Academic literature on the topic 'Length Contraction'

The active isometric force developed by a muscle decreases at muscle lengths below an optimal length (Lo). However, when the length of an actively contracting muscle is abruptly decreased, a lower level of isometric force is reached during force redevelopment than when the contraction is initiated at the shorter length. This has been attributed to a deactivation of contractile proteins caused by shortening. In this study, intracellular Ca2+ and myosin light chain (MLC) phosphorylation were measured to assess the mechanisms for the modulation of isometric force caused by changing smooth muscle length before or during isometric contraction. The decline in isometric force between Lo and 0.5Lo was associated with decreases in MLC phosphorylation and intracellular Ca2+ during contractions elicited by acetylcholine or 60 mM KCl. Quick release of the muscle during contraction depressed force redevelopment at the shorter length but not MLC phosphorylation. We conclude that decreases in Ca(2+)-calmodulin-dependent MLC phosphorylation contribute significantly to the decline in isometric force at lengths below Lo, but the depression of contractility associated with the quick release of actively contracted smooth muscle is not caused by a shortening-induced deactivation of contractile proteins.

SUMMARYMany mechanisms reduce the cost of muscle activity. Here, we describe a set of specializations that reduce the cost of contraction in the high-frequency twitches that are used by a wide variety of animals for either sound production or flight. Minimizing the cost of these contractions means that cellular ATP production can meet ATP demand and sustain the high contractile rate. Two classes of specialization are found that minimize the contractile cost. The first class reduces the muscle work required per contraction. Light appendages such as rattles, insect limbs and membranous wings that require little work for movement are used in high-frequency contractions. The second set of specializations involves processes that minimize energy use. High-frequency muscles tend to have a lower cross-bridge content, fewer attached cross-bridges and shorter length changes per contraction. The result is low muscle-specific forces (stress), small length changes (strain) and rapid contraction times that suggest that these muscles push the lower limit of contractile function. The consequence of function at this lower extreme of contraction is to minimize the contractile cost of high-frequency muscles. Thus, specializations that permit rapid contractions at a low rate of ATP use per twitch are the basis of a minimization strategy for energy saving in muscles contracting at high frequency.

It is well known that the strength of cardiac contraction is dependent on the cycle length, evidenced by the force-frequency relationship (FFR) and the existence of postrest potentiation (PRP). Because the contractile strength of the steady-state FFR and force-interval relationship involve instant intrinsic responses to cycle length as well as slower acting components such as posttranslational modification-based mechanisms, it remains unclear how cycle length intrinsically affects cardiac contraction and relaxation. To dissect the impact of cycle length changes from slower acting signaling components associated with persisting changes in cycle length, we developed a novel technique/protocol to study cycle length-dependent effects on cardiac function; twitch contractions of right ventricular rabbit trabeculae at different cycle lengths were randomized around a steady-state frequency. Patterns of cycle lengths that resulted in changes in force and/or relaxation times can now be identified and analyzed. Using this novel protocol, taking under 10 min to complete, we found that the duration of the cycle length before a twitch contraction (“primary” cycle length) positively correlated with force. In sharp contrast, the cycle length one (“secondary”) or two (“tertiary”) beats before the analyzed twitch correlated negatively with force. Using this protocol, we can quantify the intrinsic effect of cycle length on contractile strength while avoiding rundown and lengthiness that are often complications of FFR and PRP assessments. The data show that the history of up to three cycle lengths before a contraction influences myocardial contractility and that primary cycle length affects cardiac twitch dynamics in the opposite direction from secondary/tertiary cycle lengths.

Our purpose was to use small-angle X-ray diffraction to investigate the structural changes within sarcomeres at steady-state isometric contraction following active lengthening and shortening, compared to purely isometric contractions performed at the same final lengths. We examined force, stiffness, and the 1,0 and 1,1 equatorial and M3 and M6 meridional reflections in skinned rabbit psoas bundles, at steady-state isometric contraction following active lengthening to a sarcomere length of 3.0 µm (15.4% initial bundle length at 7.7% bundle length/s), and active shortening to a sarcomere length of 2.6 µm (15.4% bundle length at 7.7% bundle length/s), and during purely isometric reference contractions at the corresponding sarcomere lengths. Compared to the reference contraction, the isometric contraction after active lengthening was associated with an increase in force (i.e., residual force enhancement) and M3 spacing, no change in stiffness and the intensity ratio I1,1/I1,0, and decreased lattice spacing and M3 intensity. Compared to the reference contraction, the isometric contraction after active shortening resulted in decreased force, stiffness, I1,1/I1,0, M3 and M6 spacings, and M3 intensity. This suggests that residual force enhancement is achieved without an increase in the proportion of attached cross-bridges, and that force depression is accompanied by a decrease in the proportion of attached cross-bridges. Furthermore, the steady-state isometric contraction following active lengthening and shortening is accompanied by an increase in cross-bridge dispersion and/or a change in the cross-bridge conformation compared to the reference contractions.

Firing rates of motor units and surface EMG were measured from the triceps brachii muscles of able-bodied subjects during brief submaximal and maximal isometric voluntary contractions made at 5 elbow joint angles that covered the entire physiological range of muscle lengths. Muscle activation at the longest, midlength, and shortest muscle lengths, measured by twitch occlusion, averaged 98%, 97%, and 93% respectively, with each subject able to achieve complete activation during some contractions. As expected, the strongest contractions were recorded at 90° of elbow flexion. Mean motor unit firing rates and surface EMG increased with contraction intensity at each muscle length. For any given absolute contraction intensity, motor unit firing rates varied when muscle length was changed. However, mean motor unit firing rates were independent of muscle length when contractions were compared with the intensity of the maximal voluntary contraction (MVC) achieved at each joint angle.Key words: muscle activation, length–tension relationships, force–frequency relationships.

Striated muscle contraction requires intricate interactions of microstructures. The classic textbook assumption that myosin filaments are compressed at the meshed Z-disc during striated muscle fibre contraction conflicts with experimental evidence. For example, myosin filaments are too stiff to be compressed sufficiently by the muscular force, and, unlike compressed springs, the muscle fibres do not restore their resting length after contractions to short lengths. Further, the dependence of a fibre's maximum contraction velocity on sarcomere length is unexplained to date. In this paper, we present a structurally consistent model of sarcomere contraction that reconciles these findings with the well-accepted sliding filament and crossbridge theories. The few required model parameters are taken from the literature or obtained from reasoning based on structural arguments. In our model, the transition from hexagonal to tetragonal actin filament arrangement near the Z-disc together with a thoughtful titin arrangement enables myosin filament sliding through the Z-disc. This sliding leads to swivelled crossbridges in the adjacent half-sarcomere that dampen contraction. With no fitting of parameters required, the model predicts straightforwardly the fibre's entire force–length behaviour and the dependence of the maximum contraction velocity on sarcomere length. Our model enables a structurally and functionally consistent view of the contractile machinery of the striated fibre with possible implications for muscle diseases and evolution.

The production of work by the contractile component (CC) and the storage and release of work in the elastic structures that act in series (the series elastic component, SEC) with the contractile component were measured using white muscle fibres from the dogfish Scyliorhinus canicula. Heat production was also measured because the sum of work and heat is equivalent to the energy cost of the contraction (ATP used). These energy fluxes were evaluated in contractions with constant-velocity shortening either during stimulation or during relaxation. The muscle preparation was tetanized for 0.6 s and shortened by 1 mm (approximately 15 % of L0) at 3.5 or 7.0 mm s-1 (approximately 15 or 30 % of V0), where L0 is the muscle length at which isometric force is greatest and V0 is the maximum velocity of shortening. In separate experiments, the stiffness of the SEC was characterized from measurements of force responses to step changes in the length of contracting muscle. Using the values of SEC stiffness, we evaluated separately the work and heat associated with the CC and with the SEC. The major findings were (1) that work stored in the SEC could be completely recovered as external work when shortening occurred during relaxation (none of the stored work being degraded into heat) and (2) that, when shortening occurred progressively later during the contraction, the total energy cost of the contraction declined towards that of an isometric contraction.

The slack length of a relaxed human skeletal muscle is not fixed; it can be modified by contraction and stretch. Contraction of the human vastus lateralis muscle at short lengths reduces the muscle’s slack length. Even very weak contractions are sufficient to induce this effect. The effect persists for at least 5 min but can be reduced or abolished with a large-amplitude passive stretch.

Effects of intravenous infusions of somatostatin, methionine-enkephalin, and 5-hydroxytryptophan on canine ileal motor patterns and transit of chyme were investigated postprandially. Motility was recorded by multiple closely spaced extraluminal strain gauges. By a computerized method, the length of contraction spread and other motility parameters were evaluated. Transit rates were measured fluoroscopically. Somatostatin and methionine-enkephalin initiated a mixing activity by reducing the incidence and the length of spread of contraction waves induced by a noncaloric meal. Methionine-enkephalin, but not somatostatin, decreased both the number of contractions per minute and the motility index. 5-Hydroxytryptophan converted the mixing activity induced by a nutrient meal into a propulsive pattern. The incidence and the length of spread of contraction waves as well as the number of contractions per minute, the contraction force, and the motility index were enhanced. Results suggest that somatostatin, methionine-enkephalin, and 5-hydroxytryptophan are effective modulators of ileal propulsive activity. Effects are largely similar to those observed in the proximal jejunum, although the lengths of contraction spread and the transit rates were generally less in the ileum.

Background.Muscles not only shorten during contraction to perform mechanical work, but they also bulge radially because of the isovolumetric constraint on muscle fibres. Muscle bulging may have important implications for muscle performance, however quantifying three-dimensional (3D) muscle shape changes in human muscle is problematic because of difficulties with sustaining contractions for the duration of anin vivoscan. Although two-dimensional ultrasound imaging is useful for measuring local muscle deformations, assumptions must be made about global muscle shape changes, which could lead to errors in fully understanding the mechanical behaviour of muscle and its surrounding connective tissues, such as aponeurosis. Therefore, the aims of this investigation were (a) to determine the intra-session reliability of a novel 3D ultrasound (3DUS) imaging method for measuringin vivohuman muscle and aponeurosis deformations and (b) to examine how contraction intensity influencesin vivohuman muscle and aponeurosis strains during isometric contractions.Methods.Participants (n= 12) were seated in a reclined position with their left knee extended and ankle at 90° and performed isometric dorsiflexion contractions up to 50% of maximal voluntary contraction. 3DUS scans of the tibialis anterior (TA) muscle belly were performed during the contractions and at rest to assess muscle volume, muscle length, muscle cross-sectional area, muscle thickness and width, fascicle length and pennation angle, and central aponeurosis width and length. The 3DUS scan involved synchronous B-mode ultrasound imaging and 3D motion capture of the position and orientation of the ultrasound transducer, while successive cross-sectional slices were captured by sweeping the transducer along the muscle.Results.3DUS was shown to be highly reliable across measures of muscle volume, muscle length, fascicle length and central aponeurosis length (ICC ≥ 0.98, CV < 1%). The TA remained isovolumetric across contraction conditions and progressively shortened along its line of action as contraction intensity increased. This caused the muscle to bulge centrally, predominantly in thickness, while muscle fascicles shortened and pennation angle increased as a function of contraction intensity. This resulted in central aponeurosis strains in both the transverse and longitudinal directions increasing with contraction intensity.Discussion.3DUS is a reliable and viable method for quantifying multidirectional muscle and aponeurosis strains during isometric contractions within the same session. Contracting muscle fibres do work in directions along and orthogonal to the muscle’s line of action and central aponeurosis length and width appear to be a function of muscle fascicle shortening and transverse expansion of the muscle fibres, which is dependent on contraction intensity. How factors other than muscle force change the elastic mechanical behaviour of the aponeurosis requires further investigation.

Overactive bladder (OAB), involuntary contractions during bladder filling, is a common condition affecting 17% of the adult population worldwide, and in the U.S. ranks ahead of diabetes in a list of the 10 most common chronic disorders (Mullins 2009). Mechanical mechanisms contributing to OAB are not completely understood and because of the unique function and broad volume range of the bladder, there may be mechanical characteristics that distinguish detrusor smooth muscle (DSM) in bladder from other smooth muscles. Recent studies have shown that the length-passive tension curve in DSM exhibits adjustable passive stiffness (APS) characterized by a passive curve that can be shifted along the length axis as a function of strain history and activation history; however, the mechanical mechanisms responsible for APS remain to be determined. Also, whether DSM exhibits a dynamic length-active tension relationship, as has been identified in airway and vascular smooth muscles, has not been investigated. This dissertation focused on both the passive and active length-tension relationships in DSM and the mechanical mechanisms responsible for these relationships. The first objective was to study the impact of APS on the length-total tension relationship and identify the mechanical mechanisms responsible for generating APS. The second objective was to determine whether the length-active tension relationship is adaptive and identify specific mechanical mechanisms contributing to any adaptive behavior. The results showed that a shift in the length-passive tension curve due to APS corresponded with a shift in the length-total tension curve in DSM, and that APS was 27.0±8.4% of active tension at the optimum length for active tension generation. Most importantly, low-grade rhythmic contraction (RC), which can occur spontaneously in rabbit and human bladders, regenerated APS. Results also showed that the length-active tension curve shifted due to stretch to and then activation at long lengths, as well as either multiple KCl-induced maximal contractions or RC. Thus, DSM exhibits length adaptation, and RC may contribute to both APS and length adaptation. Because increased RC has been correlated with OAB, understanding RC, APS and length-adaptation in bladder may enable the identification of specific targets for new treatments for OAB.

Titin is a giant protein spanning from the Z-disk to the M-band of the cardiac sarcomere. In the I-band titin acts as a molecular spring, contributing to passive mechanical characteristics of the myocardium throughout a heartbeat. RNA Binding Motif Protein 20 (RBM20) is required for normal titin splicing, and its absence or altered function leads to greater expression of a very large, more compliant N2BA titin isoform in Rbm20 homozygous mice (Rbm20(Delta RRm)) compared to wild-type mice (WT) that almost exclusively express the stiffer N2B titin isoform. Prior studies using Rbm20(Delta RRm) animals have shown that increased titin compliance compromises muscle ultrastructure and attenuates the Frank-Starling relationship. Although previous computational simulations of muscle contraction suggested that increasing compliance of the sarcomere slows the rate of tension development and prolongs cross-bridge attachment, none of the reported effects of Rbm20(Delta RRm) on myocardial function have been attributed to changes in cross-bridge cycling kinetics. To test the relationship between increased sarcomere compliance and cross-bridge kinetics, we used stochastic length-perturbation analysis in Ca2+-activated, skinned papillary muscle strips from Rbrn20<^>R'Rm and WT mice. We found increasing titin compliance depressed maximal tension, decreased Ca2+-sensitivity of the tension-pCa relationship, and slowed myosin detachment rate in myocardium from Rbm20(Delta RRm) vs. WT mice. As sarcomere length increased from 1.9 to 2.2 mu m, length-dependent activation of contraction was eliminated in the Rbrn20<^>R'Rm myocardium, even though myosin MgADP release rate decreased similar to 20% to prolong strong cross-bridge binding at longer sarcomere length. These data suggest that increasing N2BA expression may alter cardiac performance in a length-dependent manner, showing greater deficits in tension production and slower cross-bridge kinetics at longer sarcomere length. This study also supports the idea that passive mechanical characteristics of the myocardium influence ensemble cross-bridge behavior and maintenance of tension generation throughout the sarcomere.

Protein-coding genes evolve together with their genome and acquire changes, some of which affect the length of their protein products. This explains why equivalent proteins from different species can exhibit length differences. Variation in length of proteins during evolution arguably presents a large number of possibilities for improvement and innovation of protein structure and function. In order to contribute to an increased understanding of this process, we have studied variation caused by tandem domain duplications and insertions or deletions of intrinsically disordered residues. The study of two proteins, Nebulin and Filamin, together with a broader study of long repeat proteins (>10 domain repeats), began by confirming that tandem domains evolve by internal duplications. Next, we show that vertebrate Nebulins evolved by duplications of a seven-domain unit, yet the most recent duplications utilized different gene parts as duplication units. However, Filamin exhibits a checkered duplication pattern, indicating that duplications were followed by similarity erosions that were hindered at particular domains due to the presence of equivalent binding motifs. For long repeat proteins, we found that human segmental duplications are over-represented in long repeat genes. Additionally, domains that have formed long repeats achieved this primarily by duplications of two or more domains at a time. The study of homologous protein pairs from the well-characterized eukaryotes nematode, fruit fly and several fungi, demonstrated a link between variation in length and variation in the number of intrinsically disordered residues. Next, insertions and deletions (indels) estimated from HMM-HMM pairwise alignments showed that disordered residues are clearly more frequent among indel than non-indel residues. Additionally, a study of raw length differences showed that more than half of the variation in fungi proteins is composed of disordered residues. Finally, a model of indels and their immediate surroundings suggested that disordered indels occur in already disordered regions rather than in ordered regions.

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: In press. Paper 4: Manuscript.

Neuromuscular explosive strength (defined as rate of force development; RFD) is considered important during explosive functional human movements; however this association has been poorly documented. It is also unclear how different variants of strength training may influence RFD and its neuromuscular determinants. Furthermore, RFD has typically been measured in isometric situations, but how it is influenced by the types of contraction (isometric, concentric, eccentric) is unknown. This thesis compared neuromuscular function in explosive power athletes (athletes) and untrained controls, and assessed the relationship between RFD in isometric squats with sprint and jump performance. The athletes achieved a greater RFD normalised to maximum strength (+74%) during the initial phase of explosive contractions, due to greater agonist activation (+71%) in this time. Furthermore, there were strong correlations (r2 = 0.39) between normalised RFD in the initial phase of explosive squats and sprint performance, and between later phase absolute explosive force and jump height (r2 = 0.37), confirming an association between explosive athletic performance and RFD. This thesis also assessed the differential effects of short-term (4 weeks) training for maximum vs. explosive strength, and whilst the former increased maximum strength (+20%) it had no effect on RFD. In contrast explosive strength training improved explosive force production over short (first 50 ms; +70%) and long (>50 ms; +15%) time periods, due to improved agonist activation (+65%) and maximum strength (+11%), respectively. Explosive strength training therefore appears to have greater functional benefits than maximum strength training. Finally, the influence of contraction type on RFD was assessed, and the results provided unique evidence that explosive concentric contractions are 60% more effective at utilising the available force capacity of the muscle, that was explained by superior agonist activation. This work provides a comprehensive analysis of the association between athletic performance and RFD, the differential effects of maximum vs. explosive strength training, and the influence of contraction type on the capacity for RFD.

L’objectif de ce travail était de comparer l’importance de la composante nerveuse dans les différences de production de force entre les enfants et les adultes en tenant compte des conditions mécaniques qui sont susceptibles de l’influencer (longueur musculaire, mode et vitesse de contraction) et des effets de la fatigue. Trente et un garçons pré-pubères de 8-12 ans et 37 hommes de 18-30 ans ont été recrutés et répartis dans les 3 études composant ce projet. La première étude portait sur les effets de la longueur musculaire et de la fatigue sur la production de force et le niveau d’activation maximal volontaire (VA) des extenseurs du genou.La seconde traitait des effets de la longueur musculaire sur la production de force et le VA des fléchisseurs plantaires. Enfin, la troisième portait sur les effets du mode et de la vitesse de contraction sur le VA des extenseurs du genou. Les propriétés neuromusculaires ont été évaluées à l’aide de stimulations magnétiques périphériques et de contractions maximales volontaires. Les résultats montrent un VA des extenseurs du genou supérieur chez l’adulte lors de grande longueur musculaire (90°-100° de flexion) (étude 1), mais aucune différence avec l’enfant à courte longueur musculaire (20° de flexion). En revanche, sur des groupes musculaires tels que les fléchisseurs plantaires, aucun effet de la longueur sur le VA n’a été observé chez les enfants et les adultes (étude 2). En ce qui concerne les effets du mode de contraction, nos résultats montrent que le VA est inférieur en conditions excentrique et concentrique par rapport à la condition isométrique, mais aucune différence n’était observée entre les groupes (étude 3). Toutefois, l’effet de cette baisse de VA sur la production de force variait entre les groupes ; une diminution de force accompagnant la diminution de VA n’était retrouvée que chez l’adulte. Enfin,nous avons montré que la baisse du VA au cours d’un protocole de fatigue est plus conséquente chez les enfants par rapport aux adultes (étude 1), ce qui témoigne de l’existence d’une fatigue centrale majorée chez l’enfant. Une maturation inaboutie et les propriétés musculo-tendineuses particulières de l’enfant (compliance supérieure) pourraient expliquer les résultats obtenus au cours de ce travail
The purpose of this PhD thesis was to compare the contribution of the maximal voluntary activation level (VA) of the motor units to force production differences between children and adults in various mechanical conditions that affect force production (muscle length, contraction mode and velocity), and in fatigue condition. Thirty one pre-pubertal 8 to 12-year old boys and 37 men (18-30 years) were recruited and allocated into the 3 studies ofthis project. The first study was devoted to compare the effects of muscle length and fatigue on the VA and force generating capacity of the knee extensors between children and adults. In the second study, we compared the effects of muscle length on the VA and force generating capacity of the plantar flexors between children and adults. The third study analysed the effect of contraction mode and velocity on the VA and force generating capacity of the knee extensors between children and adults. Neuromuscular properties were assessed with peripheral magnetic stimulation and maximal voluntary contractions. Results showed a higher VA of the knee extensors in adults at long muscle length (90° and 100° knee flexion) but no difference between children and adults at short length (20° knee flexion; study 1). However, the VA of the plantar flexors was not affected by muscle length changes whatever the age group considered (study 2). Results also showed a higher VA in isometric mode compared to eccentric and concentric conditions whatever the age group (study 3). However, the effect of this VA reduction on force generating capacity differed between groups, a concomitant force reduction being observed in adults, but not in children. Finally, we observed a greater VA reduction and therefore greater central fatigue in children during the fatiguing protocol (study 1). A relative immaturity and the particular musculo-tendinous properties of children (higher compliance) may account for the reported results

Smooth muscle is an essential component of the walls of numerous hollow or tubular organs throughout the body, including blood vessels, airways, and the bladder. Proper physiological functioning of these organs relies heavily on the appropriate activation and contraction of the smooth muscle tissue. Pathophysiological conditions may arise from both excessive and insufficient smooth muscle contraction. Muscle function is closely associated with muscle structure. More specifically, during a contraction, cyclic interactions between myosin cross-bridges and actin filaments allow for muscle shortening and force generation. Myosin molecules from smooth muscle and non-muscle cells are known to self-assemble into side-polar filaments in vitro. However the in situ mechanism of filament assembly is not clear and the question of whether there is a unique length for myosin filaments in smooth muscle is still under debate. In this study we measured the lengths of 16,587 myosin filaments in three types of smooth muscle cells using serial electron microscopy (EM). Sheep airway and pulmonary arterial smooth muscle as well as rabbit carotid arterial smooth muscle were fixed for EM and serial ultra-thin (50-60 nm) sections were obtained. Myosin filaments were traced in consecutive sections to determine their lengths. The results indicate that there is not a single length for the myosin filaments; instead there is a wide variation in lengths. The plots of observation frequency versus myosin filament length follow an exponential decay pattern. The most significant finding of this study is that myosin filaments in smooth muscle do not have a uniform length and analysis suggests that the distribution of filament length is a result of a dynamic equilibrium between polymerization and de-polymerization of myosin molecules driven by predictable probabilities of the myosin dimers to bind with and dissociate from each other.
Medicine, Faculty of
Medicine, Department of
Experimental Medicine, Division of
Graduate

In Chapter 6, we discovered that different frames mix time and space differently; in this chapter, we will discover that speed causes time to run slowly and space to contract. Of course, the truth is a bit subtler than that, because when two frames are in relative motion each frame measures the other as the high‐speed frame where time runs slowly and space contracts.We deduce time dilation and length contraction in multiple ways: first with a light clock as a thinking tool, and then with spacetime diagrams and a new thinking tool called the symmetric frame. We also examine the experimental proof of these effects. By the end of this chapter, you will understand how time dilation and length contraction fit together with time skew in a fully consistent model that beautifully fits a wide range of evidence.

Before explaining how the God’s-eye view resolves the impasse of theoretical physics and foundations of physics created by the ant’s-eye view, the book presents a detailed argument for the block universe. Accordingly, the main thread of chapter 2 shows how the relativity of simultaneity resolves the paradoxes associated with time dilation and length contraction that result from special relativity. A short argument is then presented showing how the relativity of simultaneity implies a block universe, that is, the co-reality or co-existence of the past, present, and future. Philosophy of Physics for Chapter 2 provides a detailed argument for block universe, taking into account all counterarguments and assumptions of the abridged argument in the main thread. Foundational Physics for Chapter 2 shows how the second postulate of special relativity leads to time dilation and length contraction, and it contains the Lorentz transformations for the spacetime events used in the main thread.

The main driving mechanism in asthmatic attacks is the contraction of airway smooth muscle (ASM). Physiological loading of ASM by normal breathing and deep inspiration has been shown to reduce ASM contractile force. Our research involves the development of a mathematical model to describe the dynamic behavior of ASM. In this model length adaptation is described by an evolving filament network of passive and active elements in parallel with a soft glassy material. Contractile forces are described by the evolving force length characteristics of some of the network links. It is envisaged that the model will bridge the gap between passive ASM dynamics and contractile dynamics. The model will be validated using experimental data from testing of ASM tissues in vitro.

Direct measurement techniques are employed to quantify the kinematics of DNA flows in micro-contraction devices. Flow through micro-contractions subjects the fluid to large spatial gradients in velocity, thereby eliciting viscoelastic effects. Additionally, in this microfluidic flow environment, the fully extended length of the macromolecule L will approach the characteristic length scale of the channel geometry h. This is a unique flow environment that is not yet well understood. Knowledge of the fundamental physics that govern this flow regime will have a profound impact on optimization of lab-on-a-chip systems incorporating macromolecular flows. This study investigates the flow of semi-dilute λ-DNA solutions in a 2:1 micro-contraction where L/h ∼ 0.32. Video microscopy and streak images of semi-dilute DNA flows reveal large vortex regions in the corners of the contraction, which are indicative of strong elastic behavior. Velocity fields constructed using Digital Particle Image Velocimetry (DPIV) demonstrate the first use of this tool for obtaining velocity measurements of viscoelastic flows in microfluidic systems.

Braided pneumatic actuators (BPAs) are attractive for use in bio-robots because they offer many muscle-like properties, especially when compared to most other commercially available robotic actuators. Unfortunately, the same properties that make these actuators similar to muscles make them more difficult to control. One such actuator manufactured by Festo, the MXAM-10-AA, is frequently utilized in robotics because of its commercial availability, durability, and force capability. Although models for BPAs exist, the properties that make this actuator more durable also make its behavior less like other braided pneumatic actuators, especially for shorter actuator lengths. Length specific models that do exist for Festo fluidic muscles have numerous parameters that can only be found experimentally by taking hundreds to thousands of data points and performing a lengthy optimization process to fit parameter values for each actuator in the system. This lack of generalizability makes it difficult to build a new robot and begin testing new control systems without significant startup time and cost. The key contribution of this work is the development of a generalizable actuator model that accounts for the geometry and limitations of the actuator at shorter lengths. This empirical model relates internal pressure, strain, stretching or contracting state, and applied force on the MXAM-10-AA actuator. The model is scalable to different length actuators by measuring their resting length at zero pressure and their minimum contraction length at maximum air pressure, and can be used for feedforward length control. The model is evaluated on a robot leg with three joints and 6 actuators, each with different length. The developed controller, using the actuator model, controls the joints to within ± 3 degrees of the desired position for different desired torques only using internal actuator pressure feedback. We also demonstrate control speed by cycling a joint over 40 degrees of rotation at varying frequencies.

A few analytical models for a simply-supported cylinder subjected to a concentric annular flow have been developed considering small perturbations in the flow components. The flow perturbation is believed to play a key role in causing instabilities or reducing the effective tube damping. However, it is generally accepted that there is no dynamic instability of a simply-supported cylinder subjected to concentric annular flows; the dynamic instability may materialize only with a very flexible rod where traveling wave solutions are expected. The dynamic behavior of a concentric annular flow in a finite-length-narrow gap at the mid-span of a cylinder is studied to investigate the possibility of a dynamic instability in the case of a simply-supported cylinder. In this study, typically used stainless steel tubing and geometry in engineering application are applied with some fluid boundary conditions at the finite-length-narrow gap such as a contraction entrance and a diffuser exit. The research results from this study are as follows: (1) determination of an analytical solution for tube dynamics in terms of fluid boundary conditions such as contraction loss at the entrance and pressure recovery at the exit of the finite-length-annular gap, (2) confirmation of the effect of leakage-flow to system dynamics, and (3) confirmation of flutter instability (negative damping type) at the fluid boundary conditions.

In-line measurements and sample stream withdrawals for on-line and/or at-line measurements of slurries flowing in horizontal pipes can be complicated by nonuniform slurry profiles. More uniform profiles would improve measurements. Area contractions are a common means used to produce more uniform velocity fields for single phase flows. For example, contractions are used to condition the flow entering wind tunnel test sections and make velocity profiles more uniform at venturi throats. It was desired to determine whether area contractions could be used to make slurry concentration profiles more uniform in horizontal pipe flows. An ASME flow nozzle with a contraction diameter ratio of 0.5 was chosen as a well defined geometry to consider in a Computational Fluid Dynamic (CFD) study of the effects of a contraction on slurry concentration profiles. The pipe was 2.8 m long with a 50.8 mm diameter. The entrance of the contraction was placed at 35 pipe diameters from the inlet in fully developed flow. A length of 20 diameters followed the contraction. The slurry had a xylene liquid phase and an ADP solid phase with a density ratio of 1.7. The simulations were performed at primary phase velocities of 2 m/s and 4 m/s, corresponding to Reynolds numbers of 1.4E05 and 2.8E05. Spherical particle diameters of 38, 75, and 150 μm were used at concentrations of 0.05, 0.2, and 0.3. ANSYS FLUENT 12 software was used with the standard k-ε turbulence model and standard wall function. The mixture multi-phase model was used for the two-phase flow. An unstructured tetrahedral meshing scheme was used with 1.4 million elements. The grid was adjusted until the condition 30 < y+ <60 for the mesh point nearest the wall was satisfied. A grid refinement study was performed to insure grid independence. The computational scheme first was validated by comparing pipe flow velocity and concentration profiles to results in the literature. The computations performed with the contraction showed that in all cases the concentration profiles of the solid particles displayed greater uniformity than the profiles in the pipe upstream of the contraction. The effect of the contraction was more pronounced for the larger particles. As in the case of single phase flows, the contraction caused the axial turbulence intensity to decrease. The greater uniformity of the concentration profiles at the exit plane of the nozzle, suggest that the contraction can provide better conditions for performing measurements of a particle-laden slurry.

Deformation of a droplet passing through a contraction and expansion at intermediate to large Reynolds number is predicted numerically and compared with experimental observations. The effects of fluid properties, contraction geometry and flow behaviour is considered. The experiments were performed for silicon oil droplets in water moving through a contraction. Orifice and droplet diameter were of the order of millimetres. Contraction length and flow rate were varied, while the contraction ratio was kept constant (1:4). The droplets are stretched by the contraction, after which oscillations are observed on the interface. Observation of the systems indicated that a disintegration zone exists for droplets above a critical size. Numerical simulations were carried out using an axisymmetric Volume-of-Fluid (VOF) code. Simulations were carried out to match the experimental conditions as well as for a few different Reynolds and Weber numbers. Results showed qualitative agreement with experiments. The predictions indicate that vortices within the contraction are responsible for the capillary-like waves on the stretched droplet.

The paper describes passivity-related input-output properties of a human muscle and tendon system given by a Hill type dynamic model. For a model having muscle contraction velocity as the input and force as the output, it is shown that the system is passive during the concentric phase. Also, it is shown that a negative strict passivity margin exists for the eccentric phase if the length and lengthening velocity of the contractile element are assumed bounded. Estimates of this margin are given by means of two alternative formulas. Further, it is shown that the mapping from contraction velocity to deviation from equilibrium force is passive in both concentric and eccentric phases. The paper discusses how these findings and the passivity theorem can be used to design controllers for a machine coupled to the muscle model by feedback interconnection. The simple case of a proportional-derivative force feedback regulator is considered as an example. A simulation example is given where the transient response of the coupled system crosses the eccentric region.

This contribution presents the development and design of a two-stage contraction zone and modular test section for a closed loop Organic Rankine Cycle (ORC) wind tunnel. The first contraction consists of four truncated cones, whose length and angle of inclination are derived from a two-stage optimization procedure, with the objective to minimize flow deviation and to avoid boundary-layer separation. The geometrical optimization yields a profile with minor deviation to the ideal polynomial shape, whereas the flow optimized shape minimizes flow separation at the break-points between the single conical pieces. The second contraction has to perform two major tasks, namely the acceleration of the flow up to a Mach number of Ma = 0.8 for organic fluids and the transformation of the circular inlet to a rectangular outlet cross-sectional shape, required by the working section. The circular-to-rectangular transition is accomplished by variation of the generalized ellipse, also known as Lamé curve. Smooth polynomials are then used to define the reduction of cross-sectional area. A comprehensive number of contraction geometries with fixed contraction ratio, variable length, and different points of inflection are analyzed with regards to minimum flow deviation, the avoidance of flow separation, as well as a uniform velocity field at the contraction outlet. A semi-analytical approach based on a potential flow solution in combination with the Stratford criterion is the basis for evaluating boundary-layer separation. The design of a two-part modular diffuser, based on the concept of a dumped diffuser, as commonly encountered in gas turbine design, is presented. The numerical results are compared with analytical findings and special characteristics of the different designs are explained. Finally, the overall design concept of the test section is presented.

This study investigated the causes for premature, transverse cracking on urban jointed plain concrete pavements in Illinois. A field survey of 67 sections throughout Illinois coupled with ultrasonic evaluation was completed to synthesize the extent of premature cracking on urban JPCP. The visual survey showed some transverse and longitudinal cracks were a result of improper slab geometry (excessive slab length and width). Ultrasonic tests over the contraction joints determined some notched joints had not activated and adjacent transverse cracks were likely formed as a result. Three-dimensional finite-element analyses confirmed that cracking would not develop as a result of normal environmental factors and slab-base frictional restraint. The concrete mixture also did not appear to be a contributing factor to the premature cracks. Finally, the lack of lubrication on dowel bars was determined to potentially be a primary mechanism that could restrain the transverse contraction joints, produce excessive tensile stresses in the slab, and cause premature transverse cracks to develop.

1.1 Resumen macroeconómico En el segundo trimestre de 2022 la inflación anual (9,67 %), su pronóstico y sus expectativas se incrementaron y se mantuvieron por encima de la meta. Los choques internacionales de costos, intensificados por la invasión de Rusia a Ucrania, han sido más persistentes de lo estimado y contribuyeron a elevar la inflación. A esto se suman los efectos de la indexación, unos excesos de demanda mayores de lo estimado, un mercado laboral más ajustado, unas expectativas de inflación que siguen aumentando y que superan el 3 %, y las presiones provenientes de la tasa de cambio. Los altos registros en las medidas de inflación básica y en todos los grandes grupos del índice de precios al productor (IPC) confirman una difusión significativa de los incrementos de precios. Frente a lo estimado en abril, el nuevo sendero de pronóstico de la inflación total y básica aumentaron, en parte por mayores presiones de la tasa de cambio sobre los precios y por una brecha del producto superior, que se estima positiva en lo que resta de 2022 y que se cerraría hacia finales de 2023. Además, estas sendas contemplan una indexación a tasas de inflación más altas y la persistencia de unas expectativas de inflación por encima de la meta, así como un mayor ritmo de incremento del precio interno de los combustibles por cuenta de la corrección de su rezago frente al precio de paridad y de una proyección del precio internacional del petróleo más elevada. El pronóstico supone una buena oferta interna de alimentos perecederos, aunque también unos precios internacionales de los alimentos procesados que se mantendrían altos. Para la subcanasta de bienes, el final de la emergencia sanitaria implica una reversión de la rebaja del impuesto al valor agregado (IVA) aplicada a productos de aseo e higiene, generando aumentos en los precios de este grupo. Por otro lado, el proceso de ajuste de la política monetaria y la moderación de los choques externos contribuiría a que la inflación y sus expectativas empiecen a ceder en el tiempo y retomen su convergencia hacia la meta. Así, la nueva proyección sugiere que en el segundo semestre de 2022 la inflación continuaría elevada y finalizaría en un 9,7 %, pero a lo largo de 2023 empezaría a ceder para cerrar el año en el 5,7 %. Estos pronósticos están sujetos a una gran incertidumbre, especialmente alrededor del comportamiento futuro de los choques externos de costos, del grado de indexación de los contratos nominales y de las decisiones que se tomen en cuanto al precio interno de los combustibles. La dinámica de la actividad económica continúa sorprendiendo al alza y la proyección de crecimiento para 2022 aumentó del 5 % al 6,9 %. Los nuevos pronósticos sugieren un nivel de producto mayor que continuaría superando la capacidad productiva de la economía en lo que resta de 2022. El crecimiento económico del primer trimestre resultó mejor de lo estimado en abril y los indicadores de actividad económica para el segundo sugieren que el PIB se mantendría alto, en un nivel que sería superior al del primero. La demanda interna habría mantenido una buena dinámica debido, en especial, a un consumo privado que volvería a crecer trimestralmente, como lo sugieren las cifras de matrículas de vehículos, las ventas de comercio al por menor, las compras con tarjetas de crédito y los desembolsos de préstamos para consumo. La desaceleración de las importaciones de maquinaria y equipo desde el alto nivel observado en marzo, que contrasta con el buen desempeño de las ventas y licencias de construcción de vivienda, apuntan a un nivel de la inversión similar al registrado en los primeros tres meses del año. La información de comercio exterior indica que el déficit comercial se reduciría, debido a unas importaciones que serían menores con respecto a los altos valores observados en el primer trimestre y a unos niveles estables de las exportaciones. Para lo que resta del año y 2023 se espera que el consumo se desacelere desde los niveles elevados observados en la primera mitad del año, como consecuencia, en parte, de una menor demanda represada, de unas condiciones financieras internas más apretadas y de un deterioro del ingreso disponible de los hogares por el aumento de la inflación. La inversión continuaría recuperándose de forma lenta, sin alcanzar los niveles observados antes de la pandemia. La menor dinámica esperada de la demanda interna y los altos niveles de los precios del petróleo y de otros bienes básicos queexporta el país se reflejarían en una reducción del déficit comercial. Con todo esto, el crecimiento económico para el segundo trimestre de 2022 se situaría en el 11,5 %, y en todo 2022 y 2023 en el 6,9 % y 1,1 %, respectivamente. En la actualidad, y en lo que resta de 2022, la brecha del producto sería positiva y mayor de lo estimado en abril, y se presentarían presiones de demanda sobre los precios. Estas proyecciones continúan sujetas a un alto grado de incertidumbre, asociado con las tensiones políticas globales, el ajuste esperado de la política monetaria en los países avanzados, el comportamiento de la demanda externa, los cambios en la percepción de riesgo-país y la evolución futura de la política fiscal interna, entre otros. Los elevados niveles de la inflación y de sus expectativas, que superan las metas de los principales bancos centrales del mundo, explican, en gran parte, el incremento observado y esperado de sus tasas de interés de política. En este entorno, las proyecciones de crecimiento de la demanda externa se han moderado. Las disrupciones en las cadenas de valor, el aumento en los precios internacionales de los alimentos y de la energía, y las políticas monetarias y fiscales expansivas han contribuido al aumento de la inflación y a unas expectativas crecientes que superan las metas en varios de los principales socios comerciales del país. Estos choques de costos y de precios, acentuados por los efectos de la invasión de Rusia a Ucrania, han sido más persistentes de lo estimado y se han dado en un contexto de recuperación del producto y del empleo, variables que en algunos de los países ya igualan o superan sus niveles estimados de largo plazo. Como respuesta, en los Estados Unidos la Reserva Federal (Fed) aceleró el ritmo de incremento de la tasa de interés de política y disminuyó rápidamente los niveles de liquidez en el mercado monetario. Los participantes de los mercados financieros esperan que este comportamiento continúe y, en consecuencia, incrementaron significativamente la senda promedio esperada de la tasa de interés de política de la Fed. En este contexto, en el segundo trimestre el dólar se apreció y las medidas de riesgo de los países emergentes aumentaron, comportamiento que ha sido acentuado en el caso colombiano. Con todo esto, para lo que resta de 2022 y 2023 el equipo técnico del Banco aumentó el sendero previsto para la tasa de interés de la Fed y redujo el pronóstico de crecimiento de la demanda externa del país. La senda esperada del precio del petróleo se incrementó, debido, en especial, a mayores restricciones de oferta y a la interrupción del comercio de hidrocarburos entre la Unión Europea y Rusia. Las tensiones geopolíticas globales, el ajuste más fuerte de la política monetaria en los países avanzados, el incremento en la percepción de riesgo para los mercados emergentes y los desbalances macroeconómicos en el país explican el incremento de la senda proyectada de la prima de riesgo, de su nivel tendencial y de la tasa de interés real neutral1. La incertidumbre sobre los pronósticos externos y sobre su consecuente impacto en el escenario macroeconómico del país se mantiene elevada, dada la imprevisible evolución del conflicto entre Rusia y Ucrania, de las tensiones geopolíticas, del grado de desaceleración de la economía mundial y del efecto que la respuesta a los recientes rebrotes de la pandemia en algunos países de Asia pueda tener sobre la economía global. El contexto macroeconómico de alta inflación, de pronósticos y expectativas de inflación superiores al 3 % y de brecha del producto positiva sugiere la necesidad de una postura de la política monetaria en terreno contractivo que mitigue el riesgo de desanclaje persistente de las expectativas de inflación. Frente a los pronósticos del Informe de abril, el aumento de la tendencia de la prima de riesgo implica una tasa de interés real neutral más alta y la existencia de un estímulo monetario mayor del estimado previamente. Por su parte, la demanda interna ha sido más dinámica, con un nivel observado y proyectado del producto mayor, que supera la capacidad productiva de la economía. Las sorpresas al alza de la inflación total y básica son el reflejo de choques externos más fuertes y persistentes que, junto con la fortaleza de la demanda agregada, la indexación, las mayores expectativas de inflación y las presiones cambiarias explican el incremento en la senda de pronóstico de la inflación a niveles que superan la meta en los siguientes dos años. Esto es corroborado por las medidas de expectativas de inflación de los analistas económicos y aquellas derivadas del mercado de deuda pública, que siguieron aumentando y exceden el 3 %. Todo lo anterior aumentó el riesgo de desanclaje de las expectativas de inflación y podría generar procesos de indexación generalizados que alejen la inflación de la meta por más tiempo. Este nuevo contexto macroeconómico sugiere que el ajuste de la tasa de interés debe continuar hacia un terreno contractivo de la política monetaria. de lo estimado y se han dado en un contexto de recuperación del producto y del empleo, variables que en algunos de los países ya igualan o superan sus niveles estimados de largo plazo. Como respuesta, en los Estados Unidos la Reserva Federal (Fed) aceleró el ritmo de incremento de la tasa de interés de política y disminuyó rápidamente los niveles de liquidez en el mercado monetario. Los participantes de los mercados financieros esperan que este comportamiento continúe y, en consecuencia, incrementaron significativamente la senda promedio esperada de la tasa de interés de política de la Fed. En este contexto, en el segundo trimestre el dólar se apreció y las medidas de riesgo de los países emergentes aumentaron, comportamiento que ha sido acentuado en el caso colombiano. Con todo esto, para lo que resta de 2022 y 2023 el equipo técnico del Banco aumentó el sendero previsto para la tasa de interés de la Fed y redujo el pronóstico de crecimiento de la demanda externa del país. La senda esperada del precio del petróleo se incrementó, debido, en especial, a mayores restricciones de oferta y a la interrupción del comercio de hidrocarburos entre la Unión Europea y Rusia. Las tensiones geopolíticas globales, el ajuste más fuerte de la política monetaria en los países avanzados, el incremento en la percepción de riesgo para los mercados emergentes y los desbalances macroeconómicos en el país explican el incremento de la senda proyectada de la prima de riesgo, de su nivel tendencial y de la tasa de interés real neutral1. La incertidumbre sobre los pronósticos externos y sobre su consecuente impacto en el escenario macroeconómico del país se mantiene elevada, dada la imprevisible evolución del conflicto entre Rusia y Ucrania, de las tensiones geopolíticas, del grado de desaceleración de la economía mundial y del efecto que la respuesta a los recientes rebrotes de la pandemia en algunos países de Asia pueda tener sobre la economía global. El contexto macroeconómico de alta inflación, de pronósticos y expectativas de inflación superiores al 3 % y de brecha del producto positiva sugiere la necesidad de una postura de la política monetaria en terreno contractivo que mitigue el riesgo de desanclaje persistente de las expectativas de inflación. Frente a los pronósticos del Informe de abril, el aumento de la tendencia de la prima de riesgo implica una tasa de interés real neutral más alta y la existencia de un estímulo monetario mayor del estimado previamente. Por su parte, la demanda interna ha sido más dinámica, con un nivel observado y proyectado del producto mayor, que supera la capacidad productiva de la economía. Las sorpresas al alza de la inflación total y básica son el reflejo de choques externos más fuertes y persistentes que, junto con la fortaleza de la demanda agregada, la indexación, las mayores expectativas de inflación y las presiones cambiarias explican el incremento en la senda de pronóstico de la inflación a niveles que superan la meta en los siguientes dos años. Esto es corroborado por las medidas de expectativas de inflación de los analistas económicos y aquellas derivadas del mercado de deuda pública, que siguieron aumentando y exceden el 3 %. Todo lo anterior aumentó el riesgo de desanclaje de las expectativas de inflación y podría generar procesos de indexación generalizados que alejen la inflación de la meta por más tiempo. Este nuevo contexto macroeconómico sugiere que el ajuste de la tasa de interés debe continuar hacia un terreno contractivo de la política monetaria. 1.2 Decisión de política monetaria La Junta Directiva del Banco de la República (JDBR) en sus reuniones de junio y julio de 2022 decidió continuar con el proceso de normalización de la política monetaria. En la reunión de junio la JDBR decidió por unanimidad incrementar la tasa de interés de política monetaria en 150 puntos básicos (pb) y, por mayoría, en 150 pb en su reunión de julio. Así la tasa se ubica en 9,0% 1 La tasa de interés real neutral se refiere al nivel de la tasa de interés real que no estimula ni desestimula la demanda agregada y, por tanto, no genera presiones que lleven al cierre de la brecha de producto. En una economía pequeña y abierta, como la colombiana, esta tasa depende de la tasa de interés real neutral externa, de los componentes de mediano plazo de la prima de riesgo-país y de la depreciación esperada.