Fatigue influences the dynamic stability of the torso

Fatigue in the extensor muscles of the torso affects neuromuscular recruitment and control of the spine. The goal of this study was to test whether fatigue influences stability of dynamic torso movements. A controlled laboratory experiment measured the change in the maximum finite-time Lyapunov exponent, lambda(max), before and after fatigue of the extensor muscles. Non-linear analyses were used to compute stability from the embedding dimension and Lyapunov exponent recorded during repetitive dynamic trunk flexion tasks. Torso extensor muscles were fatigued to 60% of their unfatigued isometric maximum voluntary exertion force then stability was re-measured. Independent variables included fatigue, task asymmetry and lower-limb constraint. lambda(max) values increased with fatigue suggesting poorer dynamic stability when fatigued. Embedding dimension declined with fatigue indicating reduced dynamic complexity when fatigued. Fatigue-related changes in spinal stability may contribute to the risk of low-back injury during fatiguing occupational lifting tasks. The findings reported here indicate that one mechanism by which fatigue contributes to low back disorders may be spinal instability. This information may contribute to the development of ergonomic countermeasures to help prevent low back disorders.     

Interface stability influences torso muscle recruitment and spinal load during pushing tasks

Handle or interface design can influence torso muscle recruitment and spinal load during pushing tasks. The objective of the study was to provide insight into the role of interface stability with regard to torso muscle recruitment and biomechanical loads on the spine. Fourteen subjects generated voluntary isometric trunk flexion force against a rigid interface and similar flexion exertions against an unstable interface, which simulated handle design in a cart pushing task. Normalized electromyographic (EMG) activity in the rectus abdominus, external oblique and internal oblique muscles increased with exertion effort. When using the unstable interface, EMG activity in the internal and external oblique muscle groups was greater than when using the rigid interface. Results agreed with trends from a biomechanical model implemented to predict the muscle activation necessary to generate isometric pushing forces and maintain spinal stability when using the two different interface designs. The co-contraction contributed to increased spinal load when using the unstable interface. It was concluded that handle or interface design and stability may influence spinal load and associated risk of musculoskeletal injury during manual materials tasks that involve pushing exertions.     

Interface stability influences torso muscle recruitment and spinal load during pushing tasks

Handle or interface design can influence torso muscle recruitment and spinal load during pushing tasks. The objective of the study was to provide insight into the role of interface stability with regard to torso muscle recruitment and biomechanical loads on the spine. Fourteen subjects generated voluntary isometric trunk flexion force against a rigid interface and similar flexion exertions against an unstable interface, which simulated handle design in a cart pushing task. Normalized electromyographic (EMG) activity in the rectus abdominus, external oblique and internal oblique muscles increased with exertion effort. When using the unstable interface, EMG activity in the internal and external oblique muscle groups was greater than when using the rigid interface. Results agreed with trends from a biomechanical model implemented to predict the muscle activation necessary to generate isometric pushing forces and maintain spinal stability when using the two different interface designs. The co-contraction contributed to increased spinal load when using the unstable interface. It was concluded that handle or interface design and stability may influence spinal load and associated risk of musculoskeletal injury during manual materials tasks that involve pushing exertions.     

Experienced workers exhibit distinct torso kinematics/kinetics and patterns of task dependency during repetitive lifts and lowers

Individual differences in work methods may be related to the risk of injury during manual material handling tasks, yet existing evidence comparing experienced workers and novices is mixed. This study assessed torso kinematics and kinetics among six experienced workers and six novices during repetitive lifts/lowers under different task configurations (symmetric vs. asymmetric and lift vs. lower). Several important potential confounding effects were controlled. Peak kinematic and kinetic measures were typically higher among experienced workers and suggestive of exposure to higher levels of low back injury risk, though overall exposure levels were moderate. Work methods used by experienced workers were modified between task conditions, whereas novice behaviours were more consistent. Control of torso kinematics/kinetics may thus not be a primary factor in determining experienced worker's work methods, and future investigation is needed to establish if, or under what conditions, these methods are protective and/or should be the basis for interventions including training.

Practitioner Summary: Whether lifting experience reduces low back injury risk is unclear from earlier findings. Results from a controlled experiment suggest that lifting experience may not be associated consistently with reduced physical demands or injury risk. Further investigation is needed to assess the utility of training based on the methods of experienced workers.     

动力吸振器对转子系统气流激振稳定性影响分析

针对气流激振引起离心压缩机转子振动失稳的问题,提出了采用动力吸振方法提高转子系统的稳定性.首先建立流体激励作用下转子 吸振系统动力学模型;其次采用Lyapunov理论研究附加吸振器后流体激励作用下转子系统的稳定性;分析了吸振器参数变化对转子系统稳定性的影响;最后采用数值方法进行验证.结果表明,动力吸振方法能够有效地提高转子系统失稳转速,吸振器固有频率和阻尼比对转子稳定性有较大的影响.     

Robust stability considerations in optimal design of dynamic systems under uncertainty

This paper presents a method for the incorporation of robust stability criteria in the design of dynamic systems under uncertainty. Process systems are modelled via dynamic mathematical models, variations include both uncertain parameters and time-varying disturbances, while control structure selection and controller design is considered as part of the design optimization problem. Stability criteria are included, based on the concept of the measure of a matrix, to maintain desired dynamic characteristics, in a multiperiod design formulation. A combined flexibility-stabiluty analysis step is also introduced to ensure feasible and stable operation of the dynamic system in the presence of parametric uncertainties and process disturbances. The potential of the proposed approach is illustrated with a ternary distillation column design and control problem (featuring a rigorous tray-by-tray model).     

机械式扭转疲劳试验装置的仿真分析

为了检验汽车驱动轴在一定扭矩作用下的扭转疲劳寿命,设计了一种低成本的机械式试验装置。为了预计检验该装置能否满足要求,采用了多体动力学软件ADAMS建立了该试验装置的动力学模型,并对其在各个状态下的运动情况进行了仿真分析,得到了该装置在不同台架参数配置下所能实现的扭转角曲线等数据,并针对试验中可能出现的波形畸变问题分析了产生原因和解决办法,对于进一步完善和改进试验装置设计、指导试验进行具有重要意义。     

动态神经网络的输入2状态稳定性分析

针对非自治的动态神经网络系统，建立了动态神经网络的数学模型．并将其等效成一个非线性仿射控制系统．深入分析了该系统平衡点的存在性、唯一性和全局渐近稳定性，给出了系统输入-状态稳定的充分条件，构建了ISS-Lyapunov函数，并应用该函数确保了系统的全局渐近稳定性．     

Fail-safe stability for dynamic systems using neural-network controllers

The small gain theorem is used to consider the stability of a neural network controlled system under the condition that some of the neurons may fail with attenuated outputs and possibly with the addition of a constant output bias. Sufficient conditions based on the small gain theorem and the circle criterion are obtained for fail-safe stability. A loop-transformation technique is used to overcome the conservative nature of the small gain approach.     

Physiology-based dynamic muscle fatigue model for upper limbs during construction tasks

Due to physically demanding construction tasks, workers frequently suffer from significant levels of muscle fatigue that can cause diverse detrimental effects on safety, health, and productivity. In this regard, evaluating the level of muscle fatigue prior to work is essential to take proper preventive actions before severe fatigue takes place. Although previous research efforts have quantified muscle fatigue using surveys, instruments, and mathematical models, most of them do not take into account irregularly varying muscle activation and fatigue recovery during a task. They are thus limited, especially for construction tasks that have varying forces and intermittent idling/resting periods. This study thus proposes a physiology-based modeling approach to computationally model and empirically validate dynamic muscle fatigue generation and recovery for construction workers through laboratory testing. Specifically, a muscle fatigue estimation model for upper limbs based on System Dynamics, which is a differential equation-based continuous simulation, is developed based on fundamental physiological mechanisms of the accumulation and clearance of intramuscular metabolites during muscle exertion and their effects on muscle contractile processes. Then the model is refined and validated through laboratory experiments. The results demonstrated the immense potential for the developed elbow and shoulder models to evaluate workers' muscle fatigue in upper limbs under varying workloads. The contribution of this study is to provide an analytic tool for understanding the physiological mechanisms of muscle fatigue and estimating workers' muscle fatigue levels during construction tasks, which can help to design appropriate interventions prior to work, thereby reducing undesirable results from muscle fatigue.     

Three dimensional dynamic motor performance of the normal trunk

Traditionally, human strength has been described in terms of the maximum force one could exert under isometric conditions. However, in the past decade ergonomists have attempted to find strength measures that better represent the dynamic aspects of occupational tasks. Recent efforts have focussed upon dynamic motor performance capabilities of the human body that go beyond the traditional measures of isometric strength. The objective of this study was to document the three dimensional dynamic motor performance capabilities of the normal human trunk as subjects flexed and extended their trunks as fast as they could under 'sagittaly symmetric and asymmetric “back lifting” conditions. The results indicated that the range of motion, trunk angular velocity and trunk angular acceleration decreased in the sagittal plane as the trunk became more asymmetric. Dynamic motor performance characteristics increased in the frontal and transverse planes as the trunk became more asymmetric, however, these differences were also dependent upon the device used to measure trunk motor performance.     

一种隔水管疲劳参数低功耗遥测方法

针对深水隔水管疲劳监测问题,提出一种低功耗遥测方法;研究了3000m水深环境下,基于最优中继的隔水管疲劳遥测方案;利用水声信道特性,研究了疲劳信息多跳传输机制,监测系统总功耗是单跳链路距离的凹函数,其存在一个最小值;功耗最小值对应的链路距离即为最优中继距离;当链路距离从100m到1500m变化时,做了仿真试验;仿真结果证明,与既有方法相比,所研究的方法能极大的降低隔水管疲劳监测的功耗,提高隔水管疲劳监测设备的工作寿命。     

The dynamic stability of the flow in a meander channel

The meander channel is one of the most common channel patterns in nature.The characteristics of the flow and sediment in a meander channel which have significant effect on the development of watercourse are important subjects in river dynamics.The transition of the flow patterns in a meander channel concerns with the development mode of the channel pattern and the river regime including the generation conditions of the three-dimensional coherent vortex and secondary flow,the hierarchical scale of coherent v...     

Evaluation of artificial neural network modelling to predict torso muscle activity

Owing to the complexities involved in obtaining direct measures of in vivo muscle forces, validation of predictive models of muscle activity has been difficult. An artificial neural network (ANN) model had been previously developed for the estimation of lumbar muscle activity during moderate levels of static exertion. The predictive ability of this model is evaluated in this study using several techniques, including comparison of response surfaces and composite statistical tests of values derived from model output, with multiple EMG experimental datasets. ANN-predicted activation levels were accurately modelled to within 3% across a range of experiments and levels of combined flexion/extension and lateroflexion loadings. The results indicate both a high degree of consistency in the averaged muscle activity measured in several different experiments, and substantiate the ability of the ANN model to predict generalized recruitment patterns. It also is suggested that the use of multiple comparison methods provides a better indication of model behaviour and prediction accuracy than a single evaluation criterion.     

Robust stability of a class of hybrid dynamic uncertain systems

This paper considers the robust stability of a class of hybrid dynamic uncertain systems. It derives conditions for a class of hybrid dynamic uncertain systems with uncertainties in the continuous variable dynamic systems, variations in the ‘switching’ conditional set and variations in the reset map to be stable. © 1998 John Wiley & Sons Ltd.     

The assessment of material handling strategies in dealing with sudden loading: influences of foot placement on trunk biomechanics

Sudden unexpected loading has been identified as a risk factor of work-related low back pain (LBP). This study investigated the effects of different foot placements and load-releasing locations on trunk biomechanics under an unexpected sudden loading event. Fifteen subjects experienced sudden release of a 6.8-kg external load from symmetric or asymmetric directions while maintaining four different foot placements. The results showed that subjects experienced on average 4.1° less trunk flexion, 6.6 Nm less L5/S1 joint moment and 32.0 N less shear force with staggered stance with the right foot forward (the most preferred placement) compared with wide stance (the least preferred placement). Asymmetric load-releasing positions consistently resulted in smaller impacts on trunk biomechanics than symmetric positions. The findings suggest that staggered stance and asymmetric load-holding position can be used as a protective load-handling posture against LBP caused by sudden loading.     

I型甲烷水合物晶体稳定性的分子动力学模拟

采用分子动力学方法，模拟了晶穴占有率和温度变化对I型甲烷水合物晶体稳定性的影响。通过考察晶体破坏过程中最终构象、均方位移、扩散系数、径向分布函数等分子的动力学和结构性质的变化，分析了分子动力学模拟中晶体结构失稳的微观特征，并提出了判断甲烷水合物晶体结构失稳的依据，这对水合物形成和分解过程的研究具有重要的参考价值。     

高稳定度低纹波的线性稳压电源设计

设计制作了一款基于LT1083／LT1033系列大功率低压差三端稳压芯片的高稳定度低纹波直流电源,介绍了降压、整流滤波、线性稳压、LC低通滤波等主要构成模块。测试结果表明,本电源具有输出电压稳定度高、输出电流大、低纹波、低功耗等特点。     

Ⅰ型甲烷水合物晶体稳定性的分子动力学模拟

采用分子动力学方法,模拟了晶穴占有率和温度变化对I型甲烷水合物晶体稳定性的影响。通过考察晶体破坏过程中最终构象、均方位移、扩散系数、径向分布函数等分子的动力学和结构性质的变化,分析了分子动力学模拟中晶体结构失稳的微观特征,并提出了判断甲烷水合物晶体结构失稳的依据,这对水合物形成和分解过程的研究具有重要的参考价值。     

羽毛球动态稳定性研究

为了解决羽毛球动态稳定平衡调节问题,提出了羽毛片质点影响系数调节法。该方法结合羽毛球质量模型以及羽毛片质量、弯拱度和攻角等相关要素,通过调节质心高度得到对应的质点影响系数,求取倾斜摆动平方最小值得到满足羽毛球动态稳定性要求的羽毛片的质心调节高度。与传统的羽毛球动态稳定调节须依赖经验积累相比,所提出的羽毛片质点影响系数法形成了一个理论系统,并且具有耗时少、效率高等特点。通过数值实验进一步地验证了所提方法的正确性和有效性。