Biodynamic Model Research Articles

Biodynamic modelling of the accumulation of Ag, Cd and Zn by the deposit-feeding polychaete Nereis diversicolor: Inter-population variability and a generalised predictive model

Biodynamic parameters of the ragworm Nereis diversicolor from southern Spain and south England were experimentally derived to assess the inter-population variability of physiological parameters of the bioaccumulation of Ag, Cd and Zn from water and sediment. Although there were some limited variations, these were not consistent with the local metal bioavailability nor with temperature changes. Incorporating the biodynamic parameters into a defined biodynamic model, confirmed that sediment is the predominant source of Cd and Zn accumulated by the worms, accounting in each case for 99% of the overall accumulated metals, whereas the contribution of dissolved Ag to the total accumulated by the worm increased from about 27 to about 53% with increasing dissolved Ag concentration. Standardised values of metal-specific parameters were chosen to generate a generalised model to be extended to N. diversicolor populations across a wide geographical range from western Europe to North Africa. According to the assumptions of this model, predicted steady state concentrations of Cd and Zn in N. diversicolor were overestimated, those of Ag underestimated, but still comparable to independent field measurements. We conclude that species-specific physiological metal bioaccumulation parameters are relatively constant over large geographical distances, and a single generalised biodynamic model does have potential to predict accumulated Ag, Cd and Zn concentrations in this polychaete from a single sediment metal concentration.

Optimization of Biodynamic Seated Human Models Using Genetic Algorithms

Many biodynamic models have been derived using trial and error curve-fitting technique, such that the error between the computed and measured biodynamic response functions is minimum. This study developed a biomechanical model of the human body in a sitting posture without backrest for evaluating the vibration transmissibility and dynamic response to vertical vibration direction. In describing the human body motion, a three biomechanical models are discussed (two models are 4-DOF and one model 7-DOF). Optimization software based on stochastic techniques search methods, Genetic Algorithms (GAs), is employed to determine the human model parameters imposing some limit constraints on the model parameters. In addition, an objective function is formulated comprising the sum of errors between the computed and actual values (experimental data). The studied functions are the driving-point mechanical impedance, apparent mass and seat- to-head transmissibility functions. The optimization process increased the average goodness of fit and the results of studied functions became much closer to the target values (Experimental data). From the optimized model, the resonant frequencies of the driver parts computed on the basis of biodynamic response functions are found to be within close bounds to that expected for the human body.

Optimization of Biodynamic Seated Human Models Using Genetic Algorithms

Many biodynamic models have been derived using trial and error curve-fitting technique, such that the error between the computed and measured biodynamic response functions is minimum. This study developed a biomechanical model of the human body in a sitting posture without backrest for evaluating the vibration transmissibility and dynamic response to vertical vibration direction. In describing the human body motion, a three biomechanical models are discussed (two models are 4-DOF and one model 7-DOF). Optimization software based on stochastic techniques search methods, Genetic Algorithms (GAs), is employed to determine the human model parameters imposing some limit constraints on the model parameters. In addition, an objective function is formulated comprising the sum of errors between the computed and actual values (experimental data). The studied functions are the driving-point mechanical impedance, apparent mass and seat- to-head transmissibility functions. The optimization process increased the average goodness of fit and the results of studied functions became much closer to the target values (Experimental data). From the optimized model, the resonant frequencies of the driver parts computed on the basis of biodynamic response functions are found to be within close bounds to that expected for the human body.

Influence of Support Conditions on Vertical Whole-body Vibration of the Seated Human Body

The vibration transmission to the lumbar and thoracic segments of seated human subjects exposed to whole body vibration of a vehicular nature have been mostly characterised without the back and hand supports, which is not representative of general driving conditions. This non-invasive experimental study investigated the transmission of vertical seat vibration to selected vertebrae and the head along the vertical and fore-aft axes of twelve male human subjects seated on a rigid seat and exposed to random vertical excitation in the 0.5-20 Hz range. The measurements were performed under four different sitting postures involving combinations of back support conditions and hands positions, and three difference magnitudes of vertical vibration (0.25, 0.5 and 1.0 m/s(2) rms acceleration). The results showed significant errors induced by sensor misalignment and skin effects, which required appropriate correction methodologies. The averaged corrected responses revealed that the back support attenuates vibration in the vertical axis to all the body locations while increasing the fore-aft transmissibility at the C7 and T5. The hands position generally has a relatively smaller effect, showing some influences on the C7 and L5 vibration. Sitting without a back support resulted in very low magnitude fore-aft vibration at T5, which was substantially higher with a back support, suggestive of a probable change in the body's vibration mode. The effect of back support was observed to be very small on the horizontal vibration of the lower thoracic and lumbar regions. The results suggest that distinctly different target body-segment biodynamic functions need to be defined for different support conditions in order to represent the unique contribution of the specific support condition. These datasets may then be useful for the development of biodynamic models.

Bioaccumulation of triclocarban in Lumbriculus variegatus

The antimicrobial triclocarban (TCC) has been detected in streams and municipal biosolids throughout the United States. In addition, TCC and potential TCC transformation products have been detected at high levels (ppm range) in sediments near major cities in the United States. Previous work has suggested that TCC is relatively stable in these environments, thereby raising concerns about the potential for bioaccumulation in sediment-dwelling organisms. Bioaccumulation of TCC from sediments was assessed using the freshwater oligochaete Lumbriculus variegatus. Worms were exposed to TCC in sediment spiked to 22.4 ppm to simulate the upper bound of environmental concentrations. Uptake from laboratory-spiked sediment was examined over 56 d for TCC and 4,4'-dichlorocarbanilide (DCC), a chemical impurity in and potential transformation product of TCC. The clearance of TCC from worms placed in clean sediment was also examined over 21 d after an initial 35-d exposure to TCC in laboratory-spiked sediment. Concentrations of TCC and DCC were monitored in the worms, sediment, and the overlying water using liquid chromatography/tandem mass spectrometry. Experimental data were fitted using a standard biodynamic model to generate uptake and elimination rate constants for TCC in L. variegatus. These rate constants were used to estimate steady-state lipid (lip)- and organic carbon (OC)-normalized biota-sediment accumulation factors (BSAFs) for TCC and DCC of 2.2+/-0.2 and 0.3+/-0.1 g OC/g lip (goc/glip), respectively. Alternatively, directly measured BSAFs for TCC and DCC after 56 d of exposure were 1.6+/-0.6 and 0.5+/-0.2 goc/glip, respectively. Loss of TCC from pre-exposed worms followed first-order kinetics, and the fitted elimination rate constant was identical to that determined from the uptake portion of the present study. Overall, study observations indicate that TCC bioaccumulates from sediments in a manner that is consistent with the traditional hydrophobic organic contaminant paradigm.

Effects of Posture and Movement on Vibration Transmissibility Affecting Human Reach Performance under Vehicle Vibration

Vibration transmissibility to the human body is a function of both vehicle vibration characteristics and postures associated with the performance of movements. The majority of earlier studies investigating upper body vibration transmissibility considered only a static posture excluding dynamic limb movements (Amirouche, 1987; Wei and Griffin, 1998; Rosen and Arcan, 2003; Yoshimura et al, 2005; Liang and Chiang, 2006). A few recent studies reported the effect of vehicle vibration on arm reaching movements through the description of fingertip deviation from a desired trajectory (Rider and Chaffin, 2003, 2004). The present work investigates the variation of vibration transmissibility to upper extremities as a function of dynamic posture changes along the intended reach trajectory. Dynamic reach movements in the direction of targets distributed in the right hemisphere of a vehicle operator are analyzed as a function of vibration characteristics and movement directions. Thirteen subject performed right hand reach movements in various directions to final/end target location as well as intermediate target locations selected along the trajectory of movement performed to the end target. The established database of upper body segments transmissibility is used to develop an active biodynamic human model.

Biodynamic modelling and the prediction of Ag, Cd and Zn accumulation from solution and sediment by the polychaete Nereis diversicolor

Biodynamic modelling has been used to predict bioaccumulated concentrations of Ag, Cd and Zn in the deposit-feeding polychaete Nereis diversicolor from 5 metal-contaminated estuar- ies in SW England and a relatively non-contaminated estuary in SE England. The modelling employed previously measured physiological parameters of bioaccumulation — uptake rate constant, assimilation efficiency (AE) and efflux rate constants after uptake from water and sediment inges- tion — and measured sediment metal concentrations specific for each population. AEs were consid- ered to relate to metals in the organic component of the ingested sediment and ingestion rates were therefore expressed in these terms, with the further assumption that the total sediment metal concen- tration is a proxy for the metal concentration in the sediment organic component. A range of growth rate constants was extracted from the literature, as were concentration ranges of dissolved Ag, Cd and Zn in contaminated coastal waters. The model showed that >99% Cd and >98% Zn accumulated by N. diversicolor is derived from sediment ingestion; more bioaccumulated Ag is derived from solu- tion, the percentage contribution of the dissolved source increasing from 46 to 80% with an increase in Ag dissolved concentration from low to high values for coastal waters. Bioaccumulated metal con- centrations predicted from the model generally showed excellent agreement with independently measured concentrations in field-collected worms, supporting the assumptions made in the model.

Measurement and Modeling of Polychlorinated Biphenyl Bioaccumulation from Sediment for the Marine Polychaete Neanthes arenaceodentata and Response to Sorbent Amendment

Bioaccumulation rates of polychlorinated biphenyls (PCBs) for the marine polychaete Neanthes arenaceodentata were characterized, including PCB uptake rates from water and sediment, and the effect of sorbent amendment to the sediment on PCB bioavailability, organism growth, and lipid content. Physiological parameters were incorporated into a biodynamic model to predict contaminant uptake. The results indicate rapid PCB uptake from contaminated sediment and significant organism growth dilution during time-series exposure studies. PCB uptake from the aqueous phase accounted for less than 3% of the total uptake for this deposit-feeder. Proportional increase of gut residence time and assimilation efficiency as a consequence of the organism's growth was assessed by PCB uptake and a reactor theory model of gut architecture. Pulse-chase feeding and multilabeled stable isotope tracing techniques proved high sediment ingestion rates (i.e., 6-10 times of dry body weight per day) indicating that such deposit-feeders are promising biological indicators for sediment risk assessment. Activated carbon amendment reduced PCB uptake by 95% in laboratory experiments with no observed adverse growth effects on the marine polychaete. Biodynamic modeling explained the observed PCB body burdens for N. arenaceodentata, with and without sorbent amendment.

Bioaccumulation of essential metals (Co, Mn and Zn) in the king scallop Pecten maximus: seawater, food and sediment exposures

In order to understand the bioaccumulation of essential metals in filter-feeding mollusks living in soft sediments, the uptake and depuration kinetics of three elements (Co, Mn and Zn) were investigated in the king scallop Pectenmaximus exposed via seawater, food, or sediment, using radiotracer techniques. The scallops were collected in April 2005 in the Pertuis Breton, France and acclimated to laboratory conditions for 8 weeks prior to the experimental exposures. Dissolved metals were efficiently bioconcentrated with mean concentration factors (CFs) ranging from 65 (Co) to 94 (Mn) after 7 days of exposure. Feeding experiments using microalgae Skeletonema costatum (diatom) or Isochrysis galbana (flagellate) showed that metal assimilation efficiency (AE) and retention (Tb½) were strongly influenced by food source. For Co, AE was higher when ingested with I. galbana (29 vs. 4%), whereas Mn and Zn AE was higher for S. costatum (82 vs. 44% and 86 vs. 68%, respectively). Transfer factors (TFs) in P. maximus exposed to radiolabelled sediment were 3–4 orders of magnitude lower than CFs. Nevertheless, the fraction of sediment-bound metals that was taken up was efficiently absorbed in scallop tissues (>85%). Whatever the exposure pathway, metals were strongly retained in the kidneys of P. maximus. Due to poor determination of Mn biokinetics (and related parameters) in scallops exposed through sediment, the relative contribution of the three different pathways could be determined only for Co and Zn using a biodynamic model. The particulate pathway (i.e. food or sediment) appeared to be the main route for bioaccumulation of both metals in this scallop. In addition, even though P. maximus displayed different AEs for Co and Zn according to the food, results of the model were only slightly affected, if any, by change in the dietary parameters (AE and depuration rate constant, ke).

Biodynamic modelling and the prediction of accumulated trace metal concentrations in the polychaete Arenicola marina

The use of biodynamic models to understand metal uptake directly from sediments by deposit-feeding organisms still represents a special challenge. In this study, accumulated concentrations of Cd, Zn and Ag predicted by biodynamic modelling in the lugworm Arenicola marina have been compared to measured concentrations in field populations in several UK estuaries. The biodynamic model predicted accumulated field Cd concentrations remarkably accurately, and predicted bioaccumulated Ag concentrations were in the range of those measured in lugworms collected from the field. For Zn the model showed less but still good comparability, accurately predicting Zn bioaccumulation in A. marina at high sediment concentrations but underestimating accumulated Zn in the worms from sites with low and intermediate levels of Zn sediment contamination. Therefore, it appears that the physiological parameters experimentally derived for A. marina are applicable to the conditions encountered in these environments and that the assumptions made in the model are plausible.

A Novel Two-Stage Framework for Musculoskeletal Dynamic Modeling: An Application to Multifingered Hand Movement

In this paper, we present a new computational framework for biodynamic modeling of human movement. The framework decouples the conventional dynamic modeling process into two stages: in the first stage, two-component "agonist-antagonist" torque actuators under hypothesized and testable parametric control drive the forward dynamics, and parameters are identified by tracking both kinematics and kinetics; the second stage completes the mapping from the muscle-tendon forces to the predicted joint torques. An empirical test using multifinger grasping movement data was conducted to illustrate the application of the proposed framework and showed that the model reproduced the measurement accurately in both kinematics and kinetics. The torque components exhibited consistent spatial-temporal patterns across joints, digits, and subjects. The muscle-tendon forces computed based on the model-predicted kinematics and kinetics had the peak values within the same order of magnitude as in vivo data reported in the literature. The potential to predict was also demonstrated as we applied the control parameters of one subject to another and achieved close matches.

Development and validation of a biodynamic model for predicting multi-finger movements in cylinder-grasping tasks†

This article describes the development and validation of a model for predicting multi-finger movements in grasping activities. The model builds upon a newly proposed approach that incorporates forward dynamics and a system identification procedure, and is amenable to empirical tests. A database of multi-fingered grasping movements performed by 28 subjects was established and divided into four sets, one for model development and three for model validation. In the development phase, model parameter values were estimated by the iterative system identification procedure through a physics-based heuristic algorithm. The estimated parameter values were then statistically synthesised and integrated into the prediction model. In the validation phase, the model was applied to three novel datasets containing different grasping movements involving objects of varied sizes and different subjects. The results demonstrated the model's ability to predict hand prehensile movements with error magnitudes comparable to the inter-person variability in performing such movements. New insights into the control of multi-fingered hand prehensile movements at the systems and joint levels emerged from the model development and validation process. The current study contributes to building a foundation for long-term development of realistic biodynamic simulation of multi-finger hand movements. Such simulation capabilities will aid in design of hand-operated tools, devices or hand-intensive work for proactive ergonomics and in evaluation as well as treatment of functional impairment of the hand.

The barnacle Balanus amphitrite as a biomonitor for Cd: Radiolabelled experiments

Radiolabelled experiments were carried out to measure necessary parameters in the development of a biodynamic ecotoxicological simulation model of Cd accumulation in the barnacle biomonitor Balanus amphitrite. The Cd uptake rate constant from the dissolved phase, the Cd assimilation efficiency (AE) from suspended particulate matter (SPM) and the efflux rate constant were obtained using 109Cd. A Cd uptake rate constant from the dissolved phase ( k u) of 0.0072 L g −1 h −1 was determined for the barnacle under environmentally realistic dissolved Cd concentrations (maximum of 400 ng L −1). Cd AE from SPM was determined from the barnacle feeding on SPM with low and high chl a concentrations, resulting in AEs of 39.0% and 48.7%, respectively, and an efflux rate of 0.0072 d −1. The difference between the AEs resulted from differences in chl a:SPM ratios suggesting a general tendency of higher AE when SPM is enriched with chl a. These results reinforce that the accuracy of ecotoxicological models for metal accumulation in organisms depends on how representative the selected food items are of the organism’s natural diet.

Synthesis of a Sequential Model of Muscles of a Man during Drop Landing on a Hard Platform

The objective of this paper is to identify the parameters of a new plane biodynamic model of a man jumping down on a hard base. In the modelling of this kind of dynamic phenomena, with dominant directions of human body motion and reactions, many simple and complex models have already been applied. Some of them were lumped parameter and unidirectional [Książek, 1999], [Fritz, 2001] or plane [Pandy, 1990], [Farley, 1998]. There are also spatial models, applied in the sport and medical investigations of the kinematics and dynamics of the human body [Morecki, 1990], [A. Nagano and col., 2005]. The presented approach has been founded on the results of experiments described in [Nosiadek, 2006] where body motions of 40 men volunteers were registered during drop jumping and landing from several assumed heights. By means of video cameras in 3D space and a dynamic platform linear and angular displacements of participants’ bodies and three mutually perpendicular components of the dynamical reactions between the feet and platform were measured. The numerical results of the displacements were numerically remodelled by the authors of the present paper and used for synthesis of models of muscles of a man during drop landing. Similar investigations can be found in [Pain and Challis, 2006].

Pathways of trace metal uptake in the lugworm Arenicola marina

Radiotracer techniques were used to determine the rates of trace metal (Ag, Cd and Zn) uptake and elimination (33 psu, 10 °C) from water and sediment by the deposit-feeding polychaete Arenicola marina, proposed as a test species for estuarine-marine sediments in whole-sediment toxicity tests. Metal uptake rates from solution increase with increasing dissolved metal concentrations, with uptake rate constants (± SE) (l g −1 d −1) of 1.21 ± 0.11 (Ag), 0.026 ± 0.002 (Zn) and 0.012 ± 0.001 (Cd). Assimilation efficiencies from ingested sediments were measured using a pulse-chase radiotracer feeding technique in two different lugworm populations, one from a commercial supplier (Blyth, Northumberland, UK) and the other a field-collected population from the outer Thames estuary (UK). Assimilation efficiencies ranged from 2 to 20% for Zn, 1 to 6% for Cd and 1 to 9% for Ag for the Northumberland worms, and from 3 to 22% for Zn, 6 to 70% for Cd and 2 to 15% for Ag in the case of the Thames population. Elimination of accumulated metals followed a two-compartment model, with similar efflux rate constants for Zn and Ag and lower rates of elimination of Cd from the slow pool. Efflux rate constants (± SE) of Zn and Ag accumulated from the dissolved phase were 0.037 ± 0.002 and 0.033 ± 0.006 d −1 whereas Cd was eliminated with an efflux rate constant one order of magnitude lower (0.003 ± 0.002 d −1). When metals were accumulated from ingested sediments, the efflux rate constants for the slow-exchanging compartment were of the same order of magnitude for the three metals, and of the same order of magnitude as those derived after the dissolved exposure for Zn and Ag (0.042 ± 0.004 and 0.056 ± 0.012 d −1 for Zn and 0.044 ± 0.012 and 0.069 ± 0.016 d −1 for Ag for the Northumberland and Thames populations, respectively). Cd accumulated from ingested sediments was eliminated with a rate constant not different from the fast-exchanging compartment after the water-only exposure (0.025 ± 0.012 and 0.020 ± 0.004 d −1 for the Northumberland and Thames populations, respectively). A biodynamic model was used to estimate the relative importance of the dissolved phase versus ingested sediment as source of metal for the worms, showing that more than 90% of the Zn and Cd and more than 70% of Ag in lugworms is accumulated from sediment ingestion at realistic environmental concentrations. The model also shows that metal accumulation is highly dependent on the ingestion rate and assimilation efficiency.

Toxicokinetics of Waterborne Trivalent Arsenic in the Freshwater Bivalve Corbicula fluminea

Arsenite (As(III)) uptake and elimination kinetics were studied in a freshwater bivalve, Corbicula fluminea, exposed to several nominal concentrations of As(III) (0, 100, 300, 500, and 1000 microg L(-1)) in a static 28-day assay, followed by a depuration stage of 14 days. At the end of each sampling time (days 0, 7, 28, and 42) whole-body portions were surveyed for total As concentrations and, complimentarily, surveyed for whole-body metallothionein (MT) induction to assess its role as a defense mechanism against exposure to As(III). Histochemical evaluation of the digestive gland was performed to verify As deposition and elimination in the tissue. Results show a significant increase in whole-body total As after 28 days of exposure for all treatments, followed by a decrease at the end of the depuration phase. Biodynamic kinetic models for As uptake and elimination were obtained from bioaccumulation data during the exposure phase, for all As treatments, by estimating uptake and elimination rate constants. Bioconcentration factors (BCFs) were estimated by the ratio of these constants. Results revealed that exposure to higher concentrations of As(III) causes a decrease in BCFs, suggesting that C. fluminea triggers effective regulatory mechanisms when exposed to higher concentrations of the metalloid. Significant induction of MT was detected during the exposure phase, followed by a decrease in MT concentration to control levels after depuration for all treatments. No significant differences in MT concentrations were observed between treatments. This finding may confirm the role of MT as part of the As regulation process, but its independence relative to concentrations of As(III) in water suggests that MT induction is not dose dependent. The histochemical evaluation provided clear evidence that As was effectively accumulated in the digestive gland during exposure and eliminated during depuration. The present work demonstrated that C. fluminea is capable of regulating As, even at exposures as high as 1000 microg L(-1) of waterborne As(III).

Human Body under Two-Directional Random Vibration

Human body behaviour under the influence of two-directional random vibrations is of great importance for optimal motor vehicle design. Efforts are being made in order to discover as much information about the influence of vibration on human body as possible. References show that the most attention has been paid to vertical vibration, although intensive research has been performed lately on the other sorts of excitation. This paper shows the results of the investigation of behaviour of human body, in seated position, under the influence of random two-directional (fore and aft and vertical vibration). The investigation is performed using an electro-hydraulic simulator, on a group of 18 healthy male participants. Subjects are asked to adjust level of fore and aft excitation in order to give the same sensation as vertical excitation. Experiments are performed in order to give results which might improve human body modeling in driving conditions. Excitation amplitudes (0, 55, 1, 75 and 2, 25 m/s2 R.M.S.) and seat backrest conditions (with (K) and without inclination (S)) were varied. Data results were analysed, and partial coherent and transfer functions analysis has been performed and results are given in detail. Human body transmissibility characteristics, under two directional random excitations, depend on its spatial position. Two directional random excitations affect human body transfer functions, which points out the fact that it behaves like a nonlinear dynamic system. This should be taken into consideration in synthesis of biodynamic vibration models of human body.

Three-Dimensional Joint Kinematics of the Upper Extremity in Reach Movements under Whole-Body Vibration Exposure

Simulation of human reach movements is an essential component for proactive ergonomic analysis and computer-aided engineering of biomechanical models. Most studies on reach kinematics described human movements in a static environment, however the models derived from these studies cannot be applied to the analysis of human reach movements in vibratory environments such as in-vehicle operations. Earlier studies on reach performance under vibration exposure focused mainly on fingertip end-point accuracy. This study analyzes three dimensional joint kinematics of the upper extremity in reach movements performed in static and vibratory conditions. The ultimate goal is to develop an active biodynamic model capable of simulating reach movements in vibratory environments. Thirteen seated subjects performed reach movements to four target directions distributed in the right hemisphere. The results show differences and similarities in the characteristics of movement patterns of upper body segments for static and dynamic environments. Identification of movement patterns in terms of joint kinematics can be used to determine some biodynamic principles of upper body segments coordination in reach motion.

Biodynamics model for operator head injury in stand-up lift trucks

Biodynamics and injury potential of operators in stand-up rider lift truck accidents have been investigated with a special focus on head injury. An anthropomorphic test device (ATD) model was used as an operator surrogate in computer simulations of off-the-dock (OTD) and tip-over (TO) accidents. The biomechanical model representing the ATD was developed based on rigid body segments, and then combined with a rigid body truck model in the accident simulations. The operator compartment of the truck model was enclosed with a rear door. The computed kinematics are in agreement with the results of previous experimental testing. A 2D finite element model of the head was created to compute head impact decelerations in the sagittal plane. Values of the head injury criterion for the TO cases were computed from the model and shown to compare favourably with experimental values. The results advance the state of knowledge concerning injury potential in TO and OTD accidents and simulation models for such accidents.

Seated whole body vibrations with high-magnitude accelerations—relative roles of inertia and muscle forces

Reliable computation of spinal loads and trunk stability under whole body vibrations with high acceleration contents requires accurate estimation of trunk muscle activities that are often overlooked in existing biodynamic models. A finite element model of the spine that accounts for nonlinear load- and direction-dependent properties of lumbar segments, complex geometry and musculature of the spine, and dynamic characteristics of the trunk was used in our iterative kinematics-driven approach to predict trunk biodynamics in measured vehicle's seat vibrations with shock contents of about 4 g (g: gravity acceleration of 9.8 m/s 2) at frequencies of about 4 and 20 Hz. Muscle forces, spinal loads and trunk stability were evaluated for two lumbar postures (erect and flexed) with and without coactivity in abdominal muscles. Estimated peak spinal loads were substantially larger under 4 Hz excitation frequency as compared to 20 Hz with the contribution of muscle forces exceeding that of inertial forces. Flattening of the lumbar lordosis from an erect to a flexed posture and antagonistic coactivity in abdominal muscles, both noticeably increased forces on the spine while substantially improving trunk stability. Our predictions clearly demonstrated the significant role of muscles in trunk biodynamics and associated risk of back injuries. High-magnitude accelerations in seat vibration, especially at near-resonant frequency, expose the vertebral column to large forces and high risk of injury by significantly increasing muscle activities in response to equilibrium and stability demands.