The influence of initial posture on the sit-to-stand movement

Head movements, ground reaction forces and electromyographic activity of selected muscles were recorded simultaneously from two subjects as they performed the sit-to-stand manouevre under a variety of conditions. The influence of initial leg posture on the magnitude of the various parameters under investigation was examined first. A preferred initial leg posture resulted in smaller magnitudes of head movement and ground reaction forces. EMG activity in some muscles, trapezius and erector spinae, decreased, while in others, quadriceps and hamstrings, it increased in the preferred leg posture. The decreases seen correlate with reductions in head movement observed. The effect of inhibiting habitual postural adjustments of the head and neck, by comparing "free" and "guided" movements was also examined. In guided movements there are significant reductions in head movement, ground reaction forces and EMG activity in trapezius, sternomastoid and erector spinae. It would appear that both initial leg posture and the abolition of habitual postural adjustment have a profound influence on the efficiency of the sit-to-stand manouevre. This preliminary study high-lights the practical importance of head posture in the diagnosis and treatment of movement disorders, as well as in movement education.     

Role of substrate binding forces in exchange-only transport systems: I. Transition-state theory

Summary An analysis of transition-state models for exchange-only transport shows that substrate binding forces, carrier conformational changes, and coupled substrate flow are interrelated. For a system to catalyze exchange but not net transport, addition of the substrate must convert the carrier from an immobile to a mobile form. The reduction in the energy barrier to movement is necessarily paid for out of the intrinsic binding energy between the substrate and the transport site, and is dependent on the formation of two different types of complex: a loose complex initially and a tight complex in the transition state in carrier movement. Hence the site should at first be incompletely organized for optimal binding but, following a conformational change, complementary to the substrate structure in the transition state. The conformational change, which may involve the whole protein, would be induced by cooperative interactions between the substrate and several groups within the site, involving a chelate effect. The tightness of coupling, i.e., the ratio of exchange to net transport, is directly proportional to the increased binding energy in the transition state, a relationship which allows the virtual substrate dissociation constant in the transition state to be calculated from experimental rate and half-saturation constants. Because the transition state is present in minute amount, strong bonding here does not enhance the substrate's affinity, and specificity may, therefore, be expressed in maximum exchange rates alone. However, where substrates largely convert the carrier to a transport intermediate whose mobility is the same with all substrates, specificity is also expressed in affinity. Hence the expression of substrate specificity provides evidence on the translocation mechanism.     

Role of substrate binding forces in exchange-only transport systems: II. Implications for the mechanism of the anion exchanger of red cells

Summary The transition-state theory of exchange-only membrane transport is applied to experimental results in the literature on the anion exchanger of red cells. Two central features of the system are in accord with the theory: (i) forming the transition state in translocation involves a carrier conformational change; (ii) substrate specificity is expressed in transport rates rather than affinities. The expression of specificity is consistent with other evidence for a conformational intermediate (not the transition state) formed in the translocation of all substrates. The theory, in conjunction with concepts derived from the chemistry of macrocyclic ion inclusion complexes, prescribes certain essential properties in the transport site. Separate substites are required for the preferred substrates. Cl^– and HCO ₃ ^– , to account for tight binding in the transition state (K _diss1m). Further, the following mechanism is suggested. A substrate anion initially forms a loose surface complex at one subsite, but in the transition state the subsites converge to form an inclusion complex in which the binding forces are greatly increased through a chelation effect. The conformational change at the substrate site, which is driven by the mounting forces of binding, sets in train a wider conformational change that converts the carrier from an immobile to a mobile form. Though simple, this composite-site mechanism explains many unsual features of the system. It accounts for substrate inhibition, partially noncompetitive inhibition of one substrate by another, and tunneling, which is net transport under conditions where exchange should prevail, according to other models. All three types of behavior result from the formation of a ternary complex in which substrate anions are bound at both subsites. The mechanism also accounts for the enormous range of substrate structures accepted by the system, for the complex inhibition by the organic sulfate NAP-taurine, and for the involvement of several cationic side chains and two different protein domains in the transport site.     

Displacement of soil aggregates by elongating roots and emerging shoots of crop plants

Summary Models are presented in this paper for prediction of the extent to which soil aggregates in a loose seed bed can be displaced by extending roots and shoots. For roots, the maximum applied force is considered to be limited by either elastic bending, when the angle of contact with an aggregate surface is oblique, or buckling, when the root meets an aggregate perpendicularly. For emerging shoots, only the maximum forces are known. These forces are related to the known force displacement behaviour of artificially prepared beds of graded soil aggregates.It is concluded that displacement of soil aggregates which lie between 20 and 100 mm from the surface of the aggregate bed is only likely to be important for roots with diameters of 0.5 mm or less when the diameter of the aggregates in the bed is less than 1 mm. However, for plant species which have relatively large root diameters, such as pea, significant displacement of aggregates of up to 4 mm diameter may be possible. In contrast, emerging shoots are able to displace very much larger aggregates from their paths.     

红河流域“通道-阻隔”作用下2000-2014年植被EVI变化趋势与驱动力

红河流域地表生态、水热分布格局等受到"通道-阻隔"作用的显著影响,以MODIS EVI数据作为植被定量研究指标,结合全国1：100万植被类型图、红河流域内部以及周边气象数据和研究区的DEM数据,利用趋势分析和相关性分析法,探讨在"通道-阻隔"作用的影响下流域内2000-2014年植被EVI时空变化趋势及其驱动力,重点研究了植被EVI变化对气候因子的响应规律。结果表明：（1）2000-2014年间,红河流域生长季植被EVI整体上以-0.15%/a的年际变化率呈波动减少趋势,空间异质性明显。EVI呈减少趋势区域主要集中在绿春县中部和金平县西南部;EVI呈增加趋势区域集中分布在墨江县、文山县,麻栗坡中部、广南-富宁南部区域、红河-元江一带以及藤条江西南部地区。（2）通过对生长季EVI指数与四个不同时间序列的月平均气温和月累积降水量的相关分析可知,生长季EVI指数与同期气温相关性较好;与降水量呈现明显的滞后性,滞后时间约为1个月。（3）相关分析表明,整体上红河流域生长季植被EVI与气温呈负相关关系,与降水呈正相关关系。在0.05显著性检验水平下,红河流域生长季植被EVI变化受气候影响的区域占3.11%,气温以负向驱动型为主,面积约占1.26%,降水以正向驱动型为主,面积约占0.46%,气温降水联合驱动以弱驱动为主,面积约占1.39%;大部分地区表现为受非气候因子驱动。     

Mechanobiology of embryonic skeletal development: Insights from animal models

A range of clinical conditions in which fetal movement is reduced or prevented can have a severe effect on skeletal development. Animal models have been instrumental to our understanding of the interplay between mechanical forces and skeletal development, particularly the mouse and the chick model systems. In the chick, the most commonly used means of altering the mechanical environment is by pharmaceutical agents which induce paralysis, whereas genetically modified mice with nonfunctional or absent skeletal muscle offer a valuable tool for examining the interplay between muscle forces and skeletogenesis in mammals. This article reviews the body of research on animal models of bone or joint formation in vivo in the presence of an altered or abnormal mechanical environment. In both immobilized chicks and “muscleless limb” mice, a range of effects are seen, such as shorter rudiments with less bone formation, changes in rudiment and joint shape, and abnormal joint cavitation. However, although all bones and synovial joints are affected in immobilized chicks, some rudiments and joints are unaffected in muscleless mice. We propose that extrinsic mechanical forces from movements of the mother or littermates impact on skeletogenesis in mammals, whereas the chick embryo is reliant on intrinsic movement for mechanical stimulation. The insights gained from animal models into the mechanobiology of embryonic skeletal development could provide valuable cues to prospective tissue engineers of cartilage and bone and contribute to new or improved treatments to minimize the impact on skeletal development of reduced movement in utero. Birth Defects Research (Part C) 90:203–213, 2010. © 2010 Wiley‐Liss, Inc.     

干细胞发育的生物力学调控

生物力学是采用力学方法对生物系统的结构和功能进行的研究,与生物化学信号一起是调节胚胎发育、干细胞发育分化和组织器官形成的重要因素。近年来,随着学科交叉的深入,生物力学因素越来越受到研究者的重视。目前的研究表明：在心血管和造血系统,血流产生的流体剪切力对于血管内皮和造血细胞的发育分化至关重要;此外,对于广泛研究的间充质干细胞,由细胞外基质物理特性诱导的细胞张力对于干细胞功能及其向不同子代细胞的分化也扮演了重要的角色;而在肝脏等上皮组织来源的器官,也有研究提示生物力学因素,如基质弹性等在疾病的发生发展过程中起到了不可忽视的作用。总之,在干细胞发育分化过程中,生物力学调控与生物化学信号通路怎样协同发挥作用将成为今后研究的重点。     

世界主要国家耕地动态变化及其影响因素

在世界人口持续攀升、全球耕地面积不断减少的背景下,探讨世界主要国家耕地变化其影响因素,对于分析预测未来世界耕地变化趋势、研究世界粮食安全具有积极意义。选择2050年人口过亿的17个国家和耕地面积排名前10的国家为研究对象,在分析1961—2007年耕地总量变化、人均耕地变化的基础上,探讨了耕地变化影响因素。研究结果表明,从20世纪60年代到2007年间的不同时期内,有越来越多的国家表现出耕地减少趋势,而人均耕地面积减少的国家个数高达90%以上。满足人口消费需求、城市化与经济发展是大多数国家耕地总量变化的主要动力;而人口快速增长、城市化则是导致许多国家人均耕地显著减少的重要原因。     

干旱区包气带土壤水分运移能量关系及驱动力研究评述

包气带土壤能量和水分平衡及其驱动因子是维系地下水-土壤-植物-大气连续体(GSPAC)系统中水分运移发生的关键因素。在降水稀少、水资源短缺的干旱地区,开展包气带土壤水分形态、运移过程与能量的耦合规律研究对揭示区域水资源形成和转化机理具有极其重要的现实意义。文章总结了土壤水分运移理论研究进展,探讨了水分参与水文循环过程及干旱环境下土壤水分可能表现形态及其降雨入渗、再分布、渗漏、蒸发、毛管水上升等过程驱动机制,评述了包气带土壤水分与能量过程在不同空间尺度上生态水分效应。在一个非饱和土壤系统中,水分运移受包气带结构,土壤物理特征,植物根系和土壤生化环境的综合控制,物质和能量平衡改变是驱动水分循环的源动力,而土壤环境变化是导致水分运移形态的发生变化根本原因。因此,在气候变化背景下,研究干旱区土壤与大气界面以及包气带与饱和带界面水、汽、热耦合转化形式与能量驱动过程,能够提升我们对包气带土壤水分运移规律机理的深入理解,丰富对区域气候和水文变化认知。为干旱区生态植被恢复建设和水资源精细化管理提供理论向导。     

德国人文聚落区生态单元制图国家项目

联邦德国自然保护法、德国各州自然保护法要求在自然景观区和人文聚落区保护、管理和发展自然。实现这一法律要求 ,必须以现实的、全面的、高质量的、与自然保护原则相匹配的数据为基础。实践证明 ,城乡生态单元制图是获取和解释这些数据不可或缺的手段。德国人文聚落区生态单元制图始于 1 978年 ,同年组建成立了德国联邦与各州自然保护部门参加并联合协调的“人文聚落区生态单元制图工作组”。 1 986年工作组提出了德国人文聚落区生态单元制图基本方案 ,1 993年完成基本方案修订版。基本方案的目标是在全国范围建立标准制图方法 ,使获取的科学数据可以相互比较。至 2 0 0 0年 ,德国在人文聚落区生态单元制图项目上的投资约合 2亿马克 ;至 2 0 0 1年 3月 ,德国有 2 2 3个大中城市已经完成或正在实施生态单元制图 ,人文聚落区生态单元制图已经成为德国各级政府生态规划的基本工作内容。