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At least 217 records · Page 12Linked to original sources

PubMed2004

Geometric and material nonlinearity in tensioned wires of an external fixator.

OBJECTIVE: To investigate the fundamental characteristics of a tensioned wire in an external fixator. DESIGN: Many factors that may influence the wire performance including the change in wire geometry, material hardening and yielding, loading and unloading, and levels of pre-tension are considered in order to obtain a whole spectrum of the wire characteristics. BACKGROUND: External fixation is widely used in the treatment of unstable fractures, limb lengthening, and congenital and pathological orthopedic deformities. With the use of tensioned wires, external fixation provides attractive features such as minimal invasiveness, maximum tailorability, and versatility. These seemingly simple wires actually fulfill a very complex duty. To be able to maximize the benefit of these wires, it is necessary to know their fundamental characteristics. METHODS: A single wire was isolated from an external Ilizarov ring-frame, and nonlinear elastic and plastic analyses of the wire were performed using the method of finite element analysis. RESULTS: The nonlinear behavior of the wire originates not only from the material hardening and yielding but also from the change in its geometry. Pre-tension reduces wire deflection, delays the onset of full-plastic deformation, and elevates the limiting plastic moment, but at the same time leads to early material hardening and yielding. CONCLUSIONS: An entire load-deflection curve is necessary when comes to describe the wire performance. To enhance the wire performance, further efforts should be directed to exploiting the benefit of geometric nonlinearity of the wire. RELEVANCE: The findings will lead to refinement of external fixation design and provide engineering information to surgeons regarding the application of pre-tension during surgery.

Bone Wires↗

PubMed2000

Aerosol Research and Inhalation Epidemiological Study (ARIES): air quality and daily mortality statistical modeling--interim results.

The Aerosol Research and Inhalation Epidemiological Study (ARIES) is an EPRI-sponsored project to collect air quality and meteorological data at a single site in northwestern Atlanta, GA. Seventy high-resolution air quality indicators (AQIs) are used to examine statistical relationships between air quality and health outcome end points. Contemporaneous mortality data are collected for Fulton and DeKalb counties in Georgia. Currently, 12 months of air quality and weather data are available for analysis, from August 1998 through July 1999. The interim mortality analysis used Poisson regression in generalized additive models (GAMs). The estimated log-linear association of mortality with various AQIs was adjusted for smoothed functions of time and meteorological data. The analysis considered daily deaths due to all nonaccidental causes, deaths to persons 65 years or older, and deaths in each of the two constituent counties. The fine particle effect associated with the four mortality subgroups, using only today (lag 0), yesterday (lag 1), 2-day average (average of today and yesterday), and first difference (today minus yesterday) measurements of the air quality relative to today's number of deaths was positive for lag 0, lag 1, and 2-day average and positive only for decedents at least 65 years of age using first difference. The t values ranged from 0.81 to 1.15 for lag 0, 1.04 to 1.53 for lag 1, 1.10 to 1.66 for 2-day average, and -0.32 to 0.33 for first difference with 346 or 347 days of data. No statistically significant estimate of the linear coefficient was found for the other 14 air quality variables in our interim analysis for the four mortality subgroups. We discuss diagnostics to support these models. These interim analyses did not include an evaluation of sensitivity to a larger set of lag structures, nonlinear model specifications, multipollutant analyses, alternative weather model and smoothing model specifications, air pollution imputation schemes, or cause-specific mortality indicators, nor did they include a full reporting of model selection or goodness-of-fit indicators. No conclusion can be drawn at this time about whether the findings from subsequent studies have sufficiently greater power to detect effects comparable to those found in other U.S. cities including at least 2 or 3 years of data.

Adolescent↗

PubMed2000

Empirical assessments of social networks, fertility and family planning programs: nonlinearities and their implications.

Empirical studies of the diffusion of modern methods of family planning have increasingly incorporated social interaction within nonlinear models such as logits. But they have not considered the full implications of these nonlinear specifications. This paper considers the implications of using nonlinear models in empirical analyses of the impact of family programs, modulated by social interaction, on reproductive behavior. Three implications of nonlinear models, in comparison with linear models, are developed. 1) With nonlinear models, there may be both low and high contraceptive-use equilibria (i.e., the ultimate level of use of modern family planning that a population can be expected to reach after the effects of a sustained change in a family planning program have worked through the population) rather than just one equilibrium as in linear models. If there are multiple equilibria, then one striking and important result is that a transitory large program effort may move a community from sustained low- to high-level contraceptive use. 2) With nonlinear models, the extent to which a social interaction multiplies program efforts depends on whether the community is at a low or high level of contraceptive use rather than being independent of the level of contraceptive use as in linear models. 3) With nonlinear models, intensified social interaction can retard or enhance the diffusion of family planning, in contrast to only enhancing diffusion as within linear models. To clarify these implications, for comparison a simple and more transparent linear model is also discussed. Illustrative estimates are presented of simple linear and nonlinear models for rural Kenya that demonstrate that some of these effects may be considerable.

Contraception↗

PubMed1995

Finite element analysis of nonlinear pulsatile suspension flow dynamics in blood vessels with aneurysm.

A nonlinear pulsatile suspension flow in a dilated vessel is numerically analysed. Two sets of highly coupled nonlinear partial differential equations governing the suspension flow are numerically solved, to simulate the suspension flow dynamics. A transient velocity-pressure (UVP) finite element method (FEM) and a stable time integration scheme, based on a predictor-corrector strategy, with constant error monitoring are employed in the flow analysis. The pulsatile suspension flow is characterized by analysing the flow, pressure and stress fields. Effects of the nonlinear particulate phase on the nonlinear suspending fluid phase are brought out by comparing the suspension flow results with those of homogeneous flow. Particles are seen to dampen the flow velocity, wall and central axis pressure, pressure gradient and wall shear stress. time-dependent recirculation regions which are sensitive to the presence of particles are seen in the dilated portion of the vessel. These recirculation regions favour thrombogenesis. The nonlinear effects due to the vessel geometry and those due to the convective terms dominate the dampening effect of the particles. These nonlinear effects are depicted through the transverse velocity and pressure plots. Wall shear stresses of suspension flow are not only high but also alternate in direction.

Algorithms↗

PubMed1996

An electromyographic study of unresisted trunk rotation with normal velocity among healthy subjects.

STUDY DESIGN: An axial rotation tester was designed and fabricated for the study. This allowed stabilization of seated subjects (hip down) and coupling of shoulders, permitting axial rotation and coupled lateral flexion. Using this device, a "flexion-extension free" axial rotation was executed for studying its characteristics. OBJECTIVES: To determine the mechanism of initiation, sustenance, and execution of axial rotation. This was planned to be done by determining the phasic relationship of various torso muscles in the initiation, execution, and termination of axial rotation. Another objective was to determine the total and relative contribution of torso muscles in axial rotation and the small segments of these activities. SUMMARY OF BACKGROUND DATA: There only are a few studies conducted on axial rotation. Generally, these have investigated isometric maximal voluntary contraction in neutral or prerotated postures. The two studies that have reported isokinetic axial rotation have investigated maximal efforts. No study in literature has reported initiation, termination, and execution of unresisted normal velocity axial rotation. METHODS: Fifty healthy young subjects executed a full cycle of axial rotation, starting from neutral position to their extreme left, continuing to their extreme right, and finally moving to the neutral posture in one smooth motion without stopping anywhere. The electromyographic results of external obliques, internal obliques, rectus abdominis, pectoralis major, erectores spinae at T10 and L3, and latissimus dorsi were measured bilaterally simultaneously during this trunk rotation. The timing and relative magnitude analyses were done to determine the global and individual muscle contributions in axial rotation. The correlation between electromyographic and angular displacement, and nonlinear curve fitting regression analyses were performed to decipher individual muscles behavior. RESULTS: The pattern of muscle activation was variable. However, contralateral external obliques, ipsilateral erector spinae, and latissimus dorsi became active before other muscles. These were agonists and the others were antagonists or stabilizers. The agonists contributed 65% of the total electromyographic output, whereas antagonists and stabilizers contributed 35%. The muscle activities during onset and offset periods were biphasic with significantly different slopes. CONCLUSIONS: It was concluded that the axial rotation is achieved through the activities of agonists, and return to neutral position is because of elastic recoil controlled by agonistic muscles. A range of approximately 10-15 degrees on either side of the anatomical midsagittal plane involves little muscle effort, but beyond this region, the osteoligamentous structures become stiff and require increasing effort to execute axial rotation.

Abdominal Muscles↗

PubMedDate not supplied

Responses of auditory nerve fibres to noise stimuli show cochlear nonlinearities.

The frequency selectivity of the peripheral auditory frequency analyser was determined on the basis of recordings from single auditory nerve fibres in the rat, using pseudorandom noise as stimulus. The impulse response function was arrived at by cross-correlating period histograms of the recorded activity with the noise. Frequency transfer functions were computed by Fourier transformation of the derived impulse response function. The impulse response function was observed to become shorter with increasing stimulus intensity. A concomitant widening of the frequency transfer function and a downward shift in the centre frequency were seen. Since the method used is insensitive to nonlinearities, the derived impulse and frequency respone functions are assumed to be valid approximations of the basilar membrane properties. The results, then, support the hypothesis that the basilar membrane is a nonlinear frequency analyser whose selectivity decreases as sound intensity increases.

Acoustic Stimulation↗

PubMed2006

Prediction of one repetition maximum strength from multiple repetition maximum testing and anthropometry.

The purpose of this study was to quantify the decrease in the load lifted from 1 to 5, 10, and 20 repetitions to failure for the flat barbell bench press (chest press; CP) and plate-loaded leg press (LP). Furthermore, we developed prediction equations for 1 repetition maximum (RM) strength from the multiple RM tests, including anthropometric data, gender, age, and resistance training volume. Seventy subjects (34 men, 36 women), 18-69 years of age, completed 1, 5, 10, and 20RM testing for each of the CPs and LPs. Regression analyses of mean data revealed a nonlinear decrease in load with increasing repetition number (CP: linear S(y.x) = 2.6 kg, nonlinear S(y.x) = 0.2 kg; LP: linear S(y.x) = 11.0 kg, nonlinear S(y.x) = 2.6 kg, respectively). Multiple regression analyses revealed that the 5RM data produced the greatest prediction accuracy, with R(2) data for 5, 10, and 20RM conditions being LP: 0.974, 0.933, 0.915; CP: 0.993, 0.976, and 0.955, respectively. The regression prediction equations for 1RM strength from 5RM data were LP: 1RM = 1.0970 x (5RM weight [kg]) + 14.2546, S(y.x) = 16.16 kg, R(2) = 0.974; CP: 1RM = 1.1307 x (5RM weight) + 0.6999, S(y.x) = 2.98 kg, R(2) = 0.993. Dynamic muscular strength (1RM) can be accurately estimated from multiple repetition testing. Data reveal that no more than 10 repetitions should be used in linear equations to estimate 1RM for the LP and CP actions.

Adolescent↗

PubMed1995

Population pharmacokinetics of docetaxel during phase I studies using nonlinear mixed-effect modeling and nonparametric maximum-likelihood estimation.

Docetaxel, a novel anticancer agent, was given to 26 patients by short i.v. infusion (1-2 h) at various dose levels (70-115 mg/m2, the maximum tolerated dose) during 2 phase I studies. Two population analyses, one using NONMEM (nonlinear mixed-effect modeling) and the other using NPML (nonparametric maximum-likelihood), were performed sequentially to determine the structural model; estimate the mean population parameters, including clearance (Cl) and interindividual variability; and find influences of demographic covariates on them. Nine covariates were included in the analyses: age, height, weight, body surface area, sex, performance status, presence of liver metastasis, dose level, and type of formulation. A three-compartment model gave the best fit to the data, and the final NONMEM regression model for Cl was Cl = BSA(Theta1 + Theta02 x AGE), expressing Cl (in liters per hour) directly as a function of body surface area. Only these two covariates were considered in the NPML analysis to confirm the results found by NONMEM. Using NONMEM [for a patient with mean AGE (52.3 years) and mean BSA (1.68 m2)] and NPML, docetaxel Cl was estimated to be 35.6 l/h (21.2 lh-1 m-2) and 37.2 l/h with interpatient coefficients of variations (CVs) of 17.4% and 24.8%, respectively. The intraindividual CV was estimated at 23.8% by NONMEM; the corresponding variability was fixed in NPML in an additive Gaussian variance error model with a 20% CV. Discrepancies were found in the mean volume at steady state (Vss; 83.21 for NPML versus 1241 for NONMEM) and in terminal half-lives, notably the mean t1/2 gamma, which was shorter as determined by NPML (7.89 versus 12.2 h), although the interindividual CV was 89.1% and 62.7% for Vss and t1/2 gamma, respectively. However, the NPML-estimated probability density function (pdf) of t1/2 gamma was bimodal (5 and 11.4 h), probably due to the imbalance of the data. Both analyses suggest a similar magnitude of mean Cl decrease with small BSA and advanced age.

Adult↗

PubMed2001

Determination of release and uptake parameters from electrically evoked dopamine dynamics measured by real-time voltammetry.

Quantifying mechanisms underlying extracellular signaling by the neurotransmitter dopamine (DA) is a difficult task, particularly in the complex extracellular microenvironment of the intact brain. In this study, two methods for evaluating release and uptake from DA dynamics monitored by real-time voltammetry are described. Both are based on a neurochemical model characterizing electrically evoked levels of DA as a balance between these opposing mechanisms. The theoretical basis of what is called here nonlinear regression and single curve analyses is given. Fitting simulated data tests the reliability of the methods. The two analyses are also compared with an experimental data set describing the effects of pharmacologically inhibiting the DA transporter in the caudate-putamen (CP) and nucleus accumbens (NAc). The results indicate that nonlinear regression and single curve analyses are suitable for quantifying release and uptake mechanisms underlying DA neurotransmission. Additionally, the most important experimental finding of this technical study was the independent confirmation of high affinity (approximately 0.2 microM) DA uptake in the intact striatum.

Action Potentials↗

PubMed1996

Deterministic--chaotic and periodic properties of heart rate and arterial pressure fluctuations and their mediation in piglets.

OBJECTIVE: Only the simultaneous analysis of periodic and nonlinear properties of heart rate fluctuations (HRF) can describe completely this complex physiological process. Up to now there is, apart from a study of our own, no systematic and correlative investigation using both parameter groups, also not in early development. Thus, we tried to describe in this manner these properties of HRF, the corresponding mean arterial pressure fluctuations (MAPF) and respiratory movements (RM) and their mutual relations in neonatal pig. METHODS: In 6 term newborn piglets, periodic properties of HRF, RM, and MAPF were analyzed by spectral and coherence analysis, and deterministic-chaotic properties by calculation of correlation dimension (CD), Lyapunov exponent (LE), and construction of phase space plots. The assumption of deterministic chaotic components was supported by Theiler's test for nonlinearity, by always positive leading LEs, and by the results of a nonlinear deterministic model. These analyses were done in sleep states, general anaesthesia, hypoxic hypoxia, in ventilated state, and during cholinergic and additional beta-adrenergic blockade. RESULTS: In all experimental states, HRF and MAPF have periodic and nonlinear, very probably deterministic-chaotic properties, but in different relations. In anaesthetized piglets, periodic properties of HRF and MAPF dominate. In hypoxia the decreasing LE and CD of HRF and CD of MAPF were connected with increasing MAPF power density. Cholinergic blockade caused a decreased overall HRF and MAPF power and a decreasing LE and CD, but beta-adrenergic blockade decreased a small part of power density of both in 0.02-0.08 Hz only. The results of CD, LE, Theiler's test and the low dimensional deterministic model data suggested mainly deterministic-chaotic properties in the nonlinear part of HRF and MAPF. CONCLUSIONS: Already in neonatal piglets, both periodic and nonlinear, very probably deterministic chaotic properties of HRF and MAPF exist which change both during hypoxia and cholinergic blockade. They are partly cholinergically and--to a small extent--also beta-adrenergically mediated. The decrease of nonlinear complexity of HRF and MAPF during hypoxia suggests characteristic pathological change even in early development.

Anesthesia, General↗

PubMed2005

Theoretical analyses of the effects on the linear and quadratic nonlinear optical properties of N-arylation of pyridinium groups in stilbazolium dyes.

N-Arylation of the pyridinium electron acceptor unit in stilbazolium chromophores has been found by previous experimental hyper-Rayleigh scattering and electronic Stark effect (electroabsorption) spectroscopic studies to lead to substantial increases in the static first hyperpolarizability beta(0). We show here that INDO/SCI calculations on the isolated cations trans-4'-(dimethylamino)-N-R-4-stilbazolium (R = methyl 1, phenyl 2, 2,4-dinitrophenyl 3, or 2-pyrimidyl 4) predict only slight red-shifts in the energy of the intramolecular charge-transfer (ICT) transition and accompanying relatively small changes in beta(0) on moving along the series. The inclusion of acetonitrile solvent using a polarizable continuum model affords a somewhat better agreement with the experimental data, especially the red-shifting of the ICT transition and the increase in beta(0) on going from 1 to 4. Time-dependent density functional theory (TD-DFT), finite field, and coupled perturbed Hartree-Fock calculations reproduce even more closely the empirical data and trends; the latter two approaches lead to the highest quadratic nonlinear optical (NLO) response of the studied chromophores for 3, for which the predicted beta(0) is ca. 50-100% larger than that of the analogous N-methylated cation 1. Although the TD-DFT and INDO/SCI approaches give quite different results for ground- and excited-state dipole moments, the overall conclusions of these two methods regarding the ICT absorption and NLO responses are similar.

Acetonitriles↗

PubMed2001

Isotopic analogues as internal standards for quantitative analyses of drugs and metabolites by GC-MS--nonlinear calibration approaches.

In order to achieve accurate quantitation of drugs and metabolites (analytes) in complex matrices, 2H- (and less commonly 13C-) labeled analogues of the analytes are now routinely adapted as the internal standards (IS) using linear calibration models to fit data generated by selected ion monitoring gas chromatography-mass spectrometry (GC-MS) protocols. In this study, the effects of cross-contribution (contribution of the IS to the intensity of the ion designated for the analyte and vice versa) on the linearity of the calibration data are examined. Nonlinear approaches that may address this problem are also studied. Two ion pairs (one with least and one with significant cross-contribution) from each of the following analyte/IS pairs are used as the exemplar systems for this study: butalbital/13C4-butalbital, butalbital/2H5-butalbital, secobarbital/13C4-secobarbital, and secobarbital/2H5-secobarbital. Analyte/IS ion intensity ratios of a series of standard solutions are correlated with the analyte/IS concentration ratios using one-point, multiple-point (unweighted and weighted) linear, and hyperbolic functions. The one-point calibration approach produces excellent calibration results in treating data derived from ion pairs with no significant cross contribution. In cases where significant cross-contribution exists, results derived from the one-point approach show, as expected, significant deviations at both ends of the concentration range. With the cross-contribution phenomenon accounted for, the hyperbolic calibration model is clearly more effective in fitting calibration data at both the lower and higher analyte concentration ends, thus significantly lowering the detection limit and extending the calibration range to a higher level. However, the calibration range cannot be extended indefinitely. At the low concentration end, noise-to-signal ratio and the cross-contribution of the IS to the intensity of the ion designated for the analyte, however insignificant, will incrementally reduce the quality of the observed ion intensity and intensity ratio data. At the high concentration end, detection saturation and the cross-contribution of the analyte to the intensity of the ion designated for the IS, however insignificant, will incrementally decrease the "slope" of the calibration curve. Thus, acceptable sensitivity (increase in analyte/IS ion-pair intensity ratio per unit increase in analyte concentration) of the calibration curve will become the limiting factor.

Algorithms↗

PubMed1991

Mechanical characteristics of the stem-cement interface.

The mechanical characteristics of the interface between a metallic stem and the surrounding poly(methyl methacrylate) bone cement were determined from experimental tests and finite element analyses. Push-through-stem tests of straight and tapered titanium alloy stems, surrounded by cement columns, were performed and the resulting load-displacement behavior and strain distribution on the surface of the cement column were measured for loading, unloading, and reloading. Test geometries were modelled using nonlinear, axisymmetric, finite element analyses, which incorporated Coulomb friction elements at the titanium alloy-cement interface. Initial residual stresses, due to curing of the cement column, were modeled by thermal contraction of the cement. Good agreement was obtained between load-displacement curves and surface strains predicted from the nonlinear analysis and those obtained from experiments, when a coefficient of friction of 0.3 was assumed for the stem-cement interface. These results show that, in the absence of chemical adhesion, the load-displacement behavior of a stem-cement composite can be described completely in terms of the friction at the interface and the residual stresses normal to the interface.

Animals↗

PubMed1983

Discrete nonlinear reduction model for horizontal cell response in the carp retina.

In this paper we present a new model for the spatial behavior of the external horizontal cells in the carp retina which elucidates more precisely the mechanisms of their intercellular connections. The area and intensity effects of a brief light stimulus on the L-type response were recorded and analysed. We discovered clear nonlinearities in the behavior of the response which could not be explained by the simple summative model presently accepted. Proposed here is a new model, the spatial reduction mechanism, which explains the experimental evidences including these nonlinearities. We also present the results of a computer simulation using our new model in parallel with the actual results obtained experimentally. The physiological implications of these findings are discussed.

Animals↗

PubMed2004

Effects of liner stiffness for trans-tibial prosthesis: a finite element contact model.

The socket liner plays a crucial role in redistribution of the interface stresses between the stump and the socket, so that the peak interface stress could be reduced. However, how the peak stress is affected by various liner stiffnesses is still unknown, especially when the phenomenon of the stump slide within the socket is considered. This study employed nonlinear contact finite element analyses to study the biomechanical reaction of the stump sliding with particular attention to the liner stiffness effects of the trans-tibial prosthesis. To validate the finite element outcomes, experimental measurements of the interface stresses and sliding distance were further executed. The results showed that the biomechanical response of the stump sliding are highly nonlinear. With a less stiff liner, the slide distance of the stump would increase with a larger contact area. However, this increase in the contact area would not ensure a reduction in the peak interface stress and this is due to the combined effects of the non-uniform shape of the socket and the various sliding distances generated by the different liner stiffnesses.

Amputation, Surgical↗

PubMed1991

Reversible inhibition of voltage-dependent outer hair cell motility and capacitance.

Outer hair cells (OHC) from the organ of Corti are capable of fast voltage-induced length changes (Santos-Sacchi and Dilger, 1988), suggesting that an associated voltage sensor should reside in the OHC plasma membrane. Voltage-dependent mechanical responses and nonlinear charge movement of isolated OHCs from the guinea pig were analyzed using the whole-cell voltage-clamp technique. Ionic currents in the cells were blocked. Nonlinear voltage-dependent charge movement or, correspondingly, voltage-dependent capacitance was measured with step or AC analysis. OHC movements were measured either from video or using a differential photodiode technique. Maximum charge movements up to 2.5 pC were measured in OHCs from the low-frequency region of the cochlea. Both AC and step analyses indicated a peak nonlinear capacitance of 16-17 pF. The voltage dependence was fit to a Boltzmann relation with the step analysis indicating a maximum nonlinear capacitance at -23 mV step potential from a holding potential of about -120 mV, and AC analysis indicating a maximum at a holding potential near -40 mV. AC analysis probably provides a more accurate evaluation of voltage dependence. Measures of OHC motility magnitude versus voltage follow the nonlinear capacitance-voltage function obtained from AC measures. Treatment of the cells with gadolinium ions (0.5-1 mM) blocked OHC motility. This treatment also produced a shift of the nonlinear capacitance function along the voltage axis in the depolarizing direction, which can be explained by membrane surface charge screening. However, maximum capacitance was reduced as well and may correspond to the reduction or abolition of OHC motility in response to gadolinium treatment. Gadolinium effects were reversible. Nonlinear capacitance is not a function of membrane deformation due to length changes, since removal of OHC cytosol via the patch pipette abolished longitudinal movement but did not reduce nonlinear charge movement. It is interesting to note that the nonlinear capacitance will dynamically influence the time constant of the OHC during acoustically evoked receptor potential generation.

Animals↗

PubMed2006

Role of reflex dynamics in spinal stability: intrinsic muscle stiffness alone is insufficient for stability.

Spinal stability is related to both the intrinsic stiffness of active muscle as well as neuromuscular reflex response. However, existing analyses of spinal stability ignore the role of the reflex response, focusing solely on the intrinsic muscle stiffness associated with voluntary activation patterns in the torso musculature. The goal of this study was to empirically characterize the role of reflex components of spinal stability during voluntary trunk extension exertions. Pseudorandom position perturbations of the torso and associated driving forces were recorded in 11 healthy adults. Nonlinear systems-identification analyses of the measured data provided an estimate of total systems dynamics that explained 81% of the movement variability. Proportional intrinsic response was less than zero in more than 60% of the trials, e.g. mean value of P(INT) during the 20% maximum voluntary exertion trunk extension exertions -415+/-354N/m. The negative value indicated that the intrinsic muscle stiffness was not sufficient to stabilize the spine without reflex response. Reflexes accounted for 42% of the total stabilizing trunk stiffness. Both intrinsic and reflex components of stiffness increased significantly with trunk extension effort. Results reveal that reflex dynamics are a necessary component in the stabilizing control of spinal stability.

Humans↗

PubMed1998

One-dimensional, nonlinear determinism characterizes heart rate pattern during paced respiration.

This study focuses on the dynamic pattern of heart rate variability in the frequency range of respiration, the so-called respiratory sinus arrhythmia. Forty experimental time series of heart rate data from four healthy adult volunteers undergoing a paced respiration protocol were used as an empirical basis. For pacing-cycle lengths >8 s, the heartbeat intervals are shown to obey a rule that can be expressed by a one-dimensional circle map (next-angle map). Circle maps are introduced as a new type of model for time series analyses to characterize the nonlinear dynamic pattern underlying the respiratory sinus arrhythmia during voluntary paced respiration. Although these maps are not chaotic, the dynamic pattern shows typical imprints of nonlinearity. By starting from a piecewise linear model, which describes the different circle maps obtained from the empirical time series for various pacing frequencies, time invariant measures can be introduced that characterize the dynamic pattern of heart rate variability during voluntary slow-paced respiration.

Adult↗