Adaptive system identification applied to the biomechanical response of the human trunk during sudden loading

Epidemiological evidence indicates that sudden loading of the torso is a risk factor for low back injury. Accurately quantifying the time-varying loading of the spine during sudden loading events and how these loading profiles are affected by workplace factors such as fatigue, expectation, and training can potentially lead to intervention strategies that can reduce these risks. Electromyographic and trunk motion data were collected from six male participants who performed a series of sudden loading trials with varying levels of expectation (no preview, 300-ms audible preview), fatigue (no fatiguing exertion preceding sudden load, short duration/high intensity fatiguing exertion preceding sudden load), and training (untrained, trained). These data were used as inputs to an adaptive system identification model wherein time-varying lower back stiffness, torque, work, and impulse magnitudes were calculated. Results indicated that expectation significantly increased peak and average stiffness by 70% and 113%, respectively, and significantly decreased peak torque, work, and impulse magnitudes by 36%, 50%, and 45%, respectively. Training significantly decreased peak torque and work by 25% and 34%, respectively. The results also showed a significant interaction between expectation and training wherein training had a positive effect during the trials with preview but no effect during the trials with no preview (increased peak stiffness by 17% and decreased impulse magnitude by 43%).     

An analytically solvable model for biomechanical response of the cornea to refractive surgery

An analttically solvable model that considers the elasticity of the cornea is developed for use in the current and novel corneal refractive surgery procedures. The model assumes that the cornea is a thin spheroid shell with an elastic response to intraocular pressure. The value of the Young's modulus of the post-operative cornea and its dependence on the geometric parameters of the ablation zone are estimated employing "best-fit" approach to nomograms currently used in corneal refractive surgery. These elasticity parameters are applied for quantitative modeling of different types of refractive surgery for myopia.     

An EMG-assisted model of loads on the lumbar spine during asymmetric trunk extensions

An EMG-assisted, low-back, lifting model is presented which simulates spinal loading as a function of dynamic, asymmetric, lifting exertions. The purpose of this study has been to develop a model which overcomes the limitations of previous models including static or isokinetic mechanics, inaccurate predictions of muscle coactivity, static interpretation of myoelectric activity, and physiologically unrealistic or variable muscle force per unit area. The present model predicts individual muscle forces from processed EMG data, normalized as a function of trunk angle and asymmetry, and modified to account for muscle length and velocity artifacts. The normalized EMGs are combined with muscle cross-sectional area and intrinsic strength capacity as determined on a per subject basis, to represent tensile force amplitudes. Dynamic internal and external force vectors are employed to predict trunk moments, spinal compression, lateral and anterior shear forces. Data from 20 subjects performing a total of 2160 exertions showed good agreement between predicted and measured values under all trunk angle, asymmetry, velocity, and acceleration conditions. The design represents a significant step toward accurate, fully dynamic modeling of the low-back in multiple dimensions. The benefits of such a model are the insights provided into the effects of motion induced, muscle co-activity on spinal loading in multiple dimensions.     

An improved biomechanical model for simulating the strain of the hand-arm system under vibration stress

In order to define relationships between the vibration stress and the strain of the human hand-arm system a biomechanical model was developed. The four masses of the model representing the hand, the forearm and the upper arm were connected by dampers and springs in two perpendicular directions. Simulating muscle activity, damped torsion springs were included additionally. The motions of the model were described by a differential matrix equation which was solved by using a ‘transfer matrix routine’ as well as by numerical integration. Thus, functions with harmonic or transient time courses could be selected as an excitation. The simulated vibrations were compared with those of other hand-arm models. The forces and torques transmitted between the masses, and the energy dissipated by the dampers were computed for several combinations of exciter frequencies and accelerations. The dependence of torques upon excitation agreed fairly well with the behaviour of the arm muscles under vibration as described by various investigators. At frequencies above 100 Hz the energy was dissipated mainly by the dampers between the masses near to the exciter. Transferring this result to the hand-arm system it shows that at high frequencies energy is dissipated by the hand and its palmar tissues and this might be one cause for the incidence of vibration-induced white finger disease.     

An enhanced agent based model of the immune system response

We describe some recent enhancements introduced in C-ImmSim, a simulator of the immune system response that we have been developing for a number of years along with preliminary results produced by the simulation of the Highly Active Anti-Retroviral Therapy in HIV-1 infected patients.     

A biomechanical model of the lumbosacral joint during lifting activities

A biomechanical model of the lumbosacral region was constructed for the purpose of systematically studying the combined stresses and strains on the local ligaments, muscles and disc tissue during sagittal plane two-handed lifting. The model was validated in two ways. The first validation was a comparison of experimental study results with model predictions. In general predictions compared very reasonably with observed values of several authors with the exception of strain predictions on the articular ligaments. Second, a sensitivity analysis was performed over a wide range of lifting tasks. The predicted stress/strain values followed anticipated patterns and were of reasonable magnitudes. On the basis of the results of the sensitivity analysis it was concluded that typical lifting tasks can lead to excessive disc compressive forces, muscle moment generation requirements, and possibly lumbodorsal fascia strains. Conversely, annulus rupture of a healthy disc due to overstrain appears very unlikely.     

On the mechanism of adaptive response in human cells

There was investigated one of the mechanisms of adaptive response, related to chromosome aberrations induced by gamma-rays, in lymphocytes of healthy donors and donors with hereditary diseases (Marfan's syndrome and homocystinurea) whose cells are repair-deficient. 3H-thymidine treatment was used as an adaptive dose in G1-period of cell cycle and 8-methoxypsoralen (8-MOP), activated with UV-light, was used as a challenge agents. Cells of healthy donors and cells of patients with Marfan's syndrome had normal adaptive response in relation to gamma-irradiation and photomutagenic action of 8-MOP. There was no induction of adaptive response in realation to gamma-irradiation and 8-MOP photomutagenic action in cells of patients with homocystinurea. The cells from donors characterised with normal repair system and lack of adaptive response 8-MOP photomutagenic action wasn't modified by 3H-thymidine. We have found parallelism of adaptive response protective effect against chromosome aberrations, induced by UV activated 8-MOP and gamma-rays in repair proficient cells of healthy donors and repair deficient cells of patients with Marfan's syndrome. These data lead us to conclusion that mechanism of adaption, at least in some cases has no connection with repair process modification.     

The adaptive response of human skin to the savanna

In the early stage of human evolution, as the hominids began to inhabit the savanna mosaic in Africa some three or four million years ago, a functional complex of skin features contributed to their effective exploitation of resources and survival in the new environment. Thermal radiation from the sun combined with internally generated heat from muscular effort posed problems of thermoregulation. As a mechanism for dissipating body heat and maintaining brain temperature, eccrine sweat glands throughout the body surface combined with reduction in body hair enhanced the evaporative cooling effects of sweating. As body hair diminished, deeply pigmented skin was selected for as a protection against harmful ultraviolet radiation. When human populations left the equatorial regions of Africa, the adaptive significance of deeply pigmented skin may have shifted in response to other factors, such as latitude, diet and cultural pratices. We view the structure and function of human skin within a comparative and evolutionary framework that focuses on the environment in which the hominids evolved.     

An electromyographical comparison of trunk muscle activity during isometric trunk and dynamic strengthening exercises

The purpose of this study was to compare rectus abdominis and erector spinae muscle activity during isometric (prone bridge [PB] and superman [SM]) and dynamic strengthening exercises (back squat, front squat [FS], and military press). Participants (n = 10, age 21.8 ± 2.6 years; body mass 82.65 ± 10.80 kg, 174.5± 7.2 cm), performed each exercise in a randomized order, using a repeated-measures design. Electromyographical (EMG) activity (sampling at 2,000 Hz) of the rectus abdominis (RA) and the erector spinae (ES) muscles was recorded throughout the duration of the exercises. Intraclass correlations demonstrated the highest levels of reliability for muscle activity during the isometric exercises; however, all exercises demonstrated high level of reliability (r = 0.764-0.998, p ≤ 0.01). The PB demonstrated significantly greater (p < 0.01) RA activity compared to all other exercises. The ES activity was significantly (p < 0.01) greater during the FS (1.010 ± 0.308 root mean square value [RMS (V)]) and SM (0.951 ± 0.217 RMS[V]) and compared to all other exercises, although there was no significant difference (p > 0.05) between the FS and the SM exercise. The PB may be the most suitable exercise for strengthening the RA, compared to dynamic exercises at a low to moderate load, because of a higher level of muscle activity. The FS may be a useful alternative to isometric exercises when strengthening the ES, because it results in slightly higher muscle activity levels when using only a light to moderate load. Because of the dynamic nature of the FS, this may also be more beneficial in transferring to activities of daily living and sporting environments.     

Testing of a biomechanical model of the lumbar muscle force distribution using quasi-static loading exercises.

The study of lumbar muscle force distribution in response to externally applied loads is based on the introduction of biomechanical models of the lumbar region. The evaluation of such models requires the execution of loading exercises while monitoring the EMG activity of certain lumbar muscles. This work uses muscle activity maps as the major design tool of such exercises, provided that the subject is constrained to an upright erect posture. The maps describe the predicted muscle force for a given combination of externally applied bending moments. A series of shoulder adduction exercises were designed and the EMG signals of eight lumbar muscles were measured while subjects performed the exercises. The results show good agreement between the model predictions and the EMG measurements, especially when the load and the muscle were contralateral to one another.     

Biomechanical response of human spleen in tensile loading

Blunt splenic injuries are most frequently caused as a result of motor vehicle collisions and are associated with high mortality rates. In order to accurately assess the risk of automotive related spleen injuries using tools such as finite element models, tissue level tolerance values and suitable material models must be developed and validated based on appropriate biomechanical data. This study presents a total of 41 tension tests performed on spleen parenchyma coupons and 29 tension tests performed on spleen capsule/parenchyma coupons. Standard dog-bone coupons were obtained from fresh human spleen and tested within 48 h of death. Each coupon was tested once to failure at one of the four loading rates to investigate the effects of rate dependence. Load and acceleration data were obtained at each of the specimen grips. High-speed video and optical markers placed on the specimens were used to measure local displacement. Failure stress and strain were calculated at the location of failure in the gage length of the coupon. The results of the study showed that both the spleen parenchyma and the capsule are rate dependent, with higher loading rates yielding higher failure stresses and lower failure strains. The results also show that the failure stress of the splenic capsule is significantly greater than that of the underlying parenchyma. Overall, this study provides novel biomechanical data that demonstrate the rate dependent tissue level tolerance values of human spleen tissue in tensile loading, which can aid in the improvement of finite element models used to assess injury risk in blunt trauma.     

Differences in trabecular bone of leptin-deficient ob/ob mice in response to biomechanical loading

Objective: It is known that bone mineral density (BMD) and the strength of bone is predicted by body mass. Fat mass is a significant predictor of bone mineral density which correlates with body weight. This suggests that body fat regulates bone metabolism first by means of hormonal factors and second that the effects of muscle and loading are signaling factors in mechanotransduction. Leptin, a peptide hormone produced predominantly by white fat cells, is one of these hormonal factors. The aim of this study was to investigate and measure by micro-CT the different effects of weight-bearing on trabecular bone formation in mice without the stimulation of leptin.     

Dynamics of the head-neck complex in response to the trunk horizontal vibration: modeling and identification

Although many studies exist concerning the influence of seat vibration on the head in the seated human body, the dynamic response of the head-neck complex (HNC) to the trunk vibration has not been well investigated. Little quantitative knowledge exists about viscoelastic parameters of the neck. In this study, the dynamics of the HNC is identified when it is exposed to the trunk horizontal (fore-and-aft) vibration. The frequency response functions between the HNC angular velocity and the trunk horizontal acceleration, corresponding to four volunteers, are obtained in the frequency range of 0.5 Hz to 10 Hz. A fourth-order mathematical model, derived by considering a double-inverted-pendulum model for the HNC, is designed to simulate the dynamic response of the HNC to the trunk horizontal vibration. The frequency domain identification method is used to determine the coefficients of the mathematical model of the HNC. Good agreement has been obtained between experimental and simulation results. This indicates that the system, similar to the designed fourth-order model, has mainly two resonance frequencies. The viscoelastic parameters of the neck, including the spring and damping coefficients, are then obtained by use of the optimization method.     

Mechanism of adaptive response: Estimation of human lymphocyte adaptive response to radiation using various criteria

Blood lymphocytes of 15 healthy donors have been investigated for their ability to reduce radiosensitivity after low-dose irradiation-radio-induced adaptive response (AR). The frequency of unstable chromosome aberrations was used to evaluate cell radiosensitivity after the irradiation of cells in low adaptive (5 cGy) and high challenge (1 Gy) doses in comparison with the effect of challenge irradiation only. Three indexes have been used, i.e., (A) the frequency of cells with aberrations per total analyzed cell, (B) the number of chromosome aberrations per one cell, (C) and the number of chromosome aberrations per one aberrant cell. It was found that the donors can be divided in the four following groups: 1. AR was not estimated any of the indexes used; 2. AR was estimated with indexes A and B, but not C; 3. AR was shown by indexes B and C; 4. AR was evident with all three indexes. The generally accepted AR repair model only explains the appearance of group-3 and-4 donors, but not group-2. For the purpose of understanding the AR mechanisms and the difference in AR estimations with various criteria, the metaphase distribution by the number of chromosome aberrations has been analyzed for each donor. It was shown that, in group-2 donors, the number of cells without aberrations after adaptive and challenge irradiations was significantly higher than after irradiation with a challenge dose only. Thus, in this group, AR is formed as a result of the changed frequency of cells in the 0 class (population shift). A similar shift is observed in the metaphase distribution in the donors of group 4, but not in group 3. The data obtained show that AR is probably a result of several processes, including the activation of the reparation of premutational genome damages, population shifts evident in the frequency of undamaged cells, and, possibly, the activation of apoptotic cell death. The complex character of AR is reflected to different degrees in each criterion of radiosensitivity.