Soft tissue wobbling affects trunk dynamic response in sudden perturbations

Soft tissue wobbling reduces the transferred impact of external loads on lower limb joints. The present study investigated whether soft tissue wobbling has similar effects on trunk dynamic response to sudden perturbations. Three healthy males were subjected to a series of anteriorly directed trunk position perturbations at three different velocities while trunk kinematics and kinetics were measured. A nonlinear active-passive finite element model of the human trunk was then used to study the effects of soft tissue wobbling on trunk response. Also investigated were the effects on model predictions of including elements simulating the apparatus (rod-harness assembly) transferring motor-generated perturbations to the trunk. Predicted and measured trunk kinematics and kinetics, when accounting for the dynamic effects of both wobbling mass and rod-harness assembly, were in good agreement for all velocities especially early (<120 ms) after the perturbations (ρ>0.97). Root mean square errors in model predictions increased considerably when neglecting the aforementioned modeling considerations. The trunk wobbling mass and connecting elements between the trunk and the perturbing device, particularly during faster perturbations, substantially attenuated the transferred impact of external loads on the spine (by 33-90 N across perturbation velocities). Such reductions in the impacts transferred, in turn, reduced the predicted demands on the neuromuscular system for control and maintenance of spinal loads and stability. As such, these features should be considered in future biodynamic models of the human trunk aimed at estimating trunk neuromuscular behaviors during sudden perturbations.     

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.     

Neuromuscular response of the trunk to inertial based sudden perturbations following whole body vibration exposure

The effects of whole body vibration exposure on the neuromuscular responses following inertial-based trunk perturbations were examined. Kinematic and surface EMG (sEMG) data were collected while subjects were securely seated on a robotic platform. Participants were either exposed to 10 min of vibration or not, which was followed by sudden inertial trunk perturbations with and without timing and direction knowledge. Amplitude of sEMG was analyzed for data collected during the vibration protocol, whereas the onset of sEMG activity and lumbar spine angle were analyzed for the perturbation protocol. Data from the vibration protocol did not show a difference in amplitude of sEMG for participants exposed to vibration and those not. The perturbation protocol data showed that those not exposed to vibration had a 14% faster muscle onset, despite data showing no difference in fatigue level.     

Effects of trunk muscle fatigue and load timing on spinal responses during sudden hand loading

The purpose of this study was to investigate the responses of the spine during sudden loading in the presence of back and abdominal muscle fatigue, with a primary focus on the implications for spinal stability. Fifteen females were studied and each received sudden loads to the hands, at both known and unknown times. Participants received these loading trials (a) while rested, (b) with back muscle fatigue, and (c) with a combination of back and abdominal muscle fatigue. Measures were taken on the EMG activity of two trunk extensor and two abdominal muscles, and on the trunk angle and centre of pressure. A 3 × 2 Repeated Measures ANOVA was also performed. There were no preparations made prior to the perturbation even when it could be anticipated. However, the peak responses that followed were greater in the unexpected versus the expected condition. In addition, trunk muscle fatigue led to an increase in the baseline activity of the trunk muscles but no additional increase in activity just prior to loading. There was increased activation of both (opposing) muscle groups when only one muscle group was fatigued. Because the peak responses following the perturbation were enhanced in the unknown timing condition, preparations must have taken place prior to the anticipated perturbations, perhaps in other segments of the body that were not measured. Also, the load impact may not have been great enough to elicit large preparations. The heightened baseline activity with fatigue suggests that there may have been increased spinal stiffness whenever the spine was fatigued, and not just immediately prior to an impending perturbation. The increased activation of opposing muscle groups is evidence of increased cocontraction in response to fatigue, possibly to maintain stability with decreasing coordination.     

Aging effects of motor prediction on protective balance and startle responses to sudden drop perturbations

This pilot study investigated the effect of age on the ability of motor prediction during self-triggered drop perturbations (SLF) to modulate startle-like first trial response (FTR) magnitude during externally-triggered (EXT) drop perturbations. Ten healthy older (71.4 ± 1.44 years) and younger adults (26.2 ± 1.63 years) stood atop a moveable platform and received blocks of twelve consecutive EXT and SLF drop perturbations. Following the last SLF trial, participants received an additional EXT trial spaced 20 min apart to assess retention (EXT RTN) of any modulation effects. Electromyographic (EMG) activity was recorded bilaterally over the sternocleidomastoid (SCM), vastus lateralis (VL), biceps femoris (BF), medial gastrocnemius (MG), and tibialis anterior (TA). Whole-body kinematics and kinetic data were recorded. Stability in the antero-posterior direction was quantified using the margin of stability (MoS). Compared with EXT trials, both groups reduced SCM peak amplitude responses during SLF and EXT RTN trials. VL/BF and TA/MG coactivation were reduced during SLF FTR compared to EXT FTR (p < 0.05) with reduced peak vertical ground reaction forces (vGRF) in both younger and older adults (p < 0.05). Older adults increased their MoS during SLF FTR compared to EXT FTR (p < 0.05). Both groups performed more eccentric work during SLF trials compared to EXT (p < 0.05). These findings indicate that abnormal startle effects with aging may interfere with balance recovery and increase risk of injury with external balance perturbations. Motor prediction may be used to acutely mitigate abnormal startle/postural responses with aging.     

Rheological of blood and oxygen transport in long-term adaptation to muscular load

The blood viscosity became reduced after a long-term muscular adaptation in dogs. The main adaptation mechanism is associated with an autoregulated haemodilution and improvement of the red blood cells' microrheology (deformities and aggregation). The findings suggest that reduction in the haemodilution and the blood oxygen capacity are accompanied by a heightened efficiency of the oxygen transport. A sufficient correlation exists between the blood fluidity parameters and the oxygen balance in the body. Value of the optimum haematocrit in oxygen transport, is discussed.     

Inferring gene networks from steady-state response to single-gene perturbations

Inferring gene networks from gene expression data is an important step in understanding the molecular machinery of life. Three methods for establishing and quantifying causal relationships between genes based on steady-state measurements in single-gene perturbation experiments have recently been proposed: the regulatory strength method, the local regulatory strength method, and Gardner's method. The theoretical basis of these methods is presented here in a thorough and consistent fashion. In principle, for the same data set all three methods would generate identical networks, but they would quantify the strengths of connections in different ways. The regulatory strength method is shown here to be topology-dependent. It adopts the format of the data collected in gene expression microarray experiments and therefore can be immediately used with this technology. The regulatory strengths obtained by this method can also be used to compute local regulatory strengths. In contrast, Gardner's method requires both measurements of mRNA concentrations and measurements of the applied rate perturbations, which is not usually part of a standard microarray experimental protocol. The results generated by Gardner's method and by the two regulatory strengths methods differ only by scaling constants, but Gardner's method requires more measurements. On the other hand, the explicit use of rate perturbations in Gardner's approach allows one to address new questions with this method, like what perturbations caused given responses of the system. Results of the application of the three techniques to real experimental data are presented and discussed. The comparative analysis presented in this paper can be helpful for identifying an appropriate technique for inferring genetic networks and for interpreting the results of its application to experimental data.     

Interactions among stomata in response to perturbations in humidity

The existence of patchy stomatal closure suggests interactions among neighbouring stomata that synchronize stomatal movements in small areas of a leaf. To test for such interactions, water vapour partial pressure (e_wv) for a small group of stomata was controlled independently of that for the surrounding stomata using gas flow from a small needle. The e_wv for the surrounding stomata was controlled with a larger gas flow, termed the primary flow. The spatial pattern of e_wv isobars caused by the needle flow was assessed experimentally and theoretically. Stomatal apertures were monitored following perturbations in e_wv of the primary flow and the needle flow. When e_wv of the primary flow was perturbed and that of the needle flow held constant, stomata for which there was little or no perturbation in e_wv responded similarly to stomata experiencing the perturbation. When the e_wv of the needle flow was perturbed and that of the primary flow held constant, many stomata experiencing little or no perturbation responded similarly to those experiencing a large perturbation. The results are discussed in relation to a mechanism for stomatal interactions that has been proposed in a previous study [Haefner, Buckley & Mott (1997) Plant, Cell and Environment 20, 1087–1097, this issue].     

Endogenous hormones subtly alter women's response to heat stress

The thermoregulatory responses of menstruant women to exercise in dry heat (dry-bulb temperature/wet-bulb temperature = 48/25 degrees C) were evaluated at three times during the menstrual cycle: menstrual flow (MF), 3-5 days during midcycle including ovulation (OV), and in the middle of the luteal phase (LU). Serum concentrations of estradiol-17 beta (E2), progesterone (Pg), luteinizing hormone (LH), and follicle-stimulating hormone (FSH) were measured by radioimmunoassay, and these values were used to determine the dates of OV (peak LH and FSH) and LU (peak postovulatory Pg). After heat acclimation, subjects received heat stress tests (HST) consisting of a 2-h cycle-ergometer exercise at 30% of maximal O2 consumption in the heat. Rectal (Tre) and mean skin (Tsk) temperatures, heart rate (HR), and sweat rate on the chest and thigh were recorded continuously. Total sweat loss (Msw), as indicated by weight loss, was recorded every 20 min, and equivalent water replacement was given. Steady-state exercise metabolic rate (M) was measured at 45 and 110 min. Seven of eight subjects had ovulatory cycles during experimental months. At rest, Tre was lowest at OV and significantly higher at LU. During steady-state exercise both Tre and Tsk were lowest at OV and significantly higher at LU. There were no differences between phases in Msw, sweat rate on the chest and thigh or M. Despite higher Tre and Tsk at LU, all subjects were able to complete the 2-h of exercise.(ABSTRACT TRUNCATED AT 250 WORDS)     

Explosive voluntary torque is related to whole-body response to unexpected perturbations

Explosive torque has been demonstrated to relate to static balance. However, sports injuries occur dynamically and unpredictably, yet the relationship between explosive torque and balance response to dynamic perturbations is unknown. This study investigated the relationship between explosive torque of the plantar flexors and knee extensors and the centre of mass (COM) response to unexpected perturbations. Thirty-three healthy subjects (17 females, 16 males) were assessed for maximal and explosive isometric knee extension (KE) and plantar flexion (PF) torque and COM response (velocity (COMV), displacement (COMD)) to unexpected platform translations. Relationships between explosive torque and balance measures were investigated using Pearson’s correlation and multiple regression. A negative relationship between PF explosive torque at 50, 100, and 150 ms and COMV at 300, 400, and 500 ms (r = −0.363 to −0.508, p ≤ 0.049), and COMD at 400 and 500 ms (r = −0.349 to −0.416, p ≤ 0.046) was revealed. A negative relationship between KE explosive torque at 50, 100, and 150 ms and COMV at 400 ms (r = −0.381 to −0.411, p ≤ 0.029) but not COMD was also revealed. Multiple regression found PF 100 ms predicted 17.3% of variability in COMD at 500 ms and 25.8% of variability in COMV at 400 ms. These results suggest that producing torque rapidly may improve COM response to unexpected perturbation.     

Inflammatory response to strenuous muscular exercise in man

Based on the humoral and cellular changes occurring during strenuous muscular work in humans, the concept of inflammatory response to exercise (IRE) is developed. The main indices of IRE consist of signs of an acute phase response, leucocytosis and leucocyte activation, release of inflammatory mediators, tissue damage and cellular infiltrates, production of free radicals, activation of complement, and coagulation and fibrinolytic pathways. Depending on exercise intensity and duration, it seems likely that muscle and/or associated connective tissue damage, contact system activation due to shear stress on endothelium and endotoxaemia could be the triggering mechanisms of IRE. Although this phenomenon can be considered in most cases as a physiological process associated with tissue repair, exaggerated IRE could have physiopathological consequences. On the other hand, the influence of several factors such as age, sex, training, hormonal status, nutrition, anti-inflammatory drugs, and the extent to which IRE could be a potential risk for subjects undergoing intense physical training require further study.     

Calcium influx determines the muscular response to electrotransfer

Cell membrane permeabilization by electric pulses (electropermeabilization), results in free exchange of ions across the cell membrane. The role of electrotransfer-mediated Ca(2+)-influx on muscle signaling pathways involved in degeneration (β-actin and MurF), inflammation (IL-6 and TNF-α), and regeneration (MyoD1, myogenin, and Myf5) was investigated, using pulse parameters of both electrochemotherapy (8 HV) and DNA delivery (HVLV). Three pulsing conditions were used: 8 high-voltage pulses (8 HV), resulting in large permeabilization and ion flux, and a combination of one high-voltage pulse and one low-voltage pulse (HVLV), either alone or in combination with injection of DNA. Mice and rats were anesthetized before pulsing. At the times given, animals were killed, and intact tibialis cranialis muscles were excised for analysis. Uptake of Ca(2+) was assessed using (45)Ca as a tracer. Using gene expression analyses and histology, we showed a clear association between Ca(2+) influx and muscular response. Moderate Ca(2+) influx induced by HVLV pulses results in activation of pathways involved in immediate repair and hypertrophy. This response could be attenuated by intramuscular injection of EGTA reducing Ca(2+) influx. Larger Ca(2+) influx as induced by 8-HV pulses leads to muscle damage and muscle fiber regeneration through recruitment of satellite cells. The extent of Ca(2+) influx determines the muscular response to electrotransfer and, thus, the success of a given application. In the case of electrochemotherapy, in which the objective is cell death, a large influx of Ca(2+) may be beneficial, whereas for DNA electrotransfer, muscle recovery should occur without myofiber loss to ensure preservation of plasmid DNA.     

Genome-wide gene expression profiles in response to plastid division perturbations

Plastids are vital organelles involved in important metabolic functions that directly affect plant growth and development. Plastids divide by binary fission involving the coordination of numerous protein components. A tight control of the plastid division process ensures that: there is a full plastid complement during and after cell division, specialized cell types have optimal plastid numbers; the division rate is modulated in response to stress, metabolic fluxes and developmental status. However, how this control is exerted by the host nucleus is unclear. Here, we report a genome-wide microarray analysis of three accumulation and replication of chloroplasts (arc) mutants that show a spectrum of altered plastid division characteristics. To ensure a comprehensive data set, we selected arc3, arc5 and arc11 because they harbour mutations in protein components of both the stromal and cytosolic division machinery, are of different evolutionary origin and display different phenotypic severities in terms of chloroplast number, size and volume. We show that a surprisingly low number of genes are affected by altered plastid division status, but that the affected genes encode proteins important for a variety of fundamental plant processes.