Force Relaxation Research Articles

Chronic Activation of Tubulin Tyrosination Improves Heart Function.

Hypertrophic cardiomyopathy (HCM) is the most common cardiac genetic disorder caused by sarcomeric gene variants and associated with left ventricular hypertrophy and diastolic dysfunction. The role of the microtubule network has recently gained interest with the findings that microtubule detyrosination (dTyr-MT) is markedly elevated in heart failure. Acute reduction of dTyr-MT by inhibition of the detyrosinase (VASH [vasohibin]/SVBP [small VASH-binding protein] complex) or activation of the tyrosinase (TTL [tubulin tyrosine ligase]) markedly improved contractility and reduced stiffness in human failing cardiomyocytes and thus posed a new perspective for HCM treatment. In this study, we tested the impact of chronic tubulin tyrosination in an HCM mouse model (Mybpc3 knock-in), in human HCM cardiomyocytes, and in SVBP-deficient human engineered heart tissues (EHTs). Adeno-associated virus serotype 9-mediated TTL transfer was applied in neonatal wild-type rodents, in 3-week-old knock-in mice, and in HCM human induced pluripotent stem cell-derived cardiomyocytes. We show (1) TTL for 6 weeks dose dependently reduced dTyr-MT and improved contractility without affecting cytosolic calcium transients in wild-type cardiomyocytes; (2) TTL for 12 weeks reduced the abundance of dTyr-MT in the myocardium, improved diastolic filling, compliance, cardiac output, and stroke volume in knock-in mice; (3) TTL for 10 days normalized cell area in HCM human induced pluripotent stem cell-derived cardiomyocytes; (4) TTL overexpression activated transcription of tubulins and other cytoskeleton components but did not significantly impact the proteome in knock-in mice; (5) SVBP-deficient EHTs exhibited reduced dTyr-MT levels, higher force, and faster relaxation than TTL-deficient and wild-type EHTs. RNA sequencing and mass spectrometry analysis revealed distinct enrichment of cardiomyocyte components and pathways in SVBP-deficient versus TTL-deficient EHTs. This study provides the first proof of concept that chronic activation of tubulin tyrosination in HCM mice and in human EHTs improves heart function and holds promise for targeting the nonsarcomeric cytoskeleton in heart disease.

Influence of bolt creep induced pre-tightening force relaxation on dynamic response of the bolted joint system

The bolted joint is an important link that affects the accuracy maintenance of the heavy-duty machine tool. Due to bolts of the crossbeam are under a preload of several hundred kN for an extended period, the bolt gradually creeps and elongates as the machine tool's service time increases. This relaxation of the preload results in a degradation of the bolted stiffness, weakening the dynamic response of the bolted joint system. To reveal that the degradation law of crossbeam bolting performance of the heavy-duty machine tool induced by bolt creep, based on the Norton-Bailey(NB) model, a method for analyzing the creep of bolts with precision threads is proposed, and the influence of thread geometry and friction coefficient on the creep stress relaxation of bolts is studied. Considering the interface contact, an interface contact stiffness model is derived based on fractal theory. The dynamic equation of the bolted beam system is established using the Matrix27 stiffness matrix. The dynamic response of the bolted system, resulting from the degradation of interface stiffness due to bolt creep, is analyzed. The results show that the stress relaxation of bolt creep is not only related to creep parameters, but also related to the mate-thread interaction. The dynamic model with Matrix27 stiffness matrix is comparable to experiment in calculation accuracy and efficiency, which provides technical guidance for machine tool dynamics analysis and effectively solves the scientific problem of beam bolting performance degradation during the service of heavy-duty machine tools.

Remarkable Insensitivity of Protein Diffusion to Protein Charge.

Friction to translational diffusion of ionic particles in polar liquids should scale linearly with the squared ion charge, according to standard theories. Substantial slowing of translational diffusion is expected for proteins in water. In contrast, our simulations of charge mutants of green fluorescent proteins in water show remarkable insensitivity of the translational diffusion constant to protein's charge in the range of charges between -29 and +35. The friction coefficient is given as a product of the force variance and the memory function relaxation time. We find remarkably accurate equality between the variance of the electrostatic force and the negative cross-correlation of the electrostatic and van der Waals forces. The charge invariance of the diffusion constant is a combined effect of the force variance and relaxation time invariances with the protein charge. The temperature dependence of the protein diffusion constant is highly non-Arrhenius, with a fragile-to-strong crossover at the glass transition.

Non destructive qualification of bovine meat tenderness

Tenderness is one of the most important criteria in bovine meat, as it determines whether the meat is used for grilling, long-time cooking or post-processing. It is therefore of great interest for the industry to measure it. Common systems require the extraction and destruction of samples, inducing time and material expenses. In this article, a new nondestructive characterization apparatus, based on monitored indentation, relaxation, and recovery, is proposed. Samples from two cuts known for their difference in tenderness were used from the same carcass. Their tenderness were assessed via compression tests and then compared to indicators from the force–displacement measurements. Results showed that the indentation and recovery speed, and the force relaxation enable the differentiation of the two cuts (p<0.05). These results were obtained on samples from a single carcass. Measurements on other carcasses should be performed to ensure that the results presented in this article can be generalized to bovine meat.

Myosin Isoform-Dependent Effect of Omecamtiv Mecarbil on the Regulation of Force Generation in Human Cardiac Muscle.

Omecamtiv mecarbil (OM) is a small molecule that has been shown to improve the function of the slow human ventricular myosin (MyHC) motor through a complex perturbation of the thin/thick filament regulatory state of the sarcomere mediated by binding to myosin allosteric sites coupled to inorganic phosphate (Pi) release. Here, myofibrils from samples of human left ventricle (β-slow MyHC-7) and left atrium (α-fast MyHC-6) from healthy donors were used to study the differential effects of μmolar [OM] on isometric force in relaxing conditions (pCa 9.0) and at maximal (pCa 4.5) or half-maximal (pCa 5.75) calcium activation, both under control conditions (15 °C; equimolar DMSO; contaminant inorganic phosphate [Pi] ~170 μM) and in the presence of 5 mM [Pi]. The activation state and OM concentration within the contractile lattice were rapidly altered by fast solution switching, demonstrating that the effect of OM was rapid and fully reversible with dose-dependent and myosin isoform-dependent features. In MyHC-7 ventricular myofibrils, OM increased submaximal and maximal Ca2+-activated isometric force with a complex dose-dependent effect peaking (40% increase) at 0.5 μM, whereas in MyHC-6 atrial myofibrils, it had no effect or-at concentrations above 5 µM-decreased the maximum Ca2+-activated force. In both ventricular and atrial myofibrils, OM strongly depressed the kinetics of force development and relaxation up to 90% at 10 μM [OM] and reduced the inhibition of force by inorganic phosphate. Interestingly, in the ventricle, but not in the atrium, OM induced a large dose-dependent Ca2+-independent force development and an increase in basal ATPase that were abolished by the presence of millimolar inorganic phosphate, consistent with the hypothesis that the widely reported Ca2+-sensitising effect of OM may be coupled to a change in the state of the thick filaments that resembles the on-off regulation of thin filaments by Ca2+. The complexity of this scenario may help to understand the disappointing results of clinical trials testing OM as inotropic support in systolic heart failure compared with currently available inotropic drugs that alter the calcium signalling cascade.

Long-term relaxation analysis of steel cables based on viscoelastic model

Steel cables, renowned for their lightweight and high-strength properties, stand as ubiquitous building materials in large-span structures. Serving as crucial load-bearing elements, cables endure sustained tension throughout the structure's lifecycle. To further investigate the long-term relaxation characteristics of steel cables and provide more theoretical basis for structural relaxation calculations, this paper introduces a long-term relaxation constitutive model for steel cables, formulated through theoretical derivation, experimental validation, and data fitting techniques to establish time-dependent constitutive relationships. By employing the Bingham and Kelvin viscoelastic models, a relaxation model for steel cables is presented and optimized accordingly. Experimental investigations involve long-term tests on steel cables with three different sizes and detailed fitting analysis on collected internal force relaxation data. The findings emphasize the indispensability of structural relaxation behavior and the significance of altering internal forces, particularly in large-span cable-strut structures. The developed numerical model enables accurate simulation and prediction of long-term time-dependent performance in such structures, serving as a springboard for future studies in this domain.

Abstract We025: Human Engineered Heart Tissues Demonstrate Clinically-Relevant Disease indicators of Duchenne Muscular Dystrophy

Accurate in vitro modeling of healthy and disease conditions is crucial for assessing the safety and efficacy of novel therapeutics before clinical trials. For cardiac diseases, direct evaluation of muscle contraction is a reliable indicator of overall tissue function, however, this often necessitates expensive and low-throughput non-human in vivo models. Moreover, animal models may not accurately predict human responses to compound exposure. Here, we have developed a turnkey platform for facile fabrication of 3D engineered heart tissues (EHTs) that enables label-free, push-button measurements of contractility. The system allows for customized stimulation protocols to individual constructs, while simultaneously measuring contractility across 24 tissues in parallel with minimal user input. This approach enables stratification of muscle phenotypes and facilitates compound safety and efficacy screening. We present a 3D model of Duchenne muscular dystrophy that displays functional deficits consistent with in vivo disease indicators, including reduced contractile force and slowed relaxation kinetics, though beat rate remains unchanged. We show both acute and chronic effects of cardiotoxic drugs, as well as positive inotropic responses to external calcium and Isoproterenol. This highlights the utility of using EHTs to identify toxicity and derisk drug development and clinical testing outcomes. Spontaneously beating EHTs were also subjected to in situ stretching, leading to increased levels of cardiac troponin I (cTnI) in the tissue medium, indicating stretch induced injury. These data demonstrate a first-and-only commercial platform that integrates individual, fully-customizable, well-based control of electrical stimulation across a 24-well plate of tissues to pace muscle constructs, model force-frequency, and enable exercise regimens or damage protocols. Heart organoids were stretched under load presenting a challenge for therapeutics targeting injury prevention in the myocardium. Stimulation is coupled with automated assessment of 3D muscle contraction, providing an inclusive, medium-throughput platform for studying muscle-related disorders, advancing therapeutic discovery using a modality-agnostic biosystem.

Prediction on the time-varying behavior of tunnel segment gaskets under compression

The compression force is a critical indicator for assessing the waterproofing capability of ethylene propylene diene monomer (EPDM) rubber gaskets, and the relaxation of EPDM gaskets under compression is a key factor contributing to water leakage in shield tunnels. In this study, detailed accelerated aging tests of gaskets with different precompression openings were conducted. Analytical method and simulation method for predicting the long-term performance of EPDM gaskets considering their different compression states were proposed. First, the compression curve of the gasket was divided into three stages to analyze the mechanism of compression force degradation in depth. Under the action of a compression force, the unrecoverable deformation caused by open holes shortened stage III of the compression curve, which constituted the primary factor influencing gasket compression force relaxation. In addition, the Arrhenius equation was used to establish a conversion relationship between the accelerated aging test conditions and actual service time, and an analytical expression (AE) method for predicting the compression force degradation pattern of gaskets was proposed. Finally, a finite element (FE) method based on the relaxation constitutive model was proposed, enabling simulation of deteriorating tunnel segment gasket performance under complex compression states. Then, the effectiveness and accuracy of the proposed relaxation model were validated by comparing the compression force curves and deformation states obtained from experiments and simulations. Based on the time-varying relaxation constitutive parameters, the variation in the compression force of EPDM gaskets with different joint openings over time was simulated. The results show that the minimum compression force retention rates of gaskets after 100 years of service predicted by the AE method and FE method are 28.5 % and 30.6 %, respectively. This study provides two effective means for predicting the long-term mechanical performance of compressed EPDM gaskets, offering pivotal insights for long-term waterproofing design of shield tunnel joints.

Reduction in motor error by presenting subthreshold somatosensory information during visuomotor tracking tasks

Weak sensory noise acts on the nervous system and promotes sensory and motor functions. This phenomenon is called stochastic resonance and is expected to be applied for improving biological functions. This study investigated the effect of electrical stimulation on grip force adjustment ability. The coefficient of variation and absolute motor error in grip force was measured during a visuomotor tracking task under different intensities of somatosensory noise. Depending on the style of force exertion, the grip movement used in the visuomotor tracking task consisted of force generation (FG), force relaxation (FR), and constant contraction (Constant) phases. The subthreshold condition resulted in significantly lower coefficient of variation in the Constant phase and motor errors in the FG and Constant phases than the no-noise condition. However, the differences among the other conditions were insignificant. Additionally, we examined the correlation between the motor error in the condition without electrical stimulation and the change in motor error induced by subthreshold electrical stimulation. Significant negative correlations were observed in all FG, FR, and Constant phases. These results indicated that somatosensory noise had a strong effect on subjects with large motor errors and enhanced the grip force adjustment ability. By contrast, subjects with small motor errors had weak improvement in motor control. Although the effect of subthreshold noise varies depending on the individual differences, stochastic resonance is effective in improving motor control ability.

Designed wrinkles for optical encryption and flexible integrated circuit carrier board

Patterns on polymers usually have different mechanical properties as those of the substrates, causing deformation or distortion and even detachment of the patterns from the polymer substrates. Herein, we present a wrinkling strategy, which utilizes photolithography to define the area of stress distribution by light-induced physical crosslinking of polymers and controls diffusion of residual solvent to redistribute the stress and then offers the same material for patterns as substrate by thermal polymerization, providing uniform wrinkles without worrying about force relaxation. The strategy allows the recording and hiding of up to eight switchable images in one place that can be read by the naked eye without crosstalk, applying the wrinkled polymer for optical anti-counterfeiting. The wrinkled polyimide film was also utilized to act as a substrate for the creation of fine copper circuit by a full-additive process. It generates flexible integrated circuit (IC) carrier board with copper wire density of 400% higher than that of the state-of-the-art in industry while fulfilling the standards for industrialization.

Implications of using simplified finite element meshes to identify material parameters of articular cartilage

The objective of this work was to determine the effects of using simplified finite element (FE) mesh geometry in the process of performing reverse iterative fitting to estimate cartilage material parameters from in situ indentation testing. Six bovine tibial osteochondral explants were indented with sequential 5 % step-strains followed by a 600 s hold while relaxation force was measured. Three sets of porous viscohyperelastic material parameters were estimated for each specimen using reverse iterative fitting of the indentation test with (1) 2D axisymmetric, (2) 3D idealized, and (3) 3D specimen-specific FE meshes. Variable material parameters were identified using the three different meshes, and there were no systematic differences, correlation to basic geometric features, nor distinct patterns of variation based on the type of mesh used. Implementing the three material parameter sets in a separate 3D FE model of 40 % compressive strain produced differences in von Mises stresses and pore pressures up to 25 % and 50 %, respectively. Accurate material parameters are crucial in any FE model, and parameter differences influenced by idealized assumptions in initial material property determination have the potential to alter subsequent FE models in unpredictable ways and hinder the interpretation of their results.

A quantitative study of mechanics in the treatment of atlantoaxial joint disorder by tendon relaxation manipulation

To explore dose-effect relationship of biomechanical parameters in treating atlantoaxial joint disorder by slimming manipulation. From October 2022 to May 2023, 18 patients with atlantoaxial joint disorders were treated, including 10 males and 8 females;aged from 24 to 27 years old with an average of (25.50±1.10) years old;CT of cervical vertebra showed 16 patients with right side distortion and 2 patients with left side distortion. The mechanical parameters of treatment of atlantoaxial joint disorder by tendon relaxation manipulation were measured by wearing massage manipulation gloves. The magnitude, frequency and mechanical curve of force during tendon relaxation and starting force, pulling force, pulling time and mechanical curve during rehabilitation were quantified, the differences between the affected and contralateral manipulations were compared. The maximum force and frequency of Fengchi(GB20) on the affected side were (19.82±2.02) N and (116.83±14.49) times/min, and opposite side were (13.87±2.19) N and (188.89±16.03) times/min, respectively. There were statistically difference in the maximum force and frequency of both sides (P<0.05). The maximum force and frequency of Quepen (ST12) on the affected side were (14.44±3.27) N and (139.06±28.47) times/min, and those on the opposite side were (9.41±1.38) N and (142.50±28.47) times/min. There was difference in maximum force on both sides (P<0.05). The starting force, turning force and turning time of the affected side were (14.16±5.98) N, (11.56±6.63) N, (0.14±0.03) S, and the contralateral side were (8.94±3.39) N, (8.30±4.64) N, (0.18±0.04) S, respectively. The difference of starting force, turning force and turning time on both sides were statistically significant (P<0.05). By applying a light relaxation force on the affected side, the mechanical balance between cervical vertebrae could be restored, and recovery trend of atlantoaxial joint disorder could be strengthened. On this basis, the atlantoaxial odontoid process could be reversed by applying a light rotation force, which reflects the characteristics of high safety of the manipulation.

Piperine enhances contractile force in slow- and fast-twitch muscle.

Piperine has been shown to bind to myosin and shift the distribution of conformational states of myosin molecules from the super-relaxed state to the disordered relaxed state. However, little is known about the implications for muscle force production and potential underlying mechanisms. Muscle contractility experiments were performed using isolated muscles and single fibres from rats and mice. The dose-response effect of piperine on muscle force was assessed at several stimulation frequencies. The potentiation of muscle force was also tested in muscles fatigued by eccentric contractions. Potential mechanisms of force potentiation were assessed by measuring Ca2+ levels during stimulation in enzymatically dissociated muscle fibres, while myofibrillar Ca2+ sensitivity was assessed in chemically skinned muscle fibres. Piperine caused a dose-dependent increase in low-frequency force with no effect on high-frequency force in both slow- and fast-twitch muscle, with similar relative increases in twitch force, rate of force development and relaxation rate. The potentiating effect of piperine on low-frequency force was reversible, and piperine partially recovered low-frequency force in fatigued muscle. Piperine had no effect on myoplasmic free [Ca2+] levels in mouse muscle fibres, whereas piperine substantially augmented the force response to submaximal levels of [Ca2+] in rat MyHCII fibres and MyHCI fibres along with a minor increase in maximum Ca2+-activated force. Piperine enhances low-frequency force production in both fast- and slow-twitch muscle. The effects are reversible and can counteract muscle fatigue. The primary underlying mechanism appears to be an increase in Ca2+ sensitivity. KEY POINTS: Piperine is a plant alkaloid derived from black pepper. It is known to bind to skeletal muscle myosin and enhance resting ATP turnover but its effects on contractility are not well known. We showed for the first time a piperine-induced force potentiation that was pronounced during low-frequency electrical stimulation of isolated muscles. The effect of piperine was observed in both slow and fast muscle types, was reversible, and could counteract the force decrements observed after fatiguing muscle contractions. Piperine treatment caused an increase in myofibrillar Ca2+ sensitivity in chemically skinned muscle fibres, while we observed no effect on intracellular Ca2+ concentrations during electrical stimulation in enzymatically dissociated muscle fibres.

Preliminary characterization of anelastic effects in the flexure mechanism for a new Kibble balance at NIST

A new Kibble balance is being built at the National Institute of Standards and Technology (NIST). For the first time in one of the highly accurate versions of this type of balance, a single passive flexure mechanism is used for both modes of operation: the weighing mode and the velocity mode. The mechanism is at the core of the new balance design as it represents a paradigm shift for NIST away from using knife edge-based balance mechanisms, which exhibit hysteresis in the measurement procedure of the weighing mode. Mechanical hysteresis may be a limiting factor in the performance of highly accurate Kibble balances approaching single digit nanonewton repeatability on a nominal 100g mass, as targeted in this work. Flexure-based mechanisms are known to have very good static hysteresis when used as a null detector. However, for larger and especially longer lasting deformations, flexures are known to exhibit anelastic drift. We seek to characterize, and ideally compensate for, this anelastic behavior after deflections during the velocity mode to enable a 10−8 accurate Kibble balance-measurement on a nominal 100g mass artifact with a single flexure-based balance mechanism. A measurement of the anelastic after-effect after static excitation hints that the apparatus produced a result for anelastic relaxation comparable to previously published work. Furthermore, a series of oscillatory displacements similar to those occurring in a velocity mode of a Kibble balance measurement are imposed upon the flexure mechanism and show a significant anelastic relaxation torque resulting in multiple micronewton of force relaxation. The amplitude of this force relaxation could be reduced by counterbending the flexures before performing a force measurement.

Passive mechanical properties of adipose tissue and skeletal muscle from C57BL/6J mice

Skeletal muscle and adipose tissue are characterized by unique structural features finely tuned to meet specific functional demands. In this study, we investigated the passive mechanical properties of soleus (SOL), extensor digitorum longus (EDL) and diaphragm (DIA) muscles, as well as subcutaneous (SAT), visceral (VAT) and brown (BAT) adipose tissues from 13 C57BL/6J mice. Thereto, alongside stress-relaxation assessments we subjected isolated muscles and adipose tissues (ATs) to force-extension tests up to 10% and 30% of their optimal length, respectively. Peak passive stress was highest in the DIA, followed by the SOL and lowest in the EDL (p < 0.05). SOL displayed also the highest Young's modulus and hysteresis among muscles (p < 0.05). BAT demonstrated highest peak passive stress and Young's modulus followed by VAT (p < 0.05), while SAT showed the highest hysteresis (p < 0.05). When comparing data across all six biological specimens at fixed passive force intervals (i.e., 20–40 and 50–70 mN), skeletal muscles exhibited significantly higher peak stresses and strains than ATs (p < 0.05). Young's modulus was higher in skeletal muscles than in ATs (p < 0.05). Muscle specimens exhibited slower force relaxation in the first phase compared to ATs (p < 0.05), while there was no significant difference in behavior between muscles and AT in the second phase of relaxation. The study revealed distinctive mechanical behaviors specific to different tissues, and even between different muscles and ATs. These variations in mechanical properties are likely such to optimize the specific functions performed by each biological tissue.

Effects of Partial Knockout of Skeletal Muscle Dynamin-Related Protein 1 on Skeletal Muscle Characteristics in Lean and Obese Mice

Dynamin-related protein 1 (Drp1) is a key regulator of mitochondrial fission and plays an essential role in maintaining skeletal muscle homeostasis. Recent studies reported muscle atrophy in mice when skeletal muscle Drp1 content was reduced. However, whether such atrophic phenotype also exists in diet-induced obese mice remains unknown. We recently reported that reducing skeletal muscle Drp1 resulted in improved whole-body glucose homeostasis and insulin sensitivity in diet-induced obese mice, but not lean healthy mice, suggesting Drp1 may exert divergent effects on physiological phenotypes under different conditions. OBJECTIVE: To determine the effects of partial knockout of skeletal muscle-specific heterozygous Drp1 (mDrp1+/−) on body composition, energy expenditure and skeletal muscle physiology in lean and obese mice. HYPOTHESIS: mDrp1+/− exhibits divergent alterations in physiological characteristics between lean healthy and obese mice. Methods: Tamoxifen-inducible mDrp1+/− mouse model was generated. Nine-week-old male and female mDrp1+/− and wildtype (WT) mice (n=6-8/group) were fed with either a high-fat diet (HFD) or low-fat diet (LFD) for four weeks, received tamoxifen injections, and remained on their respective diet for another four to six weeks. A DXA scan was conducted to measure body composition. Mice were housed in Promethion metabolic cages for 48 hours to measure energy expenditure. Isolated soleus (SOL) and extensor digitorum longus (EDL) muscles were used for assessing muscle cross sectional area (CSA) and contractility. Results: HFD-fed mice had significantly higher percentage of body fat ( p = 0.045) and lower respiratory exchange ratio ( p &lt;0.0001) than LFD-fed mice, regardless of genotype. Regarding skeletal muscle characteristics, SOL from mDrp1+/− mice exhibited larger CSA ( p = 0.052) than WT mice, regardless of diet. However, there were no differences in muscle force or fatigue resistance in SOL between groups. In contrast, EDL muscle isolated from mDrp1+/− mice fed with HFD had greater muscle force than WT mice, but interestingly the force was lower in mDrp1+/−/LFD mice than WT/LFD mice (genotype x diet interaction effect, p = 0.001). Consistently, a significant interaction was found in both twitch and tetanic rate of force development and relaxation rates (genotype x diet interaction effect, p = 0.022, p = 0.033, p = 0.040, and p = 0.020, respectively). There was no difference in EDL muscle CSA between groups. CONCLUSION: Our data suggests that partial knockout of Drp1 in skeletal muscle induced divergent effects on CSA, force production and fatigability between diet induced obese and lean healthy mice. This study was supported by the National Institute of Health (R15DK131512). This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Ability of Adjusting Grip Strength From Childhood to Adulthood

Purpose: This cross-sectional study aimed to investigate the ability to adjust grip strength by comparing the characteristics of force generation and relaxation from childhood to adulthood. Method: This study included 225 participants aged 6, 11, 17, and 19–23 years (adults) who performed isometric hand-grip force as follows: maximum, half generation, and half relaxation. The force was recorded, and relative values and errors were calculated for half tasks. Results: The maximum task values increased with age, but there was no significant age difference between 17-year-olds and adults. The difference between sexes was significant; males were stronger than females in both 17-year-olds and adults. Both sexes in all age groups had greater errors in half relaxation than in half generation tasks. Females had negatively greater constant error than males in half tasks. The errors of 6-year-olds were greater than the other age groups in half tasks. Conclusion: There is a developmental trend for producing maximal strength that is similar across sexes until adolescence when males are stronger and females plateau. The ability of force relaxation was more difficult to accurately control than force generation for all age groups and was adult-like by middle childhood.

Impaired performance of rapid grip in people with Parkinson's disease and motor segmentation

Bradykinesia, or slow movement, is a defining symptom of Parkinson's disease (PD), but the underlying neuromechanical deficits that lead to this slowness remain unclear. People with PD often have impaired rates of motor output accompanied by disruptions in neuromuscular excitation, causing abnormal, segmented, force-time curves. Previous investigations using single-joint models indicate that agonist electromyogram (EMG) silent periods cause motor segmentation. It is unknown whether motor segmentation is evident in more anatomically complex and ecologically important tasks, such as handgrip tasks. Aim 1 was to determine how handgrip rates of force change compare between people with PD and healthy young and older adults. Aim 2 was to determine whether motor segmentation is present in handgrip force and EMG measures in people with PD. Subjects performed rapid isometric handgrip pulses to 20–60% of their maximal voluntary contraction force while EMG was collected from the grip flexors and extensors. Dependent variables included the time to 90% peak force, the peak rate of force development, the duration above 90% of peak force, the number of segments in the force-time curve, the number of EMG bursts, time to relaxation from 90% of peak force, and the peak rate of force relaxation. People with PD had longer durations and lower rates of force change than young and older adults. Six of 22 people with PD had motor segmentation. People with PD had more EMG bursts compared to healthy adults and the number of EMG bursts covaried with the number of segments. Thus, control of rapid movement in Parkinson's disease can be studied using isometric handgrip. People with PD have impaired rate control compared to healthy adults and motor segmentation can be studied in handgrip.

Viscoelasticity, stiffness gradient and their effects on adhesion of an epoxy shape memory polymer

The shape memory polymer based adhesives have demonstrated excellent programmable switchable adhesion, in which the material properties play an important role. Here, the viscoelasticity, stiffness gradient, and their effects on adhesion of an epoxy shape memory polymer (ESMP) were studied. The ESMP sample and polydimethylsiloxane control sample were fabricated firstly by mold casting and curing techniques. The sample was fixed on an electric heating plate with double-sided tape to conduct the adhesion measurements against a hemispherical glass indenter under different temperature and displacement conditions by using an adhesion tester and the temperature measurements on the double-sided tape and ESMP sample surfaces by using a digital thermodetector. Based on the measured force–displacement–time data, the hysteresis behavior and internal dissipation were evaluated; the peak force relaxation was studied; the reduced modulus was calculated and the stiffness gradient features were evaluated. The results show that the ESMP sample exhibits strong viscoelasticity and significantly enhanced adhesion that strongly depends on the compressive displacement in the glass-rubber transition zone near it is glass transition temperature ( Tg ), while weaker viscoelasticity and lower adhesion in the soft rubbery state. The reduced modulus of the ESMP sample obviously decreases with the increasing compressive displacement at a preset temperature below 70 °C. It is found that there exists a linear relation between the pull-off force, effective work of adhesion, reduced modulus, and contact area. This study helps deeply understand the material properties and adhesion mechanism of the ESMP, which can guide the design of switchable adhesives.

Rho-Kinase Inhibition of Active Force and Passive Tension in Airway Smooth Muscle: A Strategy for Treating Airway Hyperresponsiveness in Asthma.

Rho-kinase inhibitors have been identified as a class of potential drugs for treating asthma because of their ability to reduce airway inflammation and active force in airway smooth muscle (ASM). Past research has revealed that, besides the effect on the ASM's force generation, rho-kinase (ROCK) also regulates actin filament formation and filament network architecture and integrity, thus affecting ASM's cytoskeletal stiffness. The present review is not a comprehensive examination of the roles played by ROCK in regulating ASM function but is specifically focused on passive tension, which is partially determined by the cytoskeletal stiffness of ASM. Understanding the molecular basis for maintaining active force and passive tension in ASM by ROCK will allow us to determine the suitability of ROCK inhibitors and its downstream enzymes as a class of drugs in treating airway hyperresponsiveness seen in asthma. Because clinical trials using ROCK inhibitors in the treatment of asthma have yet to be conducted, the present review focuses on the in vitro effects of ROCK inhibitors on ASM's mechanical properties which include active force generation, relaxation, and passive stiffness. The review provides justification for future clinical trials in the treatment of asthma using ROCK inhibitors alone and in combination with other pharmacological and mechanical interventions.