Our results challenge the viscocapillary model, and reveal that the relaxation of shear stress governs fusion dynamics. These findings likely have implications for other dynamic processes such as multiphase organization, assembly and disassembly, and aging. Phase-separated biomolecular condensates are implicated in a myriad of biological processes. Here the authors apply optical tweezers to characterize the viscoelasticity and interfacial tension of a range of condensates, finding that condensates can deviate from simple fluids in opposite directions; and identify shear relaxation as a governing measure of condensate dynamics.

In order to study the shear mechanical properties of rock joint under different unloading stress paths, the RDS-200 rock joint shear test system was used to carry out direct shear tests on concrete joint specimens with five different morphologies under the CNL path and different unloading stress paths. The unloading stress paths include unloading normal load and maintaining constant shear load (UNLCSL), unloading normal load and unloading shear load (UNLUSL), unloading normal load and increasing shear load (UNLISL). The results show that the peak shear strength, cohesion, internal friction angle, pre-peak shear stiffness and residual shear strength of concrete joints under CNL path increases with the increasing JRC and normal stress. Under the UNLCSL path, under the same initial shear stress τ 1 , instability normal stress σ i decreases with the increasing JRC , and normal stress unloading amount Δσ n increases with the increasing JRC . Under the same JRC , σ i increases with the increase of τ 1 , and Δσ n decreases with the increasing τ 1 . Under the same JRC and σ i , τ i is significantly smaller under the UNLCSL path than the CNL path. Under the same JRC , the cohesion under the UNLCSL path is less than the CNL path, and the internal friction angle is higher than that the CNL path. Under the same JRC and σ i , τ i is the largest under the path of CNL and UNLISL, followed by the UNLCSL path, and τ i under the UNLUSL path is the smallest. Compared with the CNL path, the variation range of the specimen internal friction angle is within 3% while the average decrease percentage of the specimen cohesion reaches 37.6% under the UNLCSL path, UNLISL, and UNLUSL. Therefore, it can be inferred that the decrease in cohesion caused by normal unloading is the main reason for the decrease in joint instability shear strength. After introducing the correction coefficient k of cohesion to modify the Mohr-Coulomb criterion, the maximum average relative error after correction is only 3.5%, which is significantly improved compared with the maximum average relative error of 56.9% before correction. The research conclusions can provide some reference for the accurate estimation of shear bearing capacity of rock joints under different unloading stress paths, which is of great significance to the stability evaluation and disaster prevention of rock mass engineering.

The structural integrity of cornea depends on properties of its extracellular matrix, mainly a mixture of collagen fibers and soluble proteoglycans (PGs). PGs are macromolecules of negatively charged sulphated glycosaminoglycans (GAGs) covalently attached to a protein core. GAGs appear as bridges between adjacent collagen fibers and could facilitate force transfer between them. Furthermore, GAGs are responsible for corneal hydration by attracting and maintaining water molecules into the extracellular matrix. Based on these observations, GAGs are expected to be essential for biomechanical properties of cornea. The primary objective of the present study was to determine the effects of GAGs on shear properties of cornea. For this purpose, GAGs were enzymatically removed from porcine corneal disks by keratanase II enzyme. After confirming the successful removal of GAGs by histochemical methods, torsional rheometry was performed to characterize the shear stiffness of GAG-depleted samples as a function of axial strain. It was found that the shear modulus of all samples was a function of applied shear strain and compressive strain. Beyond the range of linear viscoelastic response, the average complex shear modulus decreased with increasing the shear strain. Increasing the axial strain from 0% to 40% significantly increased the average complex shear modulus of corneal disks in all groups. Finally, the enzyme treatment with keratanase II enzyme significantly decreased the shear stiffness. The experimental measurements were discussed in terms of microstructural and compositional properties of corneal extracellular matrix and it was concluded that GAGs play a significant role in defining shear properties of cornea.

Ultrasound shear wave elastography is becoming a valuable tool for measuring mechanical properties of individual muscles. Since ultrasound shear wave elastography measures shear modulus along the principal axis of the probe (i.e., along the transverse axis of the imaging plane), the measured shear modulus most accurately represents the mechanical property of the muscle along the fascicle direction when the probe's principal axis is parallel to the fascicle direction in the plane of the ultrasound image. However, it is unclear how the measured shear modulus is affected by the probe angle relative to the fascicle direction in the same plane. The purpose of the present study was therefore to examine whether the angle between the principal axis of the probe and the fascicle direction in the same plane affects the measured shear modulus. Shear modulus in seven specially-designed tissue-mimicking phantoms, and in eleven human in-vivo biceps brachii and medial gastrocnemius were determined by using ultrasound shear wave elastography. The probe was positioned parallel or 20° obliquely to the fascicle across the B-mode images. The reproducibility of shear modulus measurements was high for both parallel and oblique conditions. Although there was a significant effect of the probe angle relative to the fascicle on the shear modulus in human experiment, the magnitude was negligibly small. These findings indicate that the ultrasound shear wave elastography is a valid tool for evaluating the mechanical property of pennate muscles along the fascicle direction.

New Zealand white rabbits are a prevalent model species used to study preclinical articular cartilage repair therapies. The composition and structure of rabbit articular cartilage have been extensively characterized, yet the local shear properties of the tissue are unknown. Characterizing the local shear properties is essential for understanding the structure–function relationship in the tissue and relating the rabbit preclinical model to human disease. Therefore, the objectives of this study were to (1) characterize the local shear properties of articular cartilage from the femoral condyles of New Zealand white rabbits, (2) determine if local protein content or matrix structure correlated with local shear properties, and (3) compare microscale shear moduli values of rabbit cartilage to those previously reported for human, equine, and bovine tissues. Local shear strains and moduli varied with rabbit cartilage tissue depth; shear modulus was highest ∼ 50 µm below the tissue surface and decreased to plateau values around 150 µm, mirroring the trend with shear strains. Local shear strains showed significant correlations with local protein content but not matrix organization. Rabbit cartilage shear properties followed similar spatial trends as bovine, equine, and human tissue in the first ∼ 100 um of the tissue depth. However, rabbit tissue then differentiated from the larger animals as shear modulus values plateaued and did not increase by an order of magnitude like that seen in the larger species. Local shear properties of rabbit articular cartilage capture the surface properties of human, equine, and bovine cartilage but mechanically lack the deep zone region.

Understanding the mechanical behavior of the biceps femoris long head (BFlh) may be insightful due to its high susceptibility to strain injuries, particularly during high-speed running in sports, such as soccer and track and field. While prior research has focused on intrinsic muscle properties, emerging evidence suggests that the biceps femoris short head (BFsh) may influence BFlh tension. Thus, we examined the effects of BFsh load application on the tensile strength and regional shear modulus of the BFlh. Seven legs from four cadaveric specimens (mean age: 83.2 ± 7.4 years) embalmed using the Thiel method were used. BFlh was secured to a mechanical testing device equipped with a load cell, whereas BFsh was connected to a custom-built mechanical apparatus. A tensile strain of 8 % was applied to the BFlh, whereas incremental loads (0, 150, 300, 450, 600, and 750 g) were gradually added to the BFsh. The tensile force and shear modulus in the three BFlh regions (proximal, central, and distal) were recorded using shear wave elastography. The results demonstrated that BFsh loading notably reduced BFlh tensile strength, with the lowest tension at 750 g (P < 0.01). The shear modulus decreased in the proximal and distal regions at loads > 450 g (P < 0.01), with no change in the central region. The distal region exhibited a greater decrease in shear modulus compared with the proximal and central regions (P < 0.01). These findings suggest that BFsh loading reduces BFlh tensile strength and alters its mechanical properties, particularly in the distal region.

The cornea is a highly specialized transparent tissue which covers the front of the eye. It is a tough tissue responsible for refracting the light and protecting the sensitive internal contents of the eye. The biomechanical properties of the cornea are primarily derived from its extracellular matrix, the stroma. The majority of previous studies have used strip tensile and pressure inflation testing methods to determine material parameters of the corneal stroma. Since these techniques do not allow measurements of the shear properties, there is little information available on transverse shear modulus of the cornea. The primary objectives of the present study were to determine the viscoelastic behavior of the corneal stroma in shear and to investigate the effects of the compressive strain. A thorough knowledge of the shear properties is required for developing better material models for corneal biomechanics. In the present study, torsional shear experiments were conducted at different levels of compressive strain (0–30%) on porcine corneal buttons. First, the range of linear viscoelasticity was determined from strain sweep experiments. Then, frequency sweep experiments with a shear strain amplitude of 0.2% (which was within the region of linear viscoelasticity) were performed. The corneal stroma exhibited viscoelastic properties in shear. The shear storage modulus, G′, and shear loss modulus, G″, were reported as a function of tissue compression. It was found that although both of these parameters were dependent on frequency, shear strain amplitude, and compressive strain, the average shear storage and loss moduli varied from 2 to 8kPa, and 0.3 to 1.2kPa, respectively. Therefore, it can be concluded that the transverse shear modulus is of the same order of magnitude as the out-of-plane Young's modulus and is about three orders of magnitude lower than the in-plane Young's modulus.

The mechanical properties of loess-steel interface are of great significance for understanding the residual strength and deformation of loess. However, the undisturbed loess has significant structural properties, while the remolded loess has weak structural properties. There are few reports on the mechanical properties of loess-steel interface from the structural point of view. This paper focused on the ring shear test between undisturbed loess as well as its remolded loess and steel interface under the same physical mechanics and test conditions (water content, shear rate and vertical pressure), and explored the influence mechanism of structure on the mechanical deformation characteristics of steel-loess interface. The results show that the shear rate has little effect on the residual strength of the undisturbed and remolded loess-steel interface. However, the water content has a significant influence on the residual strength of the loess-steel interface, moreover, the residual internal friction angle is the dominant factor supporting the residual strength of the loess-steel interface. In general, the residual strength of the undisturbed loess-steel interface is greater than that of the remolded loess specimen (for example, the maximum percentage of residual strength difference between undisturbed and remolded loess specimens under the same moisture content is 6.8%), which is because that compared with the mosaic arrangement structure of the remolded loess, the overhead arrangement structure of the undisturbed loess skeleton particles makes the loess particles on the loess-steel interface re-adjust the arrangement direction earlier and reach a stable speed relatively faster. The loess particles with angular angles in the undisturbed loess make the residual internal friction between the particles greater than the smoother particles of the remolded loess (for example, the maximum percentage of residual cohesion difference between undisturbed and remolded loess specimens under the same vertical pressure is 4.29%), and the intact cement between undisturbed loess particles brings stronger cohesion than the remolded loess particles with destroyed cement (for example, the maximum difference percentage of residual cohesion between undisturbed and remolded soil specimens under the same vertical pressure is 33.80%). The test results provide experimental basis for further revealing the influence mechanism of structure, and parameter basis for similar engineering construction.

Coronary arteries have high curvatures, and hence, flow through them causes disturbed flow patterns, resulting in stenosis and atherosclerosis. This in turn decreases the myocardial flow perfusion, causing myocardial ischemia and infarction. Therefore, in order to understand the mechanisms of these phenomena caused by high curvatures and branching of coronary arteries, we have conducted elaborate hemodynamic analysis for both (i) idealized coronary arteries with geometrical parameters representing realistic curvatures and stenosis and (ii) patient-specific coronary arteries with stenoses. Firstly, in idealized coronary arteries with approximated realistic arterial geometry representative of their curvedness and stenosis, we have computed the hemodynamic parameters of pressure drop, wall shear stress (WSS) and wall pressure gradient (WPG), and their association with the geometrical parameters of curvedness and stenosis. Secondly, we have similarly determined the wall shear stress and wall pressure gradient distributions in four patient-specific curved stenotic right coronary arteries (RCAs), which were reconstructed from medical images of patients diagnosed with atherosclerosis and stenosis; our results show high WSS and WPG regions at the stenoses and inner wall of the arterial curves. This paper provides useful insights into the causative mechanisms of the high incidence of atherosclerosis in coronary arteries. It also provides guidelines for how simulation of blood flow in patient’s coronary arteries and determination of the hemodynamic parameters of WSS and WPG can provide a medical assessment of the risk of development of atherosclerosis and plaque formation, leading to myocardial ischemia and infarction. The novelty of our paper is in our showing how in actual coronary arteries (based on their CT imaging) curvilinearity and narrowing complications affect the computed WSS and WPG, associated with risk of atherosclerosis. This is very important for cardiologists to be able to properly take care of their patients and provide remedial measures before coronary complications lead to myocardial infarctions and necessitate stenting or coronary bypass surgery. We want to go one step further and provide clinical application of our research work. For that, we are offering to cardiologists worldwide to carry out hemodynamic analysis of the medically imaged coronary arteries of their patients and compute the values of the hemodynamic parameters of WSS and WPG, so as to provide them an assessment of the risk of atherosclerosis for their patients.