A frontal plane model of the lumbar spine subjected to a follower load: implications for the role of muscles

Compression on the lumbar spine is 1000 N for standing and walking and is higher during lifting. Ex vivo experiments show it buckles under a vertical load of 80-100 N. Conversely, the whole lumbar spine can support physiologic compressive loads without large displacements when the load is applied along a follower path that approximates the tangent to the curve of the lumbar spine. This study utilized a two-dimensional beam-column model of the lumbar spine in the frontal plane under gravitational and active muscle loads to address the following question: Can trunk muscle activation cause the path of the internal force resultant to approximate the tangent to the spinal curve and allow the lumbar spine to support compressive loads of physiologic magnitudes? The study identified muscle activation patterns that maintained the lumbar spine model under compressive follower load, resulting in the minimization of internal shear forces and bending moments simultaneously at all lumbar levels. The internal force resultant was compressive, and the lumbar spine model, loaded in compression along the follower load path, supported compressive loads of physiologic magnitudes with minimal change in curvature in the frontal plane. Trunk muscles may coactivate to generate a follower load path and allow the ligamentous lumbar spine to support physiologic compressive loads.     

Psychophysiological indices in six-to eight-year-old children under information load depending on anxiety as a stable individual characteristic

The study of psychophysiological indices in children aged six to eight years under information loads of various complexity showed that anxious subjects were characterized by a high level of nonspecific activation at rest and a shift of the autonomic balance towards a relative domination of the tone of the sympathetic division of the autonomic nervous system (ANS). The information load in the “auto-rate” mode caused in children aged six to eight years an increase in the level of nonspecific activation and the activity of sympathetic regulation and an inhibition of parasympathetic regulation. An information load in the “maximum rate of work” mode caused a decrease in the quantitative and qualitative indices of mental activity in comparison with that under comfortable conditions and a subsequent increase in autonomic shifts and the level of situational anxiety. The decrease in the efficiency of intellectual work performed at a maximum rate against the background of a high level of nonspecific activation and an increase in situational anxiety in both groups apparently reflected an increase in the activity of the modulating cerebral system due to the domination of the nonproductive activation system related to defensive behavior. At the same time, in children with a high personal anxiety, autonomic manifestations of activation and situational anxiety in both modes of work were more distinct and the efficiency of work lower than in subjects with a low anxiety. This indicates that, in anxious children, due to the excess activation of the sympathetic division of the ANS, the information load has a higher physiological cost. Thus, children with a high level of personal anxiety under intense information loads are characterized by a larger increase in the activity of the sympathetic division of the ANS and the attenuation of the effect of the parasympathetic division; a considerable increase in situational anxiety; low efficiency of activity; and, hence, its high physiological cost.     

The effects of load and training pattern on acute neuromuscular responses in the upper body

The purpose of this study was to examine the effects of repetition maximum (RM) loads and training patterns on acute neuromuscular responses in the upper body. Markers of fatigue were monitored under a descending pattern (DP), in which repetitions decreased in subsequent sets, and an ascending pattern (AP), in which repetitions increased in subsequent sets. Both training patterns were performed using 5- and 10-RM loads. Fatigue was assessed by monitoring changes in force output, motor unit activation and muscle twitch characteristics (peak twitch [PT], time to PT [TPT], and ? relaxation time [RT]). All 4 protocols (5-RM DP, 5-RM AP, 10-RM DP, and 10-RM AP) produced significant decreases pre to postprotocol in force output, TPT, and ?RT. With the exception of 5-RM DP, all protocols produced significant decreases in motor unit activation. Pre to postprotocol, PT forces were potentiated under 5-RM loads, whereas they were depressed under 10-RM loads. Hence, a main effect for training protocols showed that changes in PT force were significantly different under 5-RM, as compared to 10-RM loads. The results indicate that central fatigue may be independent of load and pattern, whereas peripheral fatigue would appear to be dependent on load but not pattern.     

Trunk response analysis under sudden forward perturbations using a kinematics-driven model

Accurate quantification of the trunk transient response to sudden loading is crucial in prevention, evaluation, rehabilitation and training programs. An iterative dynamic kinematics-driven approach was used to evaluate the temporal variation of trunk muscle forces, internal loads and stability under sudden application of an anterior horizontal load. The input kinematics is hypothesized to embed basic dynamic characteristics of the system that can be decoded by our kinematics-driven approach. The model employs temporal variation of applied load, trunk forward displacement and surface EMG of select muscles measured on two healthy and one chronic low-back pain subjects to a sudden load. A finite element model accounting for measured kinematics, nonlinear passive properties of spine, detailed trunk musculature with wrapping of global extensor muscles, gravity load and trunk biodynamic characteristics is used to estimate the response under measured sudden load. Results demonstrate a delay of ～200 ms in extensor muscle activation in response to sudden loading. Net moment and spinal loads substantially increase as muscles are recruited to control the trunk under sudden load. As a result and due also to the trunk flexion, system stability significantly improves. The reliability of the kinematics-driven approach in estimating the trunk response while decoding measured kinematics is demonstrated. Estimated large spinal loads highlight the risk of injury that likely further increases under larger perturbations, muscle fatigue and longer delays in activation.     

The effect of physical loading "till breakdown" on the mental work capacity of students with different levels of motor activity

It is shown, that the physical load "to overflowing" is connected with heterogeneous changes in the mental capacity of students. In students with the high preventive level of the working capacity an increase of the mental capacity by the model physical load on the bicycle is due to not only an increase of the physical capacity, but also to accumulation of the tiredness after load. Stimulation of the mental capacity necessitates playing and cyclic loads, or the application of stepwise increasing load (2 w/kg, 6-9 min).     

Directional preference of activation of abdominal and paraspinal muscles during position-control tasks in sitting

Controversy exists in the literature regarding antagonist activity of trunk muscles during different types of trunk loading, and the direction-specificity of activation of trunk muscles, particularly the deeper trunk muscles. This study aimed to systematically compare activation of a range of trunk muscles between directions of statically applied loads, and to consider the impact of breathing in this activation. In a semi-seated position, 13 healthy male participants resisted moderate inertial loads applied to the trunk in eight different directions. Intramuscular electromyography was recorded from eight abdominal and back muscles on the right side during 1 s prior to peak inspiration/expiration. All muscles demonstrated a directional preference of activation. No muscle displayed antagonistic activation during loading conditions of an intensity that exceded that recorded in upright sitting without a load. During these moderate intensity sustained efforts, trunk muscle activation varied little between respiratory phases. Antagonistic muscle activation of amplitude equivalent to the activation recorded in upright sitting without load is sufficient to maintain control of the spine during predictable and sustained low load tasks.     

Mechanical stimuli and IL-13 interact at integrin adhesion complexes to regulate expression of smooth muscle myosin heavy chain in airway smooth muscle tissue

Airway smooth muscle phenotype may be modulated in response to external stimuli under physiological and pathophysiological conditions. The effect of mechanical forces on airway smooth muscle phenotype were evaluated in vitro by suspending weights of 0.5 or 1 g from the ends of canine tracheal smooth muscle tissues, incubating the weighted tissues for 6 h, and then measuring the expression of the phenotypic marker protein, smooth muscle myosin heavy chain (SmMHC). Incubation of the tissues at a high load significantly increased expression of SmMHC compared with incubation at low load. Incubation of the tissues at a high load also decreased activation of PKB/Akt, as indicated by its phosphorylation at Ser 473. Inhibition of Akt or phosphatidylinositol-3,4,5 triphosphate-kinase increased SmMHC expression in tissues at low load but did not affect SmMHC expression at high load. IL-13 induced a significant increase in Akt activation and suppressed the expression of SmMHC protein at both low and high loads. The role of integrin signaling in mechanotransduction was evaluated by expressing a PINCH (LIM1-2) fragment in the muscle tissues that prevents the membrane localization of the integrin-binding IPP complex (ILK/PINCH/α-parvin), and also by expressing an inactive integrin-linked kinase mutant (ILK S343A) that inhibits endogenous ILK activity. Both mutants inhibited Akt activation and increased expression of SmMHC protein at low load but had no effect at high load. These results suggest that mechanical stress and IL-13 both act through an integrin-mediated signaling pathway to oppositely regulate the expression of phenotypic marker proteins in intact airway smooth muscle tissues. The stimulatory effects of mechanical stress on contractile protein expression oppose the suppression of contractile protein expression mediated by IL-13; thus the imposition of mechanical strain may inhibit changes in airway smooth muscle phenotype induced by inflammatory mediators.     

Muscle activity and spine load during pulling exercises: Influence of stable and labile contact surfaces and technique coaching

This study examined pulling exercises performed on stable surfaces and unstable suspension straps. Specific questions included: which exercises challenged particular muscles, what was the magnitude of resulting spine load, and did technique coaching influence results. Fourteen males performed pulling tasks while muscle activity, external force, and 3D body segment motion were recorded. These data were processed and input to a sophisticated and anatomically detailed 3D model that used muscle activity and body segment kinematics to estimate muscle force, in this way the model was sensitive to each individual’s choice of motor control for each task. Muscle forces and linked segment joint loads were used to calculate spine loads. There were gradations of muscle activity and spine load characteristics to every task. It appears that suspension straps alter muscle activity less in pulling exercises, compared to studies reporting on pushing exercises. The chin-up and pull-up exercises created the highest spine load as they required the highest muscle activation, despite the body “hanging” under tractioning gravitational load. Coaching shoulder centration through retraction increased spine loading but undoubtedly adds proximal stiffness. An exercise atlas of spine compression was constructed to help with the decision making process of exercise choice for an individual.     

Melanoma differentiation-associated gene 5 is critical for protection against Theiler's virus-induced demyelinating disease

Infection of dendritic and glial cells with Theiler's murine encephalomyelitis virus (TMEV) induces various cytokines via Toll-like receptor- and melanoma differentiation-associated gene 5 (MDA5)-dependent pathways. However, the involvement and role of MDA5 in cytokine gene activation and the pathogenesis of TMEV-induced demyelinating disease are largely unknown. In this study, we demonstrate that MDA5 plays a critical role in the production of TMEV-induced alpha interferon (IFN-α) during early viral infection and in protection against the development of virus-induced demyelinating disease. Our results indicate that MDA5-deficient 129SvJ mice display significantly higher viral loads and apparent demyelinating lesions in the central nerve system (CNS) accompanied by clinical symptoms compared with wild-type 129SvJ mice. During acute viral infection, MDA5-deficient mice produced elevated levels of chemokines, consistent with increased cellular infiltration, but reduced levels of IFN-α, known to control T cell responses and cellular infiltration. Additional studies with isolated CNS glial cells from these mice suggest that cells from MDA5-deficient mice are severely compromised in the production of IFN-α upon viral infection, which results in increased cellular infiltration and viral loads in the CNS. Despite inadequate stimulation, the overall T cell responses to the viral determinants were significantly elevated in MDA5-deficient mice, reflecting the increased cellular infiltration. Therefore, the lack of MDA5-mediated IFN-α production may facilitate a massive viral load and elevated cellular infiltration in the CNS during early viral infection, leading to the pathogenesis of demyelinating disease.     

A biomechanical model to simulate the effect of a high vertical loading on trunk flexural stiffness

A human trunk model was developed to simulate the effect of a high vertical loading on trunk flexural stiffness. A force–length relationship is attributed to each muscle of the multi-body model. Trunk stiffness and muscle forces were evaluated experimentally and numerically for various applied loads. Experimental evaluation of trunk stiffness was carried out by measuring changes in reaction force following a sudden horizontal displacement at the T10 level prior to paraspinal reflexes induction. Results showed that the trunk stiffness increases under small applied loads, peaks when the loads were further increased and decreases when higher loads are applied. A sensitivity analysis to muscle force–length relationship is provided to determine the model's limitations. This model pointed out the importance of taking into account the changes in muscle length to evaluate the effect of spinal loads beyond the safe limit that cannot be evaluated experimentally and to predict the trunk instability under vertical load.     

Dynamics of viral and proviral loads of feline immunodeficiency virus within the feline central nervous system during the acute phase following intravenous infection

Animal models of human immunodeficiency virus 1, such as feline immunodeficiency virus (FIV), provide the opportunities to dissect the mechanisms of early interactions of the virus with the central nervous system (CNS). The aims of the present study were to evaluate viral loads within CNS, cerebrospinal fluid (CSF), ocular fluid, and the plasma of cats in the first 23 weeks after intravenous inoculation with FIV(GL8). Proviral loads were also determined within peripheral blood mononuclear cells (PBMCs) and brain tissue. In this acute phase of infection, virus entered the brain in the majority of animals. Virus distribution was initially in a random fashion, with more diffuse brain involvement as infection progressed. Virus in the CSF was predictive of brain parenchymal infection. While the peak of virus production in blood coincided with proliferation within brain, more sustained production appeared to continue in brain tissue. In contrast, proviral loads in the brain decreased to undetectable levels in the presence of a strengthening PBMC load. A final observation in this study was that there was no direct correlation between viral loads in regions of brain or ocular tissue and the presence of histopathology.     

Low immune activation despite high levels of pathogenic human immunodeficiency virus type 1 results in long-term asymptomatic disease

Long-term asymptomatic human immunodeficiency virus (HIV)-infected individuals (LTA) usually have low viral load and low immune activation. To discern whether viral load or immune activation is dominant in determining progression to AIDS, we studied three exceptional LTA with high viral loads. HIV type 1 isolates from these LTA were as pathogenic as viruses from progressors in organ culture. Despite high viral loads, these LTA had low levels of proliferating and activated T cells compared to progressors, like other LTA. In contrast to those in progressors, HIV-specific CD4(+) T-cell responses in these LTA were maintained. Thus, low immune activation despite a high viral load preserved HIV-specific T-cell responses and resulted in a long-term asymptomatic phenotype.     

Reduced nucleus pulposus glycosaminoglycan content alters intervertebral disc dynamic viscoelastic mechanics

The intervertebral disc functions over a range of dynamic loading regimes including axial loads applied across a spectrum of frequencies at varying compressive loads. Biochemical changes occurring in early degeneration, including reduced nucleus pulposus glycosaminoglycan content, may alter disc mechanical behavior and thus may contribute to the progression of degeneration. The objective of this study was to determine disc dynamic viscoelastic properties under several equilibrium loads and loading frequencies, and further, to determine how reduced nucleus glycosaminoglycan content alters dynamic mechanics. We hypothesized that (1) dynamic stiffness would be elevated with increasing equilibrium load and increasing frequency, (2) the disc would behave more elastically at higher frequencies, and finally, (3) dynamic stiffness would be reduced at low equilibrium loads under all frequencies due to nucleus glycosaminoglycan loss. We mechanically tested control and chondroitinase ABC injected rat lumbar motion segments at several equilibrium loads using oscillatory loading at frequencies ranging from 0.05 to 5 Hz. The rat lumbar disc behaved non-linearly with higher dynamic stiffness at elevated compressive loads irrespective of frequency. Phase angle was not affected by equilibrium load, although it decreased as frequency was increased. Reduced glycosaminoglycan decreased dynamic stiffness at low loads but not at high equilibrium loads and led to increased phase angle at all loads and frequencies. The findings of this study demonstrate the effect of equilibrium load and loading frequencies on dynamic disc mechanics and indicate possible mechanical mechanisms through which disc degeneration can progress.     

The sensitivity of phytoplankton in Loch Leven (U.K.) to changes in nutrient load and water temperature

1. Loch Leven is a shallow, eutrophic lake in Scotland, U.K. It has experienced much change over the 30 years that it has been studied; this has primarily been due to reduced nutrient loads to the lake through active catchment management. Its recovery has been slow and, therefore, we used a phytoplankton community model (PROTECH) to test its sensitivity to changing nutrient loads and water temperature.
2. PROTECH was initialized to simulate the observed phytoplankton community in 1995 and was then repeatedly run through a combination of step-wise changes in water temperature and nutrient load (two treatments were simulated for nutrient load: one changing both nitrate and phosphorus, and one changing just phosphorus). The effect on total chlorophyll- a concentration, cyanobacteria abundance and phytoplankton diversity was examined.
3. Whilst changes in temperature had little effect, variations in the nutrient load produced a range of responses. Increasing only the phosphorus load caused a large increase in Anabaena abundance and total chlorophyll- a concentration. However, the opposite response was recorded when nitrate load was changed as well, with Anabaena increasing its biomass under reduced nutrient load scenarios.
4. The key factor determining the type of response appeared to be nitrogen availability. Anabaena , a nitrogen fixer, could exploit the phosphorus resource of Loch Leven under limiting nitrogen conditions, allowing it to dominate under most of the scenarios tested apart from those supplying extra nitrogen to the lake. The model predictions agree with the observed data, which show that Anabaena continues to dominate the summer phytoplankton bloom in Loch Leven despite the considerable reduction in phosphorus supply from the catchment. This research provides a possible explanation for this.     

Mechanisms of anterior-posterior stability of the knee joint under load-bearing

The anterior-posterior (AP) stability of the knee is an important aspect of functional performance. Studies have shown that the stability increases when compressive loads are applied, as indicated by reduced laxity, but the mechanism has not been fully explained. A test rig was designed which applied combinations of AP shear and compressive forces, and measured the AP displacements relative to the neutral position. Five knees were evaluated at compressive loads of 0, 250, 500, and 750 N, with the knee at 15° flexion. At each load, three cycles of shear force at ±100 N were applied. For the intact knee under load, the posterior tibial displacement was close to zero, due to the upward slope of the anterior medial tibial surface. The soft tissues were then resected in sequence to determine their role in AP laxity. After anterior cruciate ligament (ACL) resection, the anterior tibial displacement increased significantly even under load, highlighting its importance in stability. Meniscal resection further increased displacement but also the vertical displacement increased, implying the meniscus was providing a buffering effect. The PCL had no effect on any of the displacements under load. Plowing cartilage deformation and surface friction were negligible. This work highlighted the particular importance of the upward slope of the anterior medial tibial surface and the ACL to AP knee stability under load. The results are relevant to the design of total knees which reproduce anatomic knee stability behavior.     

Analysis of large compression loads on lumbar spine in flexion and in torsion using a novel wrapping element

Axial compression on the spine could reach large values especially in lifting tasks which also involve large rotations. Experimental and numerical investigations on the spinal multi motion segments in presence of physiological compression loads cannot adequately be carried out due to the structural instability and artefact loads. To circumvent these problems, a novel wrapping cable element is used in a nonlinear finite element model of the lumbosacral spine (L1-S1) to investigate the role of moderate to large compression loads on the lumbar stiffness in flexion and axial moments/rotations. The compression loads up to 2,700 N was applied with no instability or artefact loads. The lumbar stiffness substantially increased under compression force, flexion moment, and axial torque when applied alone. The presence of compression preloads significantly stiffened the load-displacement response under flexion and axial moments/rotations. This stiffening effect was much more pronounced under larger preloads and smaller moments/rotations. Compression preloads also increased intradiscal pressure, facet contact forces, and maximum disc fibre strain at different levels. Forces in posterior ligaments were, however, diminished with compression preload. The significant increase in spinal stiffness, hence, should be considered in biomechanical studies for accurate investigation of the load partitioning, system stability, and fixation systems/disc prostheses.     

Effect of load position on muscle forces, internal loads and stability of the human spine in upright postures

A novel kinematics-based approach coupled with a non-linear finite element model was used to investigate the effect of changes in the load position and posture on muscle activity, internal loads and stability margin of the human spine in upright standing postures. In addition to 397 N gravity, external loads of 195 and 380 N were considered at different lever arms and heights. Muscle forces, internal loads and stability margin substantially increased as loads displaced anteriorly away from the body. Under same load magnitude and location, adopting a kyphotic posture as compared with a lordotic one increased muscle forces, internal loads and stability margin. An increase in the height of a load held at a fixed lever arm substantially diminished system stability thus requiring additional muscle activations to maintain the same margin of stability. Results suggest the importance of the load position and lumbar posture in spinal biomechanics during various manual material handling operations.     

Effect of the posterior cruciate ligament on posterior stability of the knee in high flexion

Most biomechanical studies of the knee have focused on knee flexion angles between 0 degrees and 120 degrees. The posterior cruciate ligament (PCL) has been shown to constrain posterior laxity of the knee in this range of flexion. However, little is known about PCL function in higher flexion angles (greater than 120 degrees ). This in vitro study examined knee kinematics before and after cutting the PCL at high flexion under a posterior tibial load and various muscle loads. The results demonstrated that although the PCL plays an important role in constraining posterior tibial translation at low flexion angles, the PCL had little effect in constraining tibial translation at 150 degrees of flexion under the applied loads.     

Partitioning average competition and extreme-genotype effects in genetically diverse infections

Competition between parasite genotypes in genetically diverse infections is widespread. However, experimental evidence on how genetic diversity influences total parasite load is variable. Here we use an additive partition equation to quantify the negative effect of inter-genotypic competition on total parasite load in diverse infections. Our approach controls for extreme-genotype effects, a process that can potentially neutralise, or even reverse, the negative effect of competition on total parasite load. A single extreme-genotype can have a disproportionate effect on total parasite load if it causes the highest parasite load in its single-infection, while increasing its performance in diverse relative to single infections. We show that in theory such disproportionate effects of extreme-genotypes can lead to a higher total parasite load in diverse infections than expected, even if competition reduces individual parasite performance on average. Controlling for the extreme-genotype effect is only possible if the competition effect on total parasite load is measured appropriately as the average difference between the realised number of each parasite genotype in mixed infections and the expected number based on single infection parasite loads. We apply this approach to sticklebacks that were experimentally infected with different trematode genotypes. On average, genetically diverse infections had lower parasite loads than expected from single-infection results. For the first time we demonstrate that competition between co-infecting genotypes per se caused the parasite load reduction, while extreme-genotype effects were not significant. We thus suggest that to correctly quantify the effect of competition alone on total parasite load in genetically diverse infections, the extreme-genotype effect has to be controlled for.     

Effect of load position on muscle forces,internal loads and stability of the human spine in upright postures

A novel kinematics-based approach coupled with a non-linear finite element model was used to investigate the effect of changes in the load position and posture on muscle activity, internal loads and stability margin of the human spine in upright standing postures. In addition to 397 N gravity, external loads of 195 and 380 N were considered at different lever arms and heights. Muscle forces, internal loads and stability margin substantially increased as loads displaced anteriorly away from the body. Under same load magnitude and location, adopting a kyphotic posture as compared with a lordotic one increased muscle forces, internal loads and stability margin. An increase in the height of a load held at a fixed lever arm substantially diminished system stability thus requiring additional muscle activations to maintain the same margin of stability. Results suggest the importance of the load position and lumbar posture in spinal biomechanics during various manual material handling operations.