A Physiology-Guided Classification of Active-Stress and Active-Strain Approaches for Continuum-Mechanical Modeling of Skeletal Muscle Tissue

The well-established sliding filament and cross-bridge theory explain the major biophysical mechanism responsible for a skeletal muscle's active behavior on a cellular level. However, the biomechanical function of skeletal muscles on the tissue scale, which is caused by the complex interplay of muscle fibers and extracellular connective tissue, is much less understood. Mathematical models provide one possibility to investigate physiological hypotheses. Continuum-mechanical models have hereby proven themselves to be very suitable to study the biomechanical behavior of whole muscles or entire limbs. Existing continuum-mechanical skeletal muscle models use either an active-stress or an active-strain approach to phenomenologically describe the mechanical behavior of active contractions. While any macroscopic constitutive model can be judged by it's ability to accurately replicate experimental data, the evaluation of muscle-specific material descriptions is difficult as suitable data is, unfortunately, currently not available. Thus, the discussions become more philosophical rather than following rigid methodological criteria. Within this work, we provide a extensive discussion on the underlying modeling assumptions of both the active-stress and the active-strain approach in the context of existing hypotheses of skeletal muscle physiology. We conclude that the active-stress approach resolves an idealized tissue transmitting active stresses through an independent pathway. In contrast, the active-strain approach reflects an idealized tissue employing an indirect, coupled pathway for active stress transmission. Finally the physiological hypothesis that skeletal muscles exhibit redundant pathways of intramuscular stress transmission represents the basis for considering a mixed-active-stress-active-strain constitutive framework.

REINFORCING I-BEAMS WITH WEB OPENINGS USING FILLET-WELDED PERFORATED PLATE

In beam-to-column connections in steel structures, it is crucial for stresses to be transmitted from the beam to column smoothly. However, openings of I-section steel beam web are often made at the end of the beam when piping facilities pass through the beam. Openings at the end of the I-section steel beam reduce its flexural strength, shear strength, and deformation capacity of the steel beam ends. If the openings are made at the end of a beam, the beam should somehow be reinforced to achieve acceptable stress transmission performance and good deformation capacity. There are several existing ways of reinforcing openings at the end of a beam in Japan; however, simpler and more economical reinforcing methods are desired. In this paper, a new reinforcing method of openings at the beam ends is proposed. The proposed reinforcement is a perforated steel plate connected to the beam web using fillet welding. The proposed method is simple, easier to produce, and easier to construct compared to conventional methods. An experiment was conducted to verify the performance of the proposed reinforcing method. The results of the experiments showed that the proposed reinforcing method can be designed conservatively using Kato’s formula.

American study shows knock-on effects of putting family first (PFF) strategy on spouses and co-workers

Purpose The authors say they invented a new construct of putting family first (PFF). They define PFF as “the voluntary behavior of intentionally putting one’s family ahead of work in a way that violates organisational norms”. They said it helped to understand how workers break rules to manage boundary conflicts. They wanted to test the impact on co-workers and spouses. Design/methodology/approach The authors carried out two studies. The first one established a scale to measure PFF. The second one tested for links between PFF and both co-workers frustration and spousal dissatisfaction. To test their theories, the authors looked for US workers with spouses and co-workers. Findings Results showed PFF correlated significantly with the co-workers’ feelings of overload, frustration with work and work-family conflict. It also correlated significantly with the spouses’ stress transmission and relationship tensions. The results provided further validity of the scale developed in Study 1, as well as demonstrating the wider repercussions of PFF. Originality/value Results showed PFF correlated significantly with the co-workers’ feelings of overload, frustration with work and work-family conflict. It also correlated significantly with the spouses’ stress transmission and relationship tensions. The results provided further validity of the scale developed in Study 1, as well as demonstrating the wider repercussions of PFF.

Intergenerational Stress Transmission is Associated with Brain Metabotranscriptome Remodeling and Mitochondrial Dysfunction

Intergenerational stress increases lifetime susceptibility to depression and other psychiatric disorders. Whether intergenerational stress transmission is a consequence of in utero neurodevelopmental disruptions vs early-life mother-infant interaction is largely unknown. Here, we demonstrated that exposure to traumatic stress in mice during pregnancy, through predator scent exposure, induces in the offspring social deficits and depressive-like behavior. We found, through cross-fostering experiments, that raising of normal pups by traumatized mothers produced a similar behavioral phenotype to that induced in pups raised by their biological traumatized mothers. Good caregiving (by non-traumatized mothers), however, did not completely protect against the prenatal trauma- induced behavioral deficits. These findings support a two-hit stress mechanism of both in utero and early-life parenting (poor caregiving by the traumatized mothers) environments. Associated with the behavioral deficits, we found profound changes in brain metabolomics and transcriptomic (metabotranscriptome). Striking increases in the mitochondrial hypoxia marker and epigenetic modifier 2-hydroxyglutaric acid, in the brains of neonatal and adult pups whose mothers were exposed to stress during pregnancy, indicated mitochondrial metabolism dysfunctions and epigenetic mechanisms. Bioinformatic analyses revealed mechanisms involving stress- and hypoxia-response metabolic pathways in the brains of the neonatal mice, which appear to lead to long-lasting alterations in mitochondrial- energy metabolism, and epigenetic processes pertaining to DNA and chromatin modifications. Most strikingly, we demonstrated that an early pharmacological intervention that can correct mitochondria metabolism - lipid metabolism and epigenetic modifications with acetyl-L-carnitine (ALCAR) supplementation - produces long-lasting protection against the behavioral deficits associated with intergenerational transmission of traumatic stress.

Mechanical stress determines morphogenesis and cell ordering in confined bacterial biofilms

Biofilms are aggregates of bacterial cells surrounded by an extracellular matrix. Much progress has been made in studying biofilm growth on solid substrates; however, little is known about how biofilms develop in three-dimensional confined environments. Here, combining single-cell imaging, continuum mechanical modeling, and mutagenesis, we reveal the key morphogenesis steps of Vibrio cholerae biofilms embedded in hydrogels. We demonstrate how mechanical stress determines the global morphology and gives rise to bipolar cell ordering in confined biofilms. Our analysis shows that cell ordering arises from stress transmission across the biofilm-environment interface, mediated by specific matrix components. Our results open an avenue to understand how organisms grow within complex environments by means of a compromise between their inherent developmental program and the constraints imposed by the environment.One sentence summaryEmbedded biofilms mechanically interact with the confining environment, leading to the emergence of an anisotropic architecture and a precise cell organization.

Beam network model for fracture of materials with hierarchical microstructure

We introduce a beam network model for hierarchically patterned materials. In these materials, load-parallel gaps intercept stress transmission in the load perpendicular direction in such a manner that damage is confined within hierarchically nested, load-carrying ‘modules’. We describe the morphological characteristics of such materials in terms of deterministically constructed, hierarchical beam network (DHBN) models and randomized variants thereof. We then use these models to analyse the process of damage accumulation (characterized by the locations and timings of beam breakages prior to global failures, and the concomitant avalanche statistics) and of global failure. We demonstrate that, irrespective of the degree of local disorder, failure of hierarchically (micro)structured materials is characterized by diffuse local damage nucleation which ultimately percolates on the network, but never by stress-driven propagation of a critical crack. Failure of non hierarchical reference networks, on the other hand, is characterized by the sequence of damage nucleation, crack formation and crack propagation. These differences are apparent at low and intermediate degrees of material disorder but disappear in very strongly disordered materials where the local failure strengths exhibit extreme scatter. We furthermore demonstrate that, independent of material disorder, the different modes of failure lead to significant differences in fracture surface morphology.