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Unloading
Kinds of Unloading Selected AbstractsAcute Adaptation to Volume Unloading of the Functional Single Ventricle in Children Undergoing Bidirectional Glenn AnastomosisCONGENITAL HEART DISEASE, Issue 2 2009Catherine Ikemba MD ABSTRACT Objective., Volume unloading of the functional single ventricle after a bidirectional Glenn anastomosis (BDG) prior to 1 year of age leads to improved global ventricular function as measured by the myocardial performance index (MPI), a Doppler-derived measurement of combined systolic and diastolic ventricular function. Systolic function remains unchanged after BDG according to previous studies; however, acute changes in global and diastolic function have not been previously investigated in this cohort. Our objective was to assess the short-term effects of the BDG on global ventricular function in patients with a functional single ventricle. Design., Echocardiograms to obtain MPI, isovolumic contraction time, and isovolumic relaxation time were performed at four time periods: in the operating room, in the operating room prior to BDG, shortly after separation from cardiopulmonary bypass, less than 24 hours postoperatively, and either prior to hospital discharge or at the first clinic follow-up visit. Results., Twenty-six patients were enrolled. There was significant ventricular dysfunction noted shortly after separation from cardiopulmonary bypass, median MPI 0.63 (0.39,0.81), that persisted in the short term postoperatively median MPI 0.50 (0.40,0.63). Isovolumic contraction time did not change, however, isovolumic relaxation time was significantly prolonged following BDG. Conclusion., In the postoperative patient after BDG, systolic function is preserved; however, there is evidence of diastolic and global ventricular dysfunction, at least in the short term. [source] The adaptive responses in several mediators linked with hypertrophy and atrophy of skeletal muscle after lower limb unloading in humansACTA PHYSIOLOGICA, Issue 2 2009K. Sakuma Abstract Aim:, To determine the adaptive changes in several molecules regulating muscle hypertrophy and atrophy after unloading, we examined whether unilateral lower limb suspension changes the mRNA and protein levels of SRF-linked (RhoA, RhoGDI, STARS and SRF), myostatin-linked (myostatin, Smad2, Smad3 and FLRG) and Foxo-linked (P-Akt, Foxo1, Foxo3a and Atrogin-1) mediators. Methods:, A single lower limb of each of eight healthy men was suspended for 20 days. Biopsy specimens were obtained from the vastus lateralis muscle pre- and post-suspension. Results:, The volume of the vastus lateralis muscle was significantly decreased after unloading. The amount of RhoA, RhoGDI or SRF protein in the muscle was not significantly changed post-suspension. An RT-PCR semiquantitative analysis showed increased levels of myostatin mRNA but not Smad2, Smad3 or FLRG mRNA. Unloading did not elicit significant changes in the amount of p-Smad3 or myostatin protein in the muscle. The amount of p-Akt protein was markedly reduced in the unloaded muscle. Lower limb suspension did not influence the expression pattern of Foxo1, Foxo3a or Atrogin-1. Conclusion:, Unloading inducing a mild degree of muscle atrophy may decrease p-Akt and increase myostatin but not SRF-linked mediators. [source] Regional Alterations of Type I Collagen in Rat Tibia Induced by Skeletal Unloading,JOURNAL OF BONE AND MINERAL RESEARCH, Issue 9 2002Masashi Shiiba Abstract Skeletal unloading induces loss of mineral density in weight-bearing bones that leads to inferior bone mechanical strength. This appears to be caused by a failure of bone formation; however, its mechanisms still are not well understood. The objective of this study was to characterize collagen, the predominant matrix protein in bone, in various regions of tibia of rats that were subjected to skeletal unloading by 4 weeks tail suspension. Sixteen male Sprague-Dawley rats (4 months old) were divided into tail suspension and ambulatory controls (eight rats each). After the tail suspension, tibias from each animal were collected and divided into five regions and collagen was analyzed. The collagen cross-linking and the extent of lysine (Lys) hydroxylation in unloaded bones were significantly altered in proximal epiphysis, diaphysis, and, in particular, proximal metaphysis but not in distal regions. The pool of immature/nonmineralized collagen measured by its extractability with a chaotropic solvent was significantly increased in proximal metaphysis. These results suggest that skeletal unloading induced an accumulation of post-translationally altered nonmineralized collagen and that these changes are bone region specific. These alterations might be caused by impaired osteoblastic function/differentiation resulting in a mineralization defect. [source] Visualization of localized store-operated calcium entry in mouse astrocytes.THE JOURNAL OF PHYSIOLOGY, Issue 3 2005Close proximity to the endoplasmic reticulum Unloading of endoplasmic reticulum (ER) Ca2+ stores activates influx of extracellular Ca2+ through ,store-operated' Ca2+ channels (SOCs) in the plasma membrane (PM) of most cells, including astrocytes. A key unresolved issue concerning SOC function is their spatial relationship to ER Ca2+ stores. Here, using high resolution imaging with the membrane-associated Ca2+ indicator, FFP-18, it is shown that store-operated Ca2+ entry (SOCE) in primary cultured mouse cortical astrocytes occurs at plasma membrane,ER junctions. In the absence of extracellular Ca2+, depletion of ER Ca2+ stores using cyclopiazonic acid, an ER Ca2+ -ATPase inhibitor, and caffeine transiently increases the sub-plasma-membrane Ca2+ concentration ([Ca2+]SPM) within a restricted space between the plasma membrane and adjacent ER. Restoration of extracellular Ca2+ causes localized Ca2+ influx that first increases [Ca2+]SPM in the same restricted regions and then, with a delay, in ER-free regions. Antisense knockdown of the TRPC1 gene, proposed to encode endogenous SOCs, markedly reduces SOCE measured with Fura-2. High resolution immunocytochemistry with anti-TRPC1 antibody reveals that these TRPC-encoded SOCs are confined to the PM microdomains adjacent to the underlying ,junctional' ER. Thus, Ca2+ entry through TRPC-encoded SOCs is closely linked, not only functionally, but also structurally, to the ER Ca2+ stores. [source] Effects of left ventricular unloading by Impella recover LP2.5 on coronary hemodynamicsCATHETERIZATION AND CARDIOVASCULAR INTERVENTIONS, Issue 4 2007Maurice Remmelink MD Abstract Objectives: We studied the effects of LV unloading by the Impella on coronary hemodynamics by simultaneously measuring intracoronary pressure and flow and the derived parameters fractional flow reserve (FFR), coronary flow velocity reserve (CFVR), and coronary microvascular resistance (MR). Background: Patients with compromised left ventricular (LV) function undergoing high-risk percutaneous coronary intervention (PCI) may benefit from LV unloading. Limited information is available on the effects of LV unloading on coronary hemodynamics. Methods: Eleven patients (mean LV ejection fraction of 35 ± 11%) underwent PCI during LV support by the LV unloading device (Impella Recover® LP2.5). Intracoronary measurements were performed in a nonstenotic coronary artery after the PCI, before and after adenosine-induced hyperemia at four different support levels (0,2.5 L/min). Results: Aortic and coronary pressure increased with increasing support levels, whereas FFR remained unchanged. Baseline flow velocity remained unchanged, while hyperemic flow velocity and CFVR increased significantly with increasing support levels (61 ± 24 to 72 ± 27 cm/sec, P = 0.001 and 1.88 ± 0.52 to 2.34 ± 0.63, P < 0.001 respectively). The difference between baseline MR and hyperemic MR significantly increased with increasing support levels (1.28 ± 1.32 to 1.89 ± 1.43 mm Hg cm,1 sec, P = 0.005). Conclusions: Unloading of the LV by the Impella increased aortic and intracoronary pressure, hyperemic flow velocity and CFVR, and decreased MR. The Impella-induced increase in coronary flow, probably results from both an increased perfusion pressure and a decreased LV volume-related intramyocardial resistance. © 2007 Wiley-Liss, Inc. [source] Mechanisms of osteoporosis in spinal cord injuryCLINICAL ENDOCRINOLOGY, Issue 5 2006Sheng-Dan Jiang Summary Osteoporosis is a known complication of spinal cord injury (SCI), but its mechanism remains unknown. The pathogenesis of osteoporosis after SCI is generally considered disuse. However, although unloading is an important factor in the pathogenesis of osteoporosis after SCI, neural lesion and hormonal changes also seem to be involved in this process. Innervation and neuropeptides play an important role in normal bone remodelling. SCI results in denervation of the sublesional bones and the neural lesion itself may play a pivotal role in the development of osteoporosis after SCI. Although upper limbs are normally loaded and innervated, bone loss also occurs in the upper extremities in patients with paraplegia, indicating that hormonal changes may be associated with osteoporosis after SCI. SCI-mediated hormonal changes may contribute to osteoporosis after SCI by different mechanisms: (1) increased renal elimination and reduced intestinal absorption of calcium leading to a negative calcium balance; (2) vitamin D deficiency plays a role in the pathogenesis of SCI-induced osteoporosis; (3) SCI antagonizes gonadal function and inhibits the osteoanabolic action of sex steroids; (4) hyperleptinaemia after SCI may contribute to the development of osteoporosis; (5) pituitary suppression of TSH may be another contributory factor to bone loss after SCI; and (6) bone loss after SCI may be caused directly, at least in part, by insulin resistance and IGFs. Thus, oversupply of osteoclasts relative to the requirement for bone resorption and/or undersupply of osteoblasts relative to the requirement for cavity repair results in bone loss after SCI. Mechanisms for the osteoporosis following SCI include a range of systems, and osteoporosis after SCI should not be simply considered as disuse osteoporosis. Unloading, neural lesion and hormonal changes after SCI result in severe bone loss. The aim of this review is to improve understanding with regard to the mechanisms of osteoporosis after SCI. The understanding of the pathogenesis of osteoporosis after SCI can help in the consideration of new treatment strategies. Because bone resorption after SCI is very high, intravenous bisphosphonates and denosumab should be considered for the treatment of osteoporosis after SCI. [source] Acute Adaptation to Volume Unloading of the Functional Single Ventricle in Children Undergoing Bidirectional Glenn AnastomosisCONGENITAL HEART DISEASE, Issue 2 2009Catherine Ikemba MD ABSTRACT Objective., Volume unloading of the functional single ventricle after a bidirectional Glenn anastomosis (BDG) prior to 1 year of age leads to improved global ventricular function as measured by the myocardial performance index (MPI), a Doppler-derived measurement of combined systolic and diastolic ventricular function. Systolic function remains unchanged after BDG according to previous studies; however, acute changes in global and diastolic function have not been previously investigated in this cohort. Our objective was to assess the short-term effects of the BDG on global ventricular function in patients with a functional single ventricle. Design., Echocardiograms to obtain MPI, isovolumic contraction time, and isovolumic relaxation time were performed at four time periods: in the operating room, in the operating room prior to BDG, shortly after separation from cardiopulmonary bypass, less than 24 hours postoperatively, and either prior to hospital discharge or at the first clinic follow-up visit. Results., Twenty-six patients were enrolled. There was significant ventricular dysfunction noted shortly after separation from cardiopulmonary bypass, median MPI 0.63 (0.39,0.81), that persisted in the short term postoperatively median MPI 0.50 (0.40,0.63). Isovolumic contraction time did not change, however, isovolumic relaxation time was significantly prolonged following BDG. Conclusion., In the postoperative patient after BDG, systolic function is preserved; however, there is evidence of diastolic and global ventricular dysfunction, at least in the short term. [source] Myocardial Perfusion As Assessed by Positron Emission Tomography During Long-Term Mechanical Circulatory SupportCONGESTIVE HEART FAILURE, Issue 2 2006George V. Letsou MD Although mechanical circulatory support (MCS) can improve myocardial function in patients with advanced heart failure, its effects on relative myocardial perfusion are unclear. Using positron emission tomographic imaging techniques, the authors assessed relative myocardial perfusion in patients with ischemic or idiopathic cardiomyopathy who were receiving chronic MCS with a left ventricular assist device (pulsatile HeartMate [n=2] [Thoratec Corporation, Pleasanton, CA] or nonpulsatile Jarvik 2000 [n=4] [Jarvik Heart, Inc., New York, NY]). Relative myocardial perfusion was compared at lower and higher levels of MCS (50 vs. 100,110 ejections/min for the HeartMate and 8000 vs. 12,000 rpm for the Jarvik 2000). The size and severity of perfusion defects at rest and after dipyridamole stress were measured objectively and subjectively by computer algorithms and visual inspection, respectively. Relative myocardial perfusion increased >5% from baseline in only one of six patients when MCS was increased. No change in relative myocardial perfusion of >5% was seen in any of the other five patients, even after subsequent dipyridamole stress positron emission tomographic imaging. These pilot study findings suggest that the decreased metabolic requirements induced by ventricular unloading correspondingly decreased blood flow requirements to physiologically inactive myocardium. [source] Protective effects of exercise preconditioning on hindlimb unloading-induced atrophy of rat soleus muscleACTA PHYSIOLOGICA, Issue 1 2009H. Fujino Abstract Aim:, A chronic decrease in the activation and loading levels of skeletal muscles as occurs with hindlimb unloading (HU) results in a number of detrimental changes. Several proteolytic pathways are involved with an increase in myofibrillar protein degradation associated with HU. Exercise can be used to counter this increase in proteolytic activity and, thus, may be able to protect against some of the detrimental changes associated with chronic decreased use. The purpose of the present study was to determine the potential of a single bout of preconditioning endurance exercise in attenuating the effects of 2 weeks of HU on the mass, phenotype and force-related properties of the soleus muscle in adult rats. Methods:, Male Wistar rats were subjected to HU for 2 weeks. One half of the rats performed a single bout of treadmill exercise for 25 min immediately prior to the 2 weeks of HU. Results:, Soleus mass, maximum tetanic tension, myofibrillar protein content, fatigue resistance and percentage of type I (slow) myosin heavy chain were decreased in HU rats. In addition, markers for the cathepsin, calpain, caspase and ATP-ubiquitin-proteasome proteolytic pathways were increased. The preconditioning endurance exercise bout attenuated all of the detrimental changes associated with HU, and increased HSP72 mRNA expression and protein levels. Conclusion:, These findings indicate that exercise preconditioning may be an effective countermeasure to the detrimental effects of chronic decreases in activation and loading levels on skeletal muscles and that an elevation in HSP72 may be one of the mechanisms associated with these responses. [source] Gravitational unloading inhibits the regenerative potential of atrophied soleus muscle in miceACTA PHYSIOLOGICA, Issue 3 2009Y. Matsuba Abstract Aim:, The present study was performed to investigate the influence of unloading on the regeneration of atrophied and injured skeletal muscle. Methods:, Male mice (C57BL/6J), aged 8 weeks, were used. Cardiotoxin (CTX) was injected into soleus muscles bilaterally. Gravitational unloading on soleus muscle was performed by hind limb suspension for 2 weeks before and additionally 6 weeks after CTX injection in one group. Soleus muscles in the remaining groups were loaded keeping the mice in the cages and were dissected 14, 28 and 42 days after the injection. Results:, Recovery of the wet weight and protein content of soleus in the CTX-injected group was inhibited by unloading. Increase in satellite cell number, induced by CTX injection and loading, was also inhibited by unloading. Disappearance of infiltration of mononucleated cells into the necrotic area was also delayed. This phenomenon suggests that regeneration, which is indicated by the appearance of fibres with central nuclei, was inhibited by unloading. Conclusion:, Results suggested that loading plays an important role in the activation of the regenerating potential of injured skeletal muscle. [source] Soleus T reflex modulation in response to spinal and tendinous adaptations to unilateral lower limb suspension in humansACTA PHYSIOLOGICA, Issue 3 2008O. R. Seynnes Abstract Aim:, To investigate the influence of tendinous and synaptic changes induced by unilateral lower limb suspension (ULLS) on the tendon tap reflex. Methods:, Eight young men underwent a 23-day period of ULLS. Muscle cross-sectional area (CSA), torque and electromyographic (EMG) activity of the plantar flexor muscles (normalized to the M wave), Achilles tendon,aponeurosis mechanical properties, soleus (SOL) H and T reflexes and associated peak twitch torques were measured at baseline, after 14 and 23 days of ULLS, and 1 week after resuming ambulatory activity. Results:, Significant decreases in muscle CSA (,9%), in maximal voluntary torque (,10%) and in the associated SOL EMG activity (,16%) were found after ULLS (P < 0.05). In addition to a 36% (P < 0.01) decrease in tendon,aponeurosis stiffness, normalized H reflex increased by 35% (P < 0.05). An increase in the slope (28%, P < 0.05) and intercept (85%, P < 0.05) of the T reflex recruitment curve pointed to an increase in the gain and to a decrease in the sensitivity of this reflex, possibly resulting from the decrease in the tendon,aponeurosis stiffness at low forces. Following ULLS, changes in tendinous stiffness correlated with changes in neuromuscular efficiency (peak twitch torque to reflex ratio) at higher tendon tap forces. Conclusion:, These findings point out the dual and antagonistic influences of spinal and tendinous adaptations upon the tendon tap reflex in humans under conditions of chronic unloading. These observations have potential implications for the sensitivity of the short-latency Ia stretch response involved in rapid compensatory contractions to unexpected postural perturbations. [source] Skeletal muscle HSP72 response to mechanical unloading: influence of endurance trainingACTA PHYSIOLOGICA, Issue 4 2004D. Desplanches Abstract Aims:, It has been shown that increased contractile activity results in heat shock protein 72 (HSP72) accumulation in various skeletal muscles. By contrast, there is no consensus for muscle HSP72 response to muscle disuse for short duration (5,8 days). On the basis of a greater constitutive HSP72 expression in slow-twitch muscles we tested the hypothesis that mechanical unloading for a longer period (2 weeks) would affect this phenotype to a greater extent. Secondly, we evaluated the effects of a physiological muscle heat shock protein (HSP) enhancer (endurance training) on HSP response to unloading and muscle remodelling. Methods:, Adult male Wistar rats were assigned randomly to four groups: (1) sedentary weight-bearing; (2) hindlimb-unloaded (HU) via tail suspension for 2 week; (3) trained on a treadmill (6 week) and (4) trained 6 week and then HU for 2 week. Results:, Unloading resulted in a preferential atrophy of slow muscles [soleus (SOL), adductor longus (AL)] and a slow-to-fast fibre transition with no change in HSP72 level. HSP72 levels were significantly lower in fast muscles [extensor digitorum longus (EDL) and plantaris (PLA)], and did not change with mechanical unloading. Endurance training was accompanied by a small (SOL) or a large (EDL, PLA) increase in HSP72 level with no change in AL. Training-induced accumulation of HSP72 disappeared with subsequent unloading in the SOL and PLA whereas HSP72 content remained elevated in EDL. Conclusion:, The results of this study indicate that (1) after 2 weeks of unloading no change occurred in HSP72 protein levels of slow-twitch muscles despite a slow-to-fast fibre transition; and (2) the training-induced increase of HSP72 content in skeletal muscles did not attenuate fibre transition. [source] An efficient three-dimensional solid finite element dynamic analysis of reinforced concrete structuresEARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICS, Issue 2 2006K. V. Spiliopoulos Abstract Most of the finite element analyses of reinforced concrete structures are restricted to two-dimensional elements. Three-dimensional solid elements have rarely been used although nearly all reinforced concrete structures are under a triaxial stress state. In this work, a three-dimensional solid element based on a smeared fixed crack model that has been used in the past mainly for monotonic static loading analysis is extended to cater for dynamic analysis. The only material parameter that needs to be input for this model is the uniaxial compressive strength of concrete. Steel bars are modelled as uniaxial elements and an embedded formulation allows them to have any orientation inside the concrete elements. The proposed strategy for loading or unloading renders a numerical procedure which is stable and efficient. The whole process is applied to two RC frames and compared against existing experiments in the literature. Results show that the proposed approach may adequately be used to predict the dynamic response of a structure. Copyright © 2005 John Wiley & Sons, Ltd. [source] Dynamic compressive properties of porcine temporomandibular joint discEUROPEAN JOURNAL OF ORAL SCIENCES, Issue 5 2003Eiji Tanaka This study aimed to evaluate the effect of the strain frequency and amplitude on the compressive properties of the porcine temporomandibular joint disc and to determine the time-dependent changes associated with energy dissipation. Seven discs were used for compressive cycle tests, including various frequencies and magnitudes of compressive strain. Each experiment consisted of 25 cycles of loading and unloading. Hysteresis and the instantaneous and steady moduli were calculated. All specimens showed a clear hysteresis and repeatable stress,strain relationships within 19 cycles. The hysteresis at the initial cycle ranged between 35% and 62%, and gradually decreased in subsequent cycles. The instantaneous modulus became larger when the strain frequency and the strain amplitude increased. The steady modulus was approximately one-third of the instantaneous one. It was concluded that the disc has an energy-dissipating function during dynamic compression. [source] Brain superoxide as a key regulator of the cardiovascular response to emotional stress in rabbitsEXPERIMENTAL PHYSIOLOGY, Issue 3 2007Dmitry N. Mayorov Cardiovascular reactivity, an abrupt increase in blood pressure and heart rate in response to emotional stress, is a risk factor for hypertension and heart disease. Brain angiotensin II (Ang II) type 1 (AT1) receptor is increasingly recognized as an important regulator of cardiovascular reactivity. Given that a wide variety of AT1 receptor signalling pathways exists in neurones, the precise molecular mechanisms that underlie central cardiovascular actions of Ang II during emotional stress are yet to be determined. Growing evidence, however, indicates that reactive oxygen species, and in particular superoxide (·O2,), are important intracellular messengers of many actions of brain Ang II. In particular, studies employing microinjection of ·O2, scavengers directly into the rostral ventrolateral medulla (RVLM) and dorsomedial hypothalamus of rabbits have shown that the activation of AT1 receptor,·O2, signalling is required for full manifestation of the cardiovascular response to emotional stress. This role of ·O2, appears to be highly specific, because ·O2, scavengers in the RVLM do not alter the sympathoexcitatory response to baroreceptor unloading or sciatic nerve stimulation. The subcellular mechanisms for the stress-induced ·O2, production are likely to include the activation of NADPH oxidase and are essentially independent of nitric oxide. This review summarizes current knowledge of redox-sensitive signalling mechanisms in the brain that regulate cardiovascular effects of stress. Additionally, it presents initial evidence that ·O2, may be less important in the activation of central pressor pathways mediating cardiovascular arousal associated with appetitive events, such as food anticipation and feeding. [source] An algorithm for evaluating crack closure from local compliance measurementsFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 3 2002M. SKORUPA ABSTRACT A numerical procedure for evaluating fatigue crack closure from load-local deformation data is presented. The local compliance is assumed to vary during the entire loading cycle, including the portion when the crack is fully open. The closure mechanism-related characteristic load levels are identified based on comparisons between the open crack compliance variations upon loading and unloading. This type of compliance analysis is conditioned by an appropriate smoothness of the measured data. Hence, the choice and optimization of the procedure for smoothing of the measured data. Hence, the choice and optimization of the procedure for smoothing the recorded signals is thoroughly addressed in this paper. The performance of the algorithm is evaluated from comparisons between the computed closure parameters and their reference values which have been previously shown to correlate the observed crack growth rate behaviour. [source] The molecular chaperone HSP47 rapidly senses gravitational changes in myoblastsGENES TO CELLS, Issue 11 2006Asami Oguro Skeletal muscle unloading induced by spaceflight or bed rest leads to muscle atrophy. It is unclear how muscle atrophy is caused and how muscles respond to microgravity. We addressed the response of collagen and its chaperone system to gravitational forces. We show here that expression of HSP47, a collagen-specific molecular chaperone, responds to gravitational changes, including microgravity and hypergravity in vitro and in vivo. By using the method hindlimb suspension of rats, which mimics microgravity conditions, we demonstrated that the expression of Hsp47 mRNA decreased within 1 day and the mRNA levels of collagen types I and IV were subsequently reduced. In contrast, hypergravity stimulated HSP47 expression. HSP47 and collagen types I and IV were localized intracellularly in the endoplasmic reticulum and/or Golgi apparatus of myoblasts, as expected. Intriguingly, Hsp47 mRNA levels in cultured myoblasts increased significantly with hypergravity treatment at 40G for 2 h, and decreased with microgravity treatment at almost 0G for 1,2 h. Collagen mRNA levels were also altered, although changes were slower and less pronounced compared with those for HSP47. The gravity-regulated HSP47 may play a role in the maintenance of the extracellular matrix by modulating collagen production at the primary stage of adaptation. [source] DIFFERENTIAL ROCK WEATHERING IN THE ,VALLEY OF THE BOULDERS', KÄRKEVAGGE, SWEDISH LAPLANDGEOGRAFISKA ANNALER SERIES A: PHYSICAL GEOGRAPHY, Issue 3 2008ROBERT G. DARMODY ABSTRACT. Kärkevagge is an alpine valley in the low arctic of Swedish Lapland. It is named after, and famous for, its large deposit of immense (c. 10,15 m) boulders that almost fill the lower valley. Above the boulder deposit, on the flanks of the valley, are more recent and generally much smaller (c. 1,3 m) individual boulders that have fallen from the valley-wall cliff face, presumably from post-glacial valley-side unloading. Some of these smaller boulders are seemingly fresh and unweathered while others have been reduced to no more than mounds in the tundra. These boulders must be younger than the larger, lower giant boulder deposit, but are not particularly recent rockfalls as they are partially buried in colluvium. Comparisons of mineralogy and chemistry indicate that the possibility exists that the incompetent, ,rotten' rocks, if not considerably older than their competent neighbors, are inherently self-destructive. They have evidence of increased sulfur content, which is a proxy for pyrite, a known weathering accelerant in Kärkevagge. [source] Sediment Distribution Around Glacially Abraded Bedrock Landforms (Whalebacks) at Lago Tranquilo, ChileGEOGRAFISKA ANNALER SERIES A: PHYSICAL GEOGRAPHY, Issue 3 2005Neil F. Glasser Whalebacks are convex landforms created by the smoothing of bedrock by glacial processes. Their formation is attributed to glacial abrasion either by bodies of subglacial sediment sliding over bedrock or by individual clasts contained within ice. This paper reports field measurements of sediment depth around two whaleback landforms in order to investigate the relationship between glacigenic deposits and whaleback formation. The study site, at Lago Tranquilo in Chilean Patagonia, is situated within the Last Glacial Maximum (LGM) ice limits. The two whalebacks are separated by intervening depressions in which sediment depths are generally 0.2 to 0.3 m. Two facies occur on and around the whalebacks. These facies are: (1) angular gravel found only on the surface of the whalebacks, interpreted as bedrock fracturing in response to unloading of the rock following pressure release after ice recession, and (2) sandy boulder-gravel in the sediment-filled depressions between the two whalebacks, interpreted as an ice-marginal deposit, with a mixture of sediment types including basal glacial and glaciofluvial sediment. Since the whalebacks have heavily abraded and striated surfaces but are surrounded by only a patchy and discontinuous layer of sediment, the implication is that surface abrasion of the whalebacks was achieved primarily by clasts entrained in basal ice, not by subglacial till sliding. [source] Regional GPS data confirm high strain accumulation prior to the 2000 June 4 Mw= 7.8 earthquake at southeast SumatraGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2001G. W. Michel Summary Site velocities derived from repeated measurements in a regional GPS network in Southeast Asia help to constrain the motion of tectonic blocks as well as slip rates along major faults in the area. Using 3-D forward dislocation modelling, the influence of seismic elastic loading and unloading on the measured site motions are approximated. Results suggest that the northwestern Sunda arc is fully coupled seismogenically, whereas its eastern part along Java shows localized deformation. Higher horizontal velocity gradients than expected from the modelling of a fully coupled plate interface west of Manila in the Philippines suggest that deformation may be localized there. Assuming that geodetically derived convergence represents long-term rates, accumulated geodetic moments are compared to those derived using seismic data from 1977 to 2000 (Harvard CMT catalogue). If areas displaying localized deformation are dominated by creep processes, the largest difference between accumulated and seismically released deformation is located where the 2000 June 4 Mw = 7.8 Sumatra earthquake occurred. [source] The stress dependent elastic properties of thermally induced microfractures in aeolian Rotliegend sandstoneGEOPHYSICAL PROSPECTING, Issue 3 2007Colin MacBeth ABSTRACT The impact of thermally induced microfractures on the stress-sensitive elastic wave properties of aeolian Rotliegend sandstone samples is analysed. It is found that to identify the effects of the microfracture contribution accurately, a correction must first be made to account for water loss (representing a mass loss of 4,6%) from the pore throats and clays due to the heating process, despite care being taken to ensure that the thermally fractured samples re-adsorb room moisture. Both the original and thermally fractured rocks are stress-sensitive at the ultrasonic wave frequencies of the laboratory. However, a distinct shift in the estimated distribution of internal rock compliance indicates that the population of thermal microfractures differs in nature from that caused solely by core-plug extraction damage. In particular, the ratio of normal to tangential compliance is observed to be higher for the thermally generated microfractures than for the broken grain-grain contacts created by extraction unloading. This can be explained by the intragranular thermal-fracture surfaces being smoother when compared to the intergranular boundaries. Mechanical hysteresis is observed between the up- and downgoing test cycles for both the original and, to a greater extent, the thermally fractured rock. This indicates that there is compaction-induced movement of the fractures in the samples during application of stress in the laboratory. [source] The cycle of instability: stress release and fissure flow as controls on gully head retreatHYDROLOGICAL PROCESSES, Issue 1 2001A. J. C. Collison Abstract Gully head and wall retreat has commonly been attributed to fluvial scour and head collapse as a result of soil saturation, sapping or piping. The empirical evidence to substantiate these conceptual models is sparse, however, and often contradictory. This paper explores the hydrological and mechanical controls on gully head and wall stability by modelling the hydrology, stability and elastic deformation of a marl gully complex in Granada Province, south-east Spain. The hydrological and slope-stability simulations show that saturated conditions can be reached only where preferential fissure flow channels water from tension cracks into the base of the gully head, and that vertical or subvertical heads will be stable unless saturation is achieved. Owing to the high unsaturated strengths of marl measured in this research, failure in unsaturated conditions is possible only where the gully head wall is significantly undercut. Head retreat thus requires the formation of either a tension crack or an undercut hollow. Finite-element stress analysis of eroding slopes reveals a build up of shear stress at the gully head base, and a second stress anomaly just upslope of the head wall. Although tension cracks on gully heads have often been attributed to slope unloading, this research provides strong evidence that the so called ,sapping hollow' commonly found in the gully headwall base is also a function of stress release. Although further research is needed, it seems possible that ,pop out' failures in river channels may be caused by the same process. The hydrological analysis shows that, once a tension crack has developed, throughflow velocity in the gully headwall will increase by an order of magnitude, promoting piping and enlargement of this weakened area. It is, therefore, possible to envisage a cycle of gully expansion in which erosion, channel incision or human action unloads the slope below a gully head, leading to stress patterns that account for the tension crack and a stress-release hollow. The tension crack promotes faster throughflow, encouraging hollow enlargement and piping, which undercut the gully head. The tension crack permits the development of positive pore-water pressures behind the gully head, leading either to failure or contributing to toppling. Finally the debris may be eroded by fluvial action, unloading a new section of slope and completing the cycle of gully head retreat. Copyright © 2001 John Wiley & Sons, Ltd. [source] A rate-dependent cohesive crack model based on anisotropic damage coupled to plasticityINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 9 2006Per-Ola Svahn Abstract In quasi-brittle material the complex process of decohesion between particles in microcracks and localization of the displacement field into macrocracks is limited to a narrow fracture zone, and it is often modelled with cohesive crack models. Since the anisotropic nature of the decohesion process in separation and sliding is essential, it is particularly focused in this paper. Moreover, for cyclic and dynamic loading the unloading, load reversal (including crack closure) and rate dependency are essential features that are included in a new model. The modelling of degradation is based on a ,localized' version of anisotropic continuum damage coupled to inelasticity. The concept of strain energy equivalence between the states in the effective and nominal settings is adopted in order to define the free energy of the interface. The proposed fracture criterion is of the Mohr type, with a smooth transition of the failure and kinematics (slip and dilatation) characteristics between tension and shear. The chosen potential, of the Lemaitre-type, for evolution of the dissipative processes is additively decomposed into plastic and damage parts, and non-associative constitutive equations are obtained. The constitutive equations are integrated by applying the backward Euler rule and by using Newton iteration. The proposed model is assessed analytically and numerically and a typical calibration procedure for concrete is proposed. Copyright © 2006 John Wiley & Sons, Ltd. [source] Fabric evolution of granular assembly under K0 loading/unloadingINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 13 2003Jyh-Chau Liou Abstract This study attempted to investigate the fabric evolution in K0 loading/unloading. The work made use of a field simulator to control K0 loading/unloading in large specimens prepared by air-pluviation. In each loading stage, wave velocities along various propagation directions were measured. On the basis of the theories of micro-mechanics and wave propagation, the microscopic parameters of the granular assembly were back calculated to investigate the fabric evolution of granular soil during K0 loading/unloading. In this study, the Geometric fabric was modelled by fabric tensors of ranks 2 and 4. The comparison of calibrated results using ranks 2 and 4 revealed the advantage of the usage of rank-4 fabric tensor in modelling fabric evolution in spite of its complexity. By comparing relative magnitudes of vertical and horizontal components of geometric fabric, it was demonstrated that relative constraint in lateral directions increased during K0 -unloading in order to maintain a K0 condition. It revealed that fabric evolution was responsible for a higher K0 in unloading than K0 in loading. Copyright © 2003 John Wiley & Sons, Ltd. [source] Modelling of anisotropic damage in brittle rocks under compression dominated stressesINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 10 2002Y.F. Lu Abstract A new model for describing induced anisotropic damage in brittle rocks is proposed. Although phenomenological, the model is based on physical grounds of micromechanical analysis. Induced damage is represented by a second rank tensor, which is related to the density and orientation of microcracks. Damage evolution is related to the propagation condition of microcracks. The onset of microcrack coalescence leading to softening behaviour is also considered. The effective elastic compliance of the damaged material is obtained from a specific form of Gibbs potential. Irreversible damage-related strains due to residual opening of microcracks after unloading are also captured. All the model's parameters could be determined from conventional triaxial compression tests. The proposed model is applied to a typical brittle rock. Comparison between test data and numerical simulations shows an overall good agreement. The proposed model is able to describe the main features related to induced microcracks in brittle geomaterials. Copyright © 2002 John Wiley & Sons, Ltd. [source] A new path-following constraint for strain-softening finite element simulationsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 2 2004E. Lorentz Abstract The application of strain-softening constitutive relations to model the failure modes of real-life structures is faced to numerical difficulties related to instabilities that appear as sharp snap-backs of the structural response. A path-following method has to complement the solution algorithm to achieve convergence despite these critical points. Because of the sharpness of the snap-backs, it is believed essential that the path-following constraint distinguish between a purely elastic unloading and a dissipative path. For that purpose, a new constraint based on the maximal value of the elastic predictor for the yield function is proposed. As it is highly non linear, a specific solution algorithm is required. The robustness of this constraint is illustrated by three applications: the study of crack propagations by means of a cohesive zone model, the failure of a structure submitted to nonlocal damage and the simulation of a nonlocal strain-softening plastic specimen. Copyright © 2004 John Wiley & Sons, Ltd. [source] Paradoxical Vasodilation During Lower Body Negative Pressure in Patients with Vasodepressor Carotid Sinus SyndromeJOURNAL OF AMERICAN GERIATRICS SOCIETY, Issue 6 2003Arduino A. Mangoni MD OBJECTIVES: To elucidate the pathophysiological mechanism of the vasodepressor form (VD) of carotid sinus syndrome (CSS) by maneuvers designed to induce generalized sympathetic activation after baroreceptor unloading (lower body negative pressure, LBNP) or direct peripheral adrenoreceptor stimulation via local administration of norepinephrine (NA). DESIGN: Subjects were identified with VD of CSS through diagnostic testing. SETTING: Research laboratory. PARTICIPANTS: Eleven young controls (YC) (mean age ± standard error of mean = 22.8 ± 0.7), eight elderly controls (EC) (72.6 ± 0.6), and eight elderly patients with VD (78.7 ± 1.7). MEASUREMENTS: Forearm arterial blood flow (FABF) was measured in the left and right arms by venous occlusion plethysmography. Measurements were performed during baseline conditions, LBNP (,20 mmHg), and intra-arterial NA infusion in the left brachial artery at three progressively increasing rates (60, 120, and 240 pmol/min). RESULTS: During LBNP, FABF significantly decreased in YC (baseline 3.61 ± 0.30 vs ,20 mmHg 2.96 ± 0.24 mL/100 g/min, P = .030) and EC (4.05 ± 0.74 vs 3.69 ± 0.65 mL/100 g/min, P = .033) but increased in elderly patients with VD (3.65 ± 0.60 vs 4.54 ± 0.80 mL/100 g/min, P = .020). During NA infusion, a significant forearm vasoconstriction occurred in YC (FABF left:right ratio 1.00 ± 0.05 at baseline; 0.81 ± 0.08 at 60 pmol/min, P = .034; 0.81 ± 0.05 at 120 pmol/min, P < .001; 0.72 ± 0.04 at 240 pmol/min, P < .001), whereas no significant FABF changes were observed in EC (1.04 ± 0.06; 0.96 ± 0.07, P = .655; 0.89 ± 0.10, P = .401; 0.94 ± 0.10, P = .590) or elderly patients with VD (1.04 ± 0.06; 1.16 ±0 .10, P = .117; 1.04 ± 0.08, P = .602; 1.11 ± 0.10, P = .305). CONCLUSION: VD of CSS is associated with a paradoxical vasodilatation during LBNP and an impairment of peripheral ,-adrenergic responsiveness, which may be age-related. [source] Some aspects of the mechanical response of BMI 5250-4 neat resin at 191°C: Experiment and modeling,JOURNAL OF APPLIED POLYMER SCIENCE, Issue 3 2008M. B. Ruggles-Wrenn Abstract The inelastic deformation behavior of BMI-5250-4 neat resin, a high-temperature polymer, was investigated at 191°C. The effects of loading rate on monotonic stress,strain behavior as well as the effect of prior stress rate on creep behavior were explored. Positive nonlinear rate sensitivity was observed in monotonic loading. Creep response was found to be significantly influenced by prior stress rate. Effect of loading history on creep was studied in stepwise creep tests, where specimens were subjected to a constant stress rate loading followed by unloading to zero stress with intermittent creep periods during both loading and unloading. The strain-time behavior was strongly influenced by prior deformation history. Negative creep was observed on the unloading path. In addition, the behavior of the material was characterized in terms of a nonlinear viscoelastic model by means of creep and recovery tests at 191°C. The model was employed to predict the response of the material under monotonic loading/unloading and multi-step load histories. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008 [source] Impaired Energetic Metabolism After Central Leptin Signaling Leads to Massive Appendicular Bone Loss in Hindlimb-Suspended Rats,JOURNAL OF BONE AND MINERAL RESEARCH, Issue 12 2008Aline Martin Abstract We previously showed in rats that the leptin effects on bone were dose dependent. Positive effects were observed when serum leptin concentration was in a physiological range. In contrast, important increases in serum leptin levels led to negative effects on bone formation similar to those reported after intracerebroventricular leptin administration in mice. To clarify whether leptin effects on bone depend on administration route and/or animal model, female rats were hindlimb unloaded or not and treated either with intracerebroventricular infusion of leptin or vehicle for 14 days. By increasing cerebrospinal fluid (CSF) leptin concentration, intracerebroventricular infusion of leptin significantly reduced food intake and consequently body weight, abdominal fat, and lean mass of the animals. Leptin infusion inhibited bone elongation over the 14 days and blunted cortical bone thickening at the femoral diaphysis site. Interestingly, leptin effects were site dependent in the cancellous bone envelopes, because tibia metaphysis BMD was lower and lumbar spine BMD was higher under intracerebroventricular leptin. Treated groups showed reduced bone remodeling independently of hindlimb unloading. Multiple downstream pathways were implicated in the mediation of these negative leptin effects on bone including not only stimulation of the sympathetic nervous system but also a decrease in somatotropic axis activity. Therefore, the intracerebroventricular leptin-induced bone loss could be largely related to the concurrent alteration of energetic and metabolic status. In summary, our study supports the hypothesis of a concentration-dependent balance between peripheral and central control of leptin on bone. [source] Capsaicin-Sensitive Sensory Neurons Contribute to the Maintenance of Trabecular Bone Integrity,JOURNAL OF BONE AND MINERAL RESEARCH, Issue 2 2005Sarah C Offley Abstract This investigation used capsaicin to selectively lesion unmyelinated sensory neurons in rats. Neuronal lesioning induced a loss of trabecular integrity, reduced bone mass and strength, and depleted neuropeptides in nerve and bone. These data suggest that capsaicin-sensitive sensory nerves contribute to trabecular bone integrity. Introduction: Familial dysautomia is an autosomal recessive disease in which patients suffer from unmyelinated sensory neuron loss, reduced BMD, and frequent fractures. It has been proposed that the loss of neurotransmitters synthesized by unmyelinated neurons adversely affects bone integrity in this hereditary syndrome. The purpose of this study was to determine whether small sensory neurons are required for the maintenance of bone integrity in rats. Materials and Methods: Ten-month-old male Sprague-Dawley rats were treated with either capsaicin or vehicle. In vivo DXA scanning and ,CT scanning, and histomorphometry were used to evaluate BMD, structure, and cellular activity. Bone strength was measured in distal femoral sections. Body weight and gastrocnemius/soleus weights were measured and spontaneous locomotor activity was monitored. Peroneal nerve morphometry was evaluated using light and electron microscopy. Substance P and calcitonin gene-related peptide (CGRP) content in the sciatic nerve and proximal tibia were determined by enzyme immunoassay (EIA). Substance P signaling was measured using a sciatic nerve stimulation extravasation assay. Results: Four weeks after capsaicin treatment, there was a loss of BMD in the metaphyses of the tibia and femur. In the proximal tibia, the osteoclast number and surface increased, osteoblast activity and bone formation were impaired, and trabecular bone volume and connectivity were diminished. There was also a loss of bone strength in the distal femur. No changes occurred in body weight, 24-h grid-crossing activity, weight bearing, or muscle mass after capsaicin treatment, indicating that skeletal unloading did not contribute to the loss of bone integrity. Capsaicin treatment destroyed 57% of the unmyelinated sensory axons, reduced the substance P and CGRP content in the sciatic nerve and proximal tibia, and inhibited neurogenic extravasation. Conclusion: These results support the hypothesis that capsaicin-sensitive sensory neurons contribute to the maintenance of trabecular bone integrity. Capsaicin-sensitive neurons have efferent functions in the tissues they innervate, effects mediated by transmitters released from the peripheral nerve terminals. We postulate that the deleterious effects of capsaicin treatment on trabecular bone are mediated by reductions in local neurotransmitter content and release. [source] | |||||||||||||||||||||||||